Why do plants need light?

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The most important concept to understand when growing plants is the rule of limiting factors, which determines plant quality.

Hydroponics cannot compensate for poor growing conditions, such as improper temperature, insufficient irrigation, nutrient deficiencies, pest and disease problems, or poor light.

Light is the most important variable influencing plant growth.

If plants do not receive enough light, they will not grow at their maximum rate or reach their maximum potential, regardless of how much of any other variable – water, growth medium or fertiliser – they receive.

Increasing light increases yield
Light is the driving force for photosynthesis, a plant process that changes sunlight into chemical energy.

During photosynthesis, water is split in a chemical reaction in which it is separated into oxygen and hydrogen, and carbon dioxide (CO2) is converted into sugar.

A general rule of thumb is that 1% more light will give you a similar percentage increase in plant growth, resulting in a 1% higher yield.

All plants require light and CO2 for photosynthesis. Adequate spacing between plants will ensure that each plant receives sufficient light in the greenhouse.

Tomato plants pruned to a single stem are spaced at 2,7 plants/ m2 to three plants/m2. Seedless cucumbers, with their larger leaves, require almost double this spacing. Hydroponic lettuce spacing varies from 2,5cm2 for first-stage seedlings to 15cm2 for final spacing.

Much work has been done on supplemental lighting to optimise plant growth, especially in countries with low light intensity and daylight hour limitations.

Sunlight is by far the cheapest option for SA growers; we have ample and should use it without shading as far as possible.

Ensuring enough light for plants
Daily light integral (DLI)
The DLI represents the total amount of light that your plants receive per day. You can compare it with rainfall precipitation, measured in mm/day, which depends on rainfall intensity (how hard it is raining) and duration (how long it rains for).

DLI also depends on the intensity of light radiation, as well as the duration (number of sunlight hours). This provides a direct indication of how much photosynthetic light your plants receive.

DLI adjustment could help to reduce the rooting time of cuttings and seedlings, and increase crop quality at reduced levels of energy.

Winter growing
Because there are fewer sunlight hours in winter than in summer, growers often heat their greenhouses during winter in their efforts to maintain summer yields.

In Cape Town, average sunshine hours drop from 11 hours, five minutes per day in December to only five hours, 42 minutes per day in July.

This 49% reduction means that greenhouse heating efforts to maintain summer yields will be less effective than in Johannesburg, which shows a drop of only 9% during winter.

Another factor that comes into play is the lower inclination of the sun’s radiation. As a result of a shorter day, morning mist and cloudy skies, light intensity is lower in winter, causing a further reduction in the DLI and a corresponding decrease in plant growth.

This is why greenhouse vegetables, whether grown in soil or in a hydroponic system, will not do as well during winter, even with the best heating system on the market.

In addition to photosynthesis, there is another light aspect that determines the development of plants from seed to flowering.

This is known as photomorphogenesis. This relies on various photo pigments to sense and respond to light colours, which range from ultraviolet to near-infrared and include all the colours of the rainbow that we see as reflected light.

Photomorphogenesis influences the following aspects of plant growth, among others:

  • Seed germination (photoblasty and photodormancy);
  • Synthesis of chlorophyll (photosynthesis);
  • Stem and leaf growth towards visible light (etiolation and phototropism);
  • Flowering time based on the length of day and night (photoperiodism);
  • Reaction to various light colours.

There is a vast difference between the human eye’s sensitivity to the different colours of the rainbow and that of plants.

Human eyes are most sensitive to colours in the yellowish-green zone of the colour spectrum, which is close to the region where plants show the worst reaction to green light.

Humans see reflected light, and the fact that most plants are green is an indication that plants reflect more of the green light radiation than the other colours in the light spectrum.

The photosynthetic reaction of plants is concentrated in the blue and red portions of the colour spectrum, including a proportion of ultraviolet (see graph).

Plants’ reactions to various colours of the light spectrum can be used to manipulate plants to satisfy different needs, including the following:

Ultraviolet radiation can be used to shorten the internodes (the part between two nodes on a stem where leaves emerge).

Blue light can be used to stimulate vegetative growth and prevent shorter-day plants from flowering during their propagation stages.

Red light can be used to induce flowering and lengthen the internodes to produce plants with longer stems and bigger flowers. Roses are an example.

Far-red radiation can be used to control the photoperiodism of plants.

Some plant species flower only when exposed to short periods of light, whereas others flower only after exposure to prolonged periods of light. This phenomenon is called photoperiodism.

The former are known as short-day plants and include chrysanthemums and strawberries.

The latter, known as long-day plants, include spinach and radishes. Day-neutral plants, such as tomatoes and cucumbers, are not affected by photoperiodism.

If you expose short-day plants to a brief period of light in the night, you can prevent flowering and bolting. Conversely, with long-day plants, the same exposure will promote flowering.

Floriculturists can therefore use supplemental artificial lighting to delay or advance the flowering of plants to meet market needs.

Traditional photoperiodic control methods include:

  • Increasing day length by using supplemental lighting;
  • Shortening day length by covering the plants with dark material just before night time;
  • Night interruption with lighting;
  • Cyclic (intermittent) lighting,

These techniques are based largely on trial and error, using different plant species and varying greenhouse operating conditions.

Email Prof Gert Venter at at .

Ornamental Production

Light is an essential factor in maintaining plants. The rate of growth and length of time a plant remains active is dependent on the amount of light it receives. Light energy is used in photosynthesis, the plant’s most basic metabolic process. When determining the effect of light on plant growth there are three areas to consider: intensity, duration and quality.

Light Intensity

Light intensity influences the manufacture of plant food, stem length, leaf color and flowering. Generally speaking, plants grown in low light tend to be spindly with light green leaves. A similar plant grown in very bright light tends to be shorter, better branches, and have larger, dark green leaves.

Light exposure

Plants can be classified according to their light needs, such as high, medium and low light requirements. The light intensity received by an indoor plant depends upon the nearness of the light source to the plant. Light intensity rapidly decreases as the distance from the light source increases. Window direction in a home or office affects the intensity of natural sunlight that plants receive. Southern exposures have the most intense light. Eastern and western exposures receive about 60 percent of the intensity of southern exposures, while northern exposures receive 20 percent of the intensity of a southern exposure. A southern exposure is the warmest, eastern and western are less warm, and a northern exposure is the coolest. Other factors such as curtains, trees outside the window, weather, season of the year, shade from other buildings and window cleanliness also effect light intensity. Reflective, light-colored surfaces inside a home or office tend to increase light intensity , while dark surfaces decrease light intensity.

Directional Exposure:

Day and Night:

Day length or duration of light received by plants is also of some importance. Poinsettias, kalanchoes and Christmas cactus flower only when days are 11 hours or less (short-day plants). Some plants only flower when days are longer than 11 hours (long-day plants), while others are not sensitive to day length at all (day-neutral plants).

Day Length:

