Keeping a greenhouse warm

For growers large and small, the ancient change of seasons has long played foil to agricultural success throughout the year. Growers in northern climates – especially those in areas expecting heavy snowfall, low (or no) daylight, and extremely cold temperatures – often forgo growing altogether during the cold season, and simply shut down operations until the spring once again shows its face.

With a greenhouse, however, growers across the world are flipping the script on winter growing.

Because a greenhouse offers growers a controlled and consistent environment regardless of the season, many can keep their crops coming up throughout the year – and that means you can keep growing all through the winter. Here’s a look at how to keep your production steady even when the temperature starts to drop.

Contents

1. Winterize… The Right Way

Spring may be synonymous with “spring cleaning,” but for greenhouse growers, early winter brings its own call to clean up a bit in anticipation of the changing weather.

The “winterizing” process is fairly routine for greenhouse owners, whether you plan on keeping a winter grow going or not. Essentially, it’s your chance to clean up completely from the late summer and fall harvest season, and get your greenhouse in top working shape before the snow falls. That means thoroughly cleaning out any and all remaining organic materials in your greenhouse, as well as scrubbing and disinfecting the walls and equipment within your greenhouse.

It’s also a good opportunity to inspect all moving parts of your greenhouse for any maintenance requirements, which should be repaired or replaced well before the snow starts to fall, which will help ensure a healthy and happy greenhouse even when the temperature drops below zero.

2. Pick The Right Crops

Ever tried to grow a mango in the arctic? Obviously, it’s not going to work – the plant needs more light and heat than the arctic environment can really ever provide. Growing through winter – even growing the crops you succeed with all year long – can offer up the same problems.

That’s why it’s critical when planning your winter grow to pick crops you know will withstand the extremes of winter conditions. While your greenhouse may mitigate many of the most damaging aspects of the winter season, growers in northern and snowy climates should still seek out hardy root vegetables and leafy greens – like spinach, carrots, collards, and others – to ensure success throughout the winter growing season.

3. Light Well

Nothing great grows in the darkness – at least, nothing edible anyway. That’s why so many crops start to struggle just when the sunlight starts to creep backward, and long days of bright sunshine start giving way to colder, later mornings and longer, darker nights. Growing cold-tolerant crops will be sustainable in the grow cycle going through the dark.

4. Control Heat & Humidity

The colder it is outside your greenhouse, the higher your demand for heat will be inside your greenhouse. Whether you’re facing mild winters down south or harsh, sub-freezing storms up north, the heat from the sun may simply not be enough to keep your crops coming until spring. That’s why an efficient and properly-sized supplemental heating system might just be necessary for your winter greenhouse if required for the crops you are growing.

But it’s not only plants that need heat to keep growing through winter. Certain diseases love a warm hideaway when the outside is inhospitable, and your warm, humid greenhouse might just be the perfect place for diseases to settle in. That’s why it’s critical to balance your humidity as well as your heat, either through proper ventilation or effective dehumidifying systems, to prevent diseases from slowing down your crops.

5. Harvest Efficiently

One of the most common mistakes first-time winter growers make is to treat the winter harvest like the spring or summer. While grow periods during other times of the year might culminate in one big harvest of all the same crop over a short time span, winter harvests should ideally be spread out to cover the entire season. This means being smart about your scheduling, harvesting when you need to, and allowing plants that can regrow to do so.

Many winter growers plan out their beds in rows which can be harvested one-by-one, allowing the first-harvested rows to re-grow and repopulate in the time the other rows are being harvested. That way, the cycle of fresh veggies being harvested – although lower in number – will last all the way through until spring comes. This way, growers can continue to provide fresh crops at farmer’s markets throughout the winter without leaving customers wanting.

Grow Strong Through Till Spring

Keeping your greenhouse fresh and productive through the traditionally-barren winter may seem impossible to the inexperienced, but with smart planning and the right tools you can keep your best produce coming in strong through the winter and well into the spring season. With a greenhouse, year-round growing is possible. Are you ready to get growing this winter?

Keeping the heat in your greenhouse in winter

A greenhouse can be invaluable in winter, keeping tender plants sheltered from the worst of the weather.

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You can also use your greenhouse in winter for growing herbs, winter salads and citrus plants, plus sowing early seeds – discover seven projects for a winter greenhouse.

Insulating your greenhouse with a layer of bubble wrap will keep frost at bay while still letting in light. Find out how to do it, below.

Buy bubble polythene

Fitting panels of bubble-wrap into panes of a greenhouse

Bubble insulation wrap is sold on big rolls – usually 75cm wide – and is available in garden centres or online.

Create ‘double glazing’

Pinning overlapping layers of bubble-wrap to the wooden frame of a greenhouse

Cut the sheets to length but rather than trimming the width, tuck the excess polythene under as an extra layer. This keeps the polythene away from the glazing, trapping a layer of air between the glass and sheet to reduce heat loss – like double glazing.

Seal off draughts

Joining sheets of bubble-wrap with clear, weatherproof tape

Use clear weather proofing tape for gap-free joins between the insulation sheets. Fit the sheets carefully around vents to block out draughts. Plug gaps in glazing and replace seals around doors if necessary.

Create a curtain

Creating a bubble-wrap curtain to divide a greenhouse

Use bubble insulation as an internal curtain to divide your greenhouse up into smaller areas. Fix tightly around the roof and sides, down to floor level and anchor it down with something heavy. Once in place, you’ll find small areas often remain warm without heating.

Add heat only when needed

A greenhouse heater

Turn heaters on when needed, off when not. An electric fan heater with a variable thermostat is a good option if you have an electricity supply. If you don’t, paraffin heaters are useful. Top up the fuel tank each evening.

Use a max-min thermometer

A digital and analogue max-min thermometer Advertisement

Use a max-min thermometer to check if heaters are working well. You could put a digital thermometer sensor in the greenhouse to transmit a reading to the display monitor in your home. Check regularly, especially in cold spells, to ensure the temperature remains above your desired level.

Other ways to grow plants in winter

If you only have a small number of plants to protect from frost, why not use a heated propagator, instead of insulating your greenhouse? Heating a greenhouse is expensive and may not be needed. Raise seedlings in an electric propagator or on a windowsill indoors, instead.

19 Ways to Heat Your Greenhouse

Photo: RustyBuggy.com

The folks at Living the Good Life shared a video discussing 10 ways to heat your greenhouse. One big tip is to not plant right up next to the wall. Leave some space.

  1. Use plastic bottles filled with water left sitting in the sun. Dark bottles hold heat longer and don’t allow gunk to grow in them. You can minimize the gunk (algae and water mold) by adding bleach. If you have a large greenhouse, black 55 gallon drums placed along the north side work well.
  2. Make a composting area using manure and kitchen scraps mixed with leaves and shredded paper. While the composting process is taking place, heat is released. Some greenhouse gardeners in other countries keep animals and plants together in their houses, you just need to watch where you step. Don’t plant directly in the composting material unless there is 6-8” of soil on top of the working mixture.
  3. Terra cotta (AKA Clay) pots stacked with tea lights will add warmth.
  4. Heat lamps can be used but you will want to add a fan to circulate the air to keep them from over heating and the closest plants from parching.
  5. Electric space heaters can also be used, but need to be away from wetness. Get one with a thermostat so you can control heating temperature.
  6. Ceramic space heater tile uses less electricity and is safe to touch; may still need a fan to spread heat around greenhouse
  7. Propane space heaters give off CO and needs to have an outside vent; you can boil large pots of water and leave them in the greenhouse overnight
  8. Geothermal heating elements are probably the most complicated. They require water circulation underground to transfer heating and cooling.
  9. Solar panels can be set up to create the electricity needed to run a heater.
  10. Wood stove will give off the most heat for little cost, but you must have wood available

Sue Sanderson of Thompson-Morgan has additional suggestions.

  1. Insulate with horticultural bubble wrap. The larger bubbles let in more light.
  2. Use a thermostat on any heater so you only heat when necessary
  3. Choose the right temperature – 45ºF to 55ºF is adequate to keep young plants growing.
  4. Use a thermometer to record maximum and minimum temperatures to work more efficiently
  5. Position heaters carefully – not so close as to dry out plants foliage
  6. Only heat the area you need. If you are not using the entire area make a cluster grouping and bubble wrap around them to make a partition
  7. Use horticultural fleece on extra cold nights; don’t forget to take it off in the morning
  8. Ventilation is important to keep air moving to prevent diseases; open vents during warm days, close before the sun goes down
  9. Use a seed germinator heating pad to stimulate seed sprouting and keep young seedlings warm

Then there is the Chinese greenhouse. This passive greenhouse style has three walls of brick, clay or earth that make up the north, east and west sides. Only the south side, covered with a transparent material, allows the sunshine in. The walls capture solar energy during the day into the thermal mass walls, which release it during the night as heat. At sunset, an insulating sheet, usually made of stray, pressed grass or canvas, can be rolled out over the plastic to further slow heat loss. These features keep the temperature inside up to 45degrees higher than the outdoors.

Billie Nicholson, Editor
November 2017

3 Methods for Heating Greenhouses for Free

How-to

The most common way to use thermal mass is water barrels, because it has such a high heat capacity. By stacking several 55 gallon drums of water in a greenhouse, the grower can incorporate a lot of thermal mass. Barrels should be stacked where they are in direct sunlight, often on a North wall. Since plants will be warmer around the water barrels, put more tender plants — like seeding trays or warm weather crops — on or near the barrels. Growing with an aquaponics system — growing fish and plants symbiotically — has the nice benefit of the fish tanks doubling as thermal mass. Other variations include building concrete or stone into the greenhouse — such as using a concrete North wall or flagstone floor. Even the soil in raised beds will add thermal mass.

While the easiest to install, thermal mass can be slow to react. It takes longer to disseminate the heat throughout the greenhouse, limiting its effectiveness. But, given the low upfront cost, adding thermal mass to a greenhouse is a popular method for extending the growing season. It may not get you year-round growth of all things, but it can certainly take your greenhouse to the next level.

2) Incorporate a heat exchanger

To go one step beyond standard thermal mass, you can incorporate a heat exchanger to circulate air through the source of mass. This idea goes by many names. It’s often called a Climate Battery or a Subterranean Heating and Cooling System (SHCS) — a name popularized by John Cruickshank of sunnyjohn.com. Ceres Greenhouse Solutions, based in Boulder, CO, also has a variation of the system called a Ground to Air Heat Transfer (GAHT) System.

There are many configurations, but the mechanism of energy transfer and storage is always the same. When the greenhouse heats up during the day, a fan pumps warm humid air from the interior of the greenhouse through a network of pipes buried up to 4’ underground (most systems consist of a couple layers of tubes buried at 4’ and 2’ below the surface). The drop in temperature forces the water vapor to condense, and in that process (called a phase change) energy is released. That energy is stored in the soil, causing the soil to heat up. Thus, the process creates a large mass of warm soil underneath the greenhouse year-round. At night, when the greenhouse drops in temperature, the fan kicks on again and extracts that heat from the soil. It’s a relatively simple, time-tested system; ground to air heat exchangers have been used in homes for decades.

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A ground to air heat exchanger works very well for two reasons: First, the amount of available mass (the size of the battery as we mentioned before) is huge. For example, there are 768 cubic feet of soil beneath a 12’ x 16’ greenhouse, assuming a 4’ depth. If you lined the whole North wall of the same greenhouse with two rows of 55 gallon water barrels (16 barrels) they would have a total of 118 cubic feet of mass. That means, using the volumetric heat capacities in the table above, the underground heat exchanger has about twice the capacity as the water barrels. Moreover, because a ground to air heat exchanger connects to the deep earth and thus theoretically has an infinite capacity. For a diagram to better understand this, see CERES Greenhouses picture here.

Secondly, because air is actively being pushed through the ‘battery’ it increases the rate of heat exchange. The hotter / cooler air is distributed around the greenhouse more evenly, preventing cold pockets. Additionally, using fans allows you to use the mass when you want: a thermostat kicks the fan on and off at certain set temperatures. I.e., the fan will start pumping warm air down into the soil when the greenhouse reaches a set temperature (say 80 F), and draw it back up when it has gone below 50 F. Thus, an underground heat exchanger gives you some control over thermal mass; it’s kind of like taking thermal mass and making it smarter.

Variations

The material of the battery can vary. Some people backfill the area underneath the greenhouse with gravel or stones instead of soil. If you already have a greenhouse, or can’t excavate on your site to do much ground work, you can create an alternative battery above ground. You can build an insulated mass of soil or other material, such as a box of river rocks in front of the greenhouse. The system works the same way, only the location of the thermal mass is different.

3) Use an efficient renewable-powered heater

The above systems show you how to harness the sun and store solar energy, which is a good first step to natural heating. If additional heating is needed, consider a highly efficient heating system that runs off of cheap and renewable fuel.

One of the common systems used in greenhouses is the rocket mass heater, a super efficient variation of a wood stove. Instead of just exhausting hot air straight out of a chimney like a standard wood stove does, the rocket mass heater first circulates the hot air through a mass of cob, brick or stone before it’s exhausted out. The air warms the mass which holds the heat and slowly radiates it back into the greenhouse over a long period of time, even after the stove is done burning. The rocket mass heater also uses a double combustion chamber, making it much more efficient than a standard wood stove — a couple hours of a burn with a small amount of wood can heat a greenhouse overnight. Most rocket mass heaters are DIY systems; you will have to investigate and design a system that fits for your greenhouse using the plethora of plans and explanations online.

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Another common greenhouse system is the compost-pile heater, which relies on the magic of aerobic bacteria to break down organic material and give off waste heat. Like the underground heat-exchanger, a compost heater also relies on a heat exchanger: water is circulated through tubes running through a large compost pile. Because of the aerobic decomposition, a compost pile can maintain temperatures of 100-160 F. The heated water is then is circulated through the greenhouse where it dispenses heat. Of all the systems, this one probably takes the most tinkering to get right and keep going. You must first build your compost pile with the right material and consistency to get it to a high temperature, and keep adding to it or re-building the pile as it decomposes. However, a large, properly constructed pile (see picture below) can keep a 1,000-2,000 sq. ft. greenhouse heated for a winter. For these reasons, compost pile heaters are often best suited for larger greenhouses.

Summary

Which way to go? Several factors play in:

What are your goals (how much space are you trying to heat, and to what degree)? Each system has a different capacity for heating. How much control do you want to have? (Some systems are active and some are passive. (i.e., You can crank up a rocket mass heater but there’s not much you can do to change water barrels).

What constraints are you already working with? (i.e., difficult/rocky soils will rule out an underground heat-exchanger.) Think about how much floor space in the greenhouse you have for things like water barrels. And most importantly think about the time and labor involved in installing each system, as well as the on-going time/labor that it can take to run each system (i.e., an underground heat exchanger can be automated, whereas a rocket mass heater cannot be). Again, while you need to do some homework upfront, having a warm greenhouse churning out fresh food throughout the winter (and free!) is the best payoff you can get.

(Top) Photos courtesy Ceres Greenhouse Solutions: Pipes in an underground heat exchanger for a 12 x 20 greenhouse. 3D model of an underground heat exchanger below ground.

(Middle) Photo courtesy Verge Permaculture: Rocket mass heater in a greenhouse.

(Bottom) Photos courtesy Golden Hoof Farm: Compost pile in mid-construction with tubing for aeration. Completed compost pile.

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Cooler temperatures mean slower growth, and that is never welcomed news in the greenhouse. In fact, raising a plant’s soil by just 10 degrees Fahrenheit has the power to increase the plant’s height – depending on the plant – by a factor of two. That’s why heat retention matters for growers looking to maximize efficiencies without spending their hard-earned capital on supplemental heating devices.

Some growers will have the means to install an integrated environmental control system with an attached heating system. This is a fantastic upfront investment that will bring major dividends come harvest time. But for others, this may not be an option. The following are five passive heating tricks to maintain natural heat in your greenhouse.

Paint Them Black

This is a great option for growers with a little extra space in the greenhouse. Paint the outside of several 55-gallon plastic containers with flat black enamel. Smaller greenhouses can achieve the same effect with painted single gallon jugs or paint buckets.

Place whatever vessel you choose in a place where it will receive the most sunlight possible throughout the day. The larger the bucket, the more heat that will be retained. As night approaches and temperatures dip, the heat captured emanates from the water and will warm your greenhouse or high tunnel.

Only fill the buckets three-quarters of the way. If the buckets are sealed tight and heat to a certain temperature, there is the potential for the lid to blow off due to increasing pressure. If done correctly, the greenhouse can stay an average of 20-30 degrees warmer than the outside temperature – even in the dead of winter!

Compost For Warmth

Compost is a grower’s best friend. Finding just the right blend for your specific crops can take years to perfect. But beyond providing the essential nutrients to your plants, compost has another beneficial use: heat. The chemical breakdown that occurs in compost releases energy in the form of heat that can rise well above 100 degrees Fahrenheit. This benefits your heat-starved plants in the winter (and speeds up their growth).

There are several ways to implement this low-tech technique. You can use more of those 55-gallon drums mentioned earlier or bales of straw.

Rescue With Row Covers

This might be less creative than compost or gallon drums, but row covers – also known as floating row covers – can protect your plants from freezing. Just ask the team at Pleasant Valley Farms in Argyle, NY. This four-season operation uses both a Matterhorn Greenhouse and two Nor’Easter High-Tunnels. Even in the bitter cold winter, the family-run farm is able to grow produce. They credit the use of row covers and a constant monitoring of weather conditions to protect their plants at night.

Keep In The Heat

Ever heard the saying “did you grow up in a barn?” This applies in the greenhouse just the same. You generated this hard-earned heat through all the creative ways we discussed above. Now make sure that you retain that heat as much as possible. To do this, survey your greenhouse for any small cracks or gaps. Where you find either, apply silicone caulk to ensure that heat cannot escape. Duct tape is also an option for a quick-fix in a bind.

Heat Only What You Need

Separating space can save you valuable resources and maximize the efficiency of your greenhouse. Especially in larger greenhouses, heating the entire space can be a time-consuming, costly endeavor. First, group delicate plants and more hardy plants in different sections of the greenhouse. Then, erect a solid Perspex partition or create bubble wrap curtains to divide the space into more easily heated spaces. This is more economical and will allow you to control the temperatures to the liking of each plant that you grow.

Four-season growing is a challenge – especially in hardiness zones 1-6. Without supplemental heat generated through oil or gas, it can be that much harder. But with these low-tech techniques it is still possible to generate much-needed heat for your plants all year round.

Do you have your own low-tech ways for generating passive heat? Share the wealth in the comments below for other growers to use and profit from.

Heating a Greenhouse

SfC Home > Physics > Thermal Energy >

by Ron Kurtus (revised 9 November 2014)

A greenhouse is a building that is heated with solar radiation and insulated to prevent loss from convection, conduction and radiation, such that it can stay warm without external heating even during cold days of winter. Such a building is used to grow plants during the winter.

Light from the Sun passes through the glass roof of the greenhouse to heat the plants and ground inside. Objects heated by sunlight emit infrared radiation.

These objects then emit infrared radiation that is absorbed or reflected by the glass roof, thus trapping the thermal energy in the greenhouse instead of letting it escape. This helps keep the building warm.

Questions you may have include:

  • What effect does the glass roof have on the solar radiation?
  • How does the glass roof affect infrared radiation?
  • How does the greenhouse stay warm at night?

This lesson will answer those questions. Useful tool: Units Conversion

Glass lets in solar radiation

A greenhouse is built with glass walls and a glass roof. Sometimes clear plastic is used instead of glass. One purpose is to provide light to the plants to help them grow and another purpose is to help keep the greenhouse warm.

Solar radiation or light from the Sun or passes through the glass and warms the plants, soil, and other things inside the building. Light is almost completely absorbed into the dark soil in the pots holding the plants, increasing the temperature of the materials.

Greenhouse heated by solar radiation

Besides letting the light energy in, the glass walls and roof act as a thermal insulator to protect the inside from cold air and winds outside the greenhouse.

Warmed air warmed is retained in the building by the roof and walls. There is no heat transferred to the outside by air convection. The glass allows only a little heat loss due to conduction of heat through its material.

(See Heat Transfer for more information on that subject.)

Glass prevents infrared from escaping

Although the glass used for a greenhouse allows visible light and short wavelength infrared radiation to pass through it, it does not transmit the longer infrared wavelengths. This means the radiation is prevented from escaping, causing a loss of heat.

Greenhouse remains warm

Typically, the greenhouse glass will transmit solar radiation of wavelengths between 280 nm and 2500 nm, while absorbing infrared or thermal radiation in the 5000 nm to 35000 nm region.

Note: nm stands for nanometer or one 10 millionth of a meter (10-9 meter). Some books denote wavelength in microns. A micron is one millionth of a meter (10-6 meter). Thus 2500 nm is the same wavelength as 2.5 microns.

Infrared radiation that does not pass through the greenhouse glass walls and roof is absorbed in the material. The glass then re-radiates the infrared back to the material inside the greenhouse, thus trapping the thermal energy inside the building and keeping it at a warmer temperature.

(See Infrared Radiation and Infrared, Gases and the Greenhouse Effect for information on those subjects.)

Ground slowly releases energy

Besides using the soil in the pots or on the ground of the greenhouse as a source of storing thermal energy, some greenhouses add materials such as containers of water or bins of sand and rock to absorb and store even more of that energy during the day. They help maintain a steady temperature in the greenhouse.

Since heated materials give off infrared radiation, the warm greenhouse soil, water, sand and other materials emanate this thermal radiation in longer wavelengths than the radiation that heated the materials. This energy is slowly released, even during the night.

A greenhouse is a building that is heated with solar radiation, such that it can stay warm even during cold days of winter. Light from the Sun passes through the glass roof to heat plants and the ground inside the greenhouse. These objects then emit infrared radiation, which is absorbed in the glass roof. Thermal energy is trapped in the greenhouse, keeping the building warm.

Be kind to your environment

Resources and references

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Best Materials for Greenhouse Roof – from WiseGeek.com

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Top-rated books on Greenhouses

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Dispelling Common Fertilizer Misconceptions

As edible gardening continues to become more mainstream, some of your clients may have opted to use a greenhouse to grow their veggies instead of raised beds.

If this is the case, they may be under the impression they won’t be able to grow anything during the winter due to the long, cold nights causing temperatures to drop. However, this sheltered spot for plants doesn’t have to go dormant if they follow these tips from you.

Let the sunshine in

If the contractor was smart about the site and space planning of the greenhouse when it was built, then the structure should already be in a location that is free from shading from trees or other buildings. But when the sun’s trajectory is lower in the sky, other obstacles could create shadows, so make sure the greenhouse is getting as much sunlight as possible.

Add thermal mass

One of the easiest and least expensive options for warming greenhouses in the winter is to create a thermal mass or heat sink. These are objects that absorb heat during the day and release it during the chilly nighttime hours. It will raise the temperature by a degree or two and it can make all the difference.

A popular method of creating a thermal mass is placing containers of water in the greenhouse. For smaller greenhouses, capped one-gallon plastic jugs filled three-quarters full of water can be placed throughout the greenhouse among the plants. The jugs can be painted black or black food coloring can be added to the water to increase heat absorption.

For larger spaces, 55-gallon barrels painted black and filled with water can be placed in areas of direct sun. They will release the absorbed energy over the night as well.

Use a germination mat

If your client is mostly wanting to use their greenhouse over the winter to get a head start on their spring plants, a germination mat can serve their needs nicely. These mats help decrease production time by maintaining optimal root-zone temperature and increase the growth rates in the early stages of a plant’s life cycle.

Cover up

Customers can go as elaborate or as simple as they want when it comes to covering plants during particularly cold nights to provide extra degrees of warmth. Tarps, horticultural fleece, row covers or sheets can all help hold moisture in, but remind clients to keep the covers off the plants themselves and to remember to remove them during the day, as the humidity level can get too high.

Create some compost

For customers who already have a pile of compost, moving it in to the middle of the greenhouse can be a win-win, as it protects the compost from the elements and is warmer. This allows the process to speed up, while raising the temperature of the greenhouse as it breaks materials down. The optimal location for the pile is in the center of the greenhouse, but if this isn’t feasible, it can be placed elsewhere and still provide warmth. Also, come springtime the homeowner will have a pile of black gold readily available.

This alternative heating method may not be ideal if your customer doesn’t know much about composting, or doesn’t like the idea of having a compost pile in the middle of the greenhouse. This option should be skipped if the greenhouse is one that is connected to the house, as they can attract mice and rats in the winter.

Add insulation

Believe it or not, bubble wrap has another purpose other than protecting fragile packages and providing entertainment for the easily amused. Attaching a layer of bubble wrap to the interior walls of the greenhouse can reduce heat loss and block winter drafts. Horticultural bubble wrap insulation can be found at garden centers and is UV stabilized and has larger bubbles, but traditional bubble wrap can suffice in a pinch.

Install a heater

Probably the most obvious option when it comes to warming a greenhouse is simply installing some heaters, but these can be expensive to run and the heat can quickly dissipate when the heater is turned off. There are space heaters designed specifically for greenhouses, and propane heaters are a good option if your client doesn’t want to have to run extension cords to the greenhouse. It is important to have fans as well to distribute the warm air throughout the space.

If using an electric heater, be sure to check the cords and connections to make sure they aren’t frayed or worn. Ventilation is very important for controlling carbon monoxide levels and to prevent overheating the space, which is rare in the winter but still possible in warmer climates.

The thought of having a greenhouse is a bit intimidating for some people. Isn’t it already tough enough to pay the power bills? Now, the plants need their own house to stay warm, too! Luckily, with a little know-how, it’s possible to design and keep a greenhouse reasonably warm without using any electricity at all. It might not be roasting and toasting, but it’ll keep temperatures above freezing and keep the wind off the plants.

The number one trick to heating a greenhouse without power is taking advantage of the sun. The sun is free energy, and it works day in and day out. However, there are times when it’s cloudy, not to mention that the hours of sunlight are reduced in winter. Luckily, there are several ways to naturally help heat up the greenhouse, as well as design wisely to make the most of the heat that’s around anyway.

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1. Make Compost in Your Greenhouse

Every gardener at one time or another has seen it. We start digging in the compost pile, and that thing is literally smoking hot. A good compost pile will get well above 100 degrees Fahrenheit, and it can stay there for a while, particularly if it is regularly turned to add oxygen. In other words, compost will naturally put out heat that can warm the greenhouse. Having paths made from (dark) wood mulch and mulching the beds will add to this heating by decomposition.

2. Utilize Thermal Mass Objects

When trying to passively heat with solar energy, thermal mass is crucial. Some objects, rocks, clay, and bricks, absorb heat when air is warm and release it when air is cold. Water also has good thermal mass. To heat for free, we’ve got to take advantage of thermal mass. It’s wise to make raised beds with stone or brick walls to absorb the heat. It’s a good idea to put some black barrels or water around as well.

3. Double Up on the Windows

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In places where times get seriously cold, it’s code that our homes have double-pane windows. That’s because they are much more energy-efficient, allowing warmth from the sun to come in during the day and prevent heat loss due to cold temperatures outside. While it’s probably too costly for most people to use double-pane windows for a greenhouse, it might not be a bad idea to add a layer of (repurposed) clear plastic to create a double-pane effect. This will help to seal the greenhouse as well, but a little caulk here and there wouldn’t hurt either.

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4. Insulate the North Side

For those of us who live in the Northern Hemisphere, there is no point in having the north side of our greenhouse be glass. The sun never shines from the northern side, so it makes a lot more sense to insulate that side, which will prevent some heat loss from inside and thwart the frigid north winds from seeping in from outside. Plus, we could put some of our thermal mass heating back against that wall to absorb more sunshine.

5. Reflect the Sun’s Light and Heat

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To add to the effectiveness of the insulated northern wall in the greenhouse, it is a good idea to have the (south-facing) interior of it be painted white or covered with a reflective material like tin. The sun will bounce off this wall and into the garden beds, providing the plants — at least in the daytime — with more sun rays and warmth.

6. Sink the Greenhouse

While most of us think of basements as dingy, cold places, the truth is that underground is often warmer than above ground in the winter. The deeper into the earth we go, the more constant the temperature becomes, and the consistency is always above freezing. So, it’s not a bad idea to sink the floor of the greenhouse to below the frostline; that way the warmer earth can help to moderate the temperature inside, and in the garden beds particularly.

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7. Install Power-free Heated Beds

As long as we are into making compost in the greenhouse, heating thermal mass barrels of water, constructing thermal mass raised beds, reflecting sunlight from the insulated north wall, and sinking the greenhouse into middle earth, it might be worth installing some solar water heating pipes on a closed loop to pump through the garden beds. These pipes can be coiled in the middle of the compost heap so that the warmth from it will heat the water, which can move through the pipes and heat the soil from within the garden beds.

By no means will these methods entirely replace a powered greenhouse kept at tropical temperatures, but they can keep a space warm enough for plants suited to temperate climates, likely several USDA zones warmer. Plus, a greenhouse like this will keep cold-hardy greens and vegetables growing all winter long, and once it’s up, it doesn’t cost a thing to keep warm.

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Heating a greenhouse in winter

Paraffin heater, Straw, Cardboard, Bubble wrap, Decomposing Hay or Straw bales, Dust bin or old oil drum filled with water, even a Rabbit hutch complete with rabbit will warm it up. Read how to use them……..

  • Mini plastic greenhouse. If you can’t be bothered to bubble wrap your greenhouse but still want to grow plants in Winter try placing a cheap mini plastic greenhouses inside the glass greenhouse.
  • Placing a Rabbit hutch in your greenhouse will help heat it up. A cheap way to heat your greenhouse, if you have a pet rabbit, is to place the hut inside the greenhouse, the heat it radiates will take the chill of the air. The rabbit might like it too.
  • Composting Hay or Straw bales will heat up a greenhouse in winter. If you have enough floor space, try composting a bale, a composting bale gives off heat. You can also plant and grow stuff in the bale, so space may not be a problem for you. More information on this page hay bale garden.
  • Dust bin or old oil drum filled with water saves greenhouse heating costs. Place a dust bin or old oil drum, inside your greenhouse and fill it with water. Water has a high specific heat (amount of energy needed to raise the temperature by one degree) and will absorb excess heat keeping the temperature down in the summer. The high specific heat of water will also allow it to hold onto the heat in winter and keep your greenhouse a few degrees warmer in the winter. Haven’t tried this but it sounds as if it should work.
  • Paraffin heaters are simple to use and providing the greenhouse is less than 8ft long and well insulated, not too expensive to run.
  • Bubble wrap. insulating a small greenhouse with bubble wrap is cheap and easy and will only take about an hour to wrap-up a small greenhouse.
  • Screening off part of the greenhouse with polythene and battens will limit the area to be heated.
  • Just a few plants. If you are not planting and growing through the winter, but just heating the whole greenhouse for a few less hardy plants, you might want to think about moving them into your house for the winter months.

DIY Projects: 4 Ways To Heat A Greenhouse 

In one of my recent articles I thoroughly discussed the “how to’s” of DIY-ing your own greenhouse.

Now, let’s address another issue: how to heat a greenhouse during the harsh winter months, because even if a greenhouse is an excellent environment for growing plants and veggies, stabilizing the temperature inside is of major importance to your crops.

If you’re wondering what I am talking about, consider this: even in October and November, the temperature inside a glass-covered greenhouse can fluctuate considerably, ranging between 30 degrees F lows and 100 degrees F highs.

This massive fluctuation in temperature happens regularly in certain climate conditions. Why? Well, the glazing of a greenhouse does a great job when it comes to letting in the sunlight and therefore the heat, but it’s also very good at letting heat out. That’s because glass or plastic does a relatively poor job in terms of insulation.

Actually, almost all greenhouses tend to overheat during the day if they’re not “temperature controlled”.

During the night when the temperature drops, the greenhouse loses all the heat, causing the plants to freeze. As you can easily imagine, plants (just like people) are not very happy in these circumstances.

So, what can you do to mitigate the problem? In order to control the temperature swings, you must install either a heater or a cooler inside the greenhouse. The cooling job is easier, as it’s basically taken care of by an efficient ventilation system.

Today we’ll take care of the heating thingy; that’s the hardest part of the job.

The smartest and also most sustainable way for mitigating the temperature swings inside a greenhouse is to capture the “extra” solar energy getting in during the day, then store it and use it later during the night when the temperature drops. That’s one solution.

Another solution is to build an efficient heating system that uses renewable or cheap fuels.

When building a greenhouse, remember to design it in such way that it doesn’t require very much cooling or heating in the first place. Good design is key and I discuss that in my article about building a greenhouse.

To revisit that topic briefly, that involves properly insulating the structure, using high-quality materials for roofing, and orienting the greenhouse facing south.

Now, let’s talk about heating solutions, tips and tricks, and the whole nine yards, right after the break!

1. Additional Insulation

Let’s begin with the simplest method: additional insulation. For blocking icy winter droughts and significantly reducing heat loss during the winter, the easiest and cheapest way is to add an insulating layer of bubble wrap, attached with clips to the inside frame of your greenhouse. This trick works very well even when it comes to unheated greenhouses.

For best results, go for horticultural bubble wrap insulation, which is available at garden centers. Unlike regular bubble wrap, this one is tougher and also UV-stabilized. Remember that the bigger the bubbles, the more light they let in.

Besides bubble wrap, you may also use horticultural fleece for further insulating your greenhouse and adding a few extra degrees for your plants during extra-cold winter nights. Just remember to remove the fleece during the day to ensure that your plants and veggies receive proper light and ventilation.

2. Heating System

Now, these are temporary, palliative solutions for heating a greenhouse. A better option is to invest in a heating system. Ideally, you should use electric fan-heaters, which can be easily moved around the greenhouse, thus preventing the apparition of cold spots and reducing the risks of plant disease.

When using an electrical heating system for your greenhouse, remember to save energy and money by investing in a thermostat, which will allow you to start the heaters only when necessary, i.e. when the temperature reaches a specific value. Also, invest in a high quality thermometer and check it daily; in this way you’ll be able to use and adjust your greenhouse heater more efficiently.

Try to avoid wasting money and energy by choosing the optimal temperature inside your greenhouse. Remember that most plants will thrive at temperatures as low as 45 degrees F and some of them even below that. The idea is not to transform your greenhouse into a tropical paradise; that’s not really necessary.

Remember to position your electric heaters carefully. Place them in a central spot, out in the open, or at one end of the greenhouse at a time, and heat only the areas that you need to.

For example, if you have a big greenhouse and only a few delicate plants, you just group them together and try to partition the greenhouse into smaller areas (use bubble wrap insulation curtains for example) which can be heated easily and economically.

But, there’s a problem with electric heaters: they are relatively expensive and they require a power supply. If you don’t have electricity nearby, you can go for paraffin heaters.

3. Heat Sink / Thermal Mass

However, if you’re a die-hard off-the-grid prepper, you should opt for building a heat sink or a thermal mass (they’re the same thing basically). The thermal mass is the smart solution I was talking about in the preamble of the article.

Thermal mass can be defined as any type of material or structure which is able to store thermal energy. And, obviously, almost any type of material is capable of doing that; it’s a basic energy conservation principle, but some materials are better than others at storing heat.

The heat sink or thermal mass works by trapping the extra heat generated by the sun during the day and releasing it slowly when the temperature drops during the night, thus heating your greenhouse free of charge. Basically, it works like a battery, storing energy during the day and releasing it during the night.

Now, how much energy you can store in your “battery” is directly dependent upon the size of the thermal mass and also the heat capacity of its building materials.

Water is excellent at storing heat when compared to concrete or soil, having a twice the specific heat capacity volume of concrete and 4 times the heat capacity volume of soil. Hence, the best and most common method for building thermal mass/heat sinks is to use water barrels, due to the water’s excellent heat storing capacity.

The general idea is to stack 55 gallon barrels filled with water inside the greenhouse. How many you use will depend on the volume and size of your greenhouse. The barrels must be located where they receive the maximum amount of direct sunlight, i.e. near a north-facing wall.

The water inside the barrels will get warm during the day and the energy (heat) stored inside will be slowly released during the night, keeping your crop warm. Easy as pie, right? And cheap as dirt, too. Well, almost.

Remember to place the tender plants (seeding trays or warm-weather crops) near the barrels, which will be the warmest place in the greenhouse, for better results.

4. Heat Exchanger

Now, if the thermal mass idea, aka the water filled barrels, are not enough, you can go to the next level and incorporate a heat exchanger into your DIY project.

The heat exchanger is also called a Climate Battery or a SHCS (subterranean heating and cooling system) and it works by circulating the air through the heating mass.

There are lots of versions and designs for heat exchangers, but they all work using the same principles. The mechanisms of energy transfer and storage are identical: as the greenhouse heats during the day, the warm and humid air from inside the greenhouse is pumped by an electric fan via a network of underground pipes. The temperature drop produces water-vapor condensation; hence energy is released during the process (it’s called phase change).

The released energy is stored in the soil in the form of heat, thus creating a big mass of warm soil under your greenhouse, regardless of the season. During the night, when the outside temperature drops, the electric fan starts over (via a thermostat) and it circulates the air again through the underground pipes, which, this time, extract the heat stored in the soil and warm the greenhouse.

There are additional methods for building a heat exchanger, as the battery material may vary. For example some people choose to dig and backfill with stones or gravel the area underneath the greenhouse, as stone and gravel are better in terms of heat storing capacity than dirt.

It sounds a little bit complicated, but it’s actually pretty straightforward. This air-heat-exchanger system is relatively simple and time-tested for decades in homes and greenhouses all around the world.

Heat-to-air-exchangers are very efficient for two main reasons: first, the size/volume of the battery/thermal mass is huge when compared to a water-filled barrel (generally speaking, two times bigger).

Secondly, because the air is pushed actively through the thermal mass, this significantly increases the rate of heat exchange, making it more efficient when compared to “static” barrels.

Also, this system does three jobs at the same time: during the day, the greenhouse gets cooler, during the night it gets warmer and on top of that, ventilation is taken care of by design, making sure there are no cold pockets inside! Awesome, right?

You can use a thermostat to kick the fan on and off when the desired temperature is reached, offering you total control over the thermal mass, and that means it’s as smart as it gets, right?

Here’s a video which depicts how a heat sink helps with keeping the greenhouse warm during cold nights.

Video first seen on Michael Dibb

Here’s another idea about solving the problem of freezing during the winter when growing inside a greenhouse, called a Zero Energy Thermal Mass Greenhouse, which requires no power and it’s totally off the grid. It will work anywhere and it allows you to grow produce even in the winter.

Video first seen on Ted Pasternack

I hope the article helped and if you have suggestions or comments, feel free to express yourself in the dedicated section below. Also make sure to comeback on Sunday as we continue to talk about our survival gardens!

This article has been written by Chris Black for Survivopedia.

How To Heat A Greenhouse In Winter

Table of Contents (Quickly Jump To Information)

Wondering how to keep a greenhouse warm in winter without investing in electric or fuel-supplied heating systems?

MY LATEST VIDEOS

Yes, it can be done. And without adding any more costs to your household budget.

I mean, who needs another bill right? Right.

Now, you might be wondering why bother keeping your greenhouse warm during the frostier months anyway – why not just enjoy the season?

Well, this girl likes her greens.

Ok, you caught me. I DO like greens, but I’m not a superfan.

I like them…but more like sprouts on a sammich. NOT full blown salads. Unless they’re Southwestern salads. Then, bring on the arugula.

ANYWAY, I like to keep growing over the winter because, well, I like to grow vegetables. Like any normal, sane person.

The other reason to keep a greenhouse warm in winter is because if you ARE growing anything, you’ll want to provide a healthier living environment for your vegetables, prevent cold spots, and reduce the risk of fungal diseases.

I have more readers growing crops in the winter, and naturally, a common question is how to heat a greenhouse in winter for free (which mean you can grow a wider variety of vegetables, too).

Understand the Basics of How to Keep a Greenhouse Warm in Winter

Before we delve into our ideas, let’s first establish some basics.

In this season where temperatures can go unpredictably low, you can only do so much. In other words, don’t try to grow oranges in sub-zero weather. You won’t be successful, right?

So, let’s talk about some basics to help you run your greenhouse in winter.

  • Choose the right crops to grow for the season. Go for low-lying greens like kale, spinach, and mustard greens that can stand below-freezing temperatures

(want more ideas? Grab my best selling book, Organic By Choice: The (Secret Rebel’s Guide To Backyard Gardening here, Use coupon code GREENHOUSE to save 10%!)

  • Invest in a good quality thermometer like this one that can read max and min temperatures throughout the day.
  • Only heat the areas necessary. Grouping plants together will help you save energy and cost.
  • Install proper ventilation to prevent the spread of fungal diseases and maintain a healthy growing greenhouse.

Here are 3 more effective strategies in controlling the temperature inside your structure without having to waste fuel or energy.

Store Thermal Energy Using Thermal Mass

Thermal mass heaters are the bee’s knees, and easy to incorporate into your greenhouse.

If you’re not sure what I’m talking about, thermal mass, sometimes called a heat sink, absorbs and stores solar heat energy.

This involves putting materials around your greenhouse that absorb heat from the sunlight during the day. These heat sinks are then capable of slowly releasing thermal energy at night time when the mercury drops like crazy.

Here are some effective methods to collect thermal mass:

Idea 1: Build a cobbled pathway across the floor of your greenhouse using dark gravel or small stones (you can reach out to a local nursery or a dealer that sells rocks for driveways). These rocks naturally absorb heat – and the release of this heat keeps your plants warmer during the dark, cold hours of winter.

Idea 2: Since water has higher heat capacity than land or soil, try putting water or rain barrels around the interior of your greenhouse. Place dark barrels at a Southern-facing location, where they can easily absorb sunlight in the day. Make sure they’re also near tender plants that need more warmth at night

Idea 3: Use cinder blocks or earthenware ceramic pots to further absorb solar heat. They can be used to support planters on table-tops and benches, and they can release their heat around the plants (this is also a good idea to keep your chicken flock’s water from freezing over the winter).

Note: Painting these materials dark (i.e. black) helps absorb more thermal mass and one additional tip on how to keep a greenhouse warm in winter.

Build an Indoor Compost Pile

This is a genius idea that’s also one of the most sustainable techniques to keep your greenhouse warm this winter.

(Psst…it’s also cost-effective since you can build it nearly for free AND you won’t have to use power or fuel to heat your greenhouse. This is what we call Win-Win-Win.)

As the material in your pile composts, bacteria that break down organic material generate a considerable amount of heat to the environment.

We cover compost piles in depth in Organic By Choice: The (Secret) Rebel’s Guide To Backyard Farming. Save 10% with coupon code GREENHOUSE right here.)

Insulate!

Insulation is another option to keep a greenhouse warm in the winter.

So what do I mean by insulate?

Well, you can insulate the entire greenhouse using plastic sheeting, OR you can add row covers (yes, row covers over crops inside your greenhouse) for added protection.

Plastic helps absorb more heat without keeping the sunlight away from your crops. Combined with the other ideas in this article, you have quite a few ways to keep a greenhouse warm in winter.

There are many other natural techniques for keeping your greenhouse thermally controlled throughout the year. In the most challenging seasons, let these suggestions guide you on how to heat a greenhouse in winter for free. You don’t have to do everything. You just need to find the right combination that will work best for your set-up.

Maat van Uitert is a backyard chicken and sustainable living expert. She is also the author of Chickens: Naturally Raising A Sustainable Flock, which was a best seller in it’s Amazon category. Maat has been featured on NBC, CBS, AOL Finance, Community Chickens, the Huffington Post, Chickens magazine, Backyard Poultry, and Countryside Magazine. She lives on her farm in Southeast Missouri with her husband, two children, and about a million chickens and ducks. You can follow Maat on Facebook here and Instagram here.

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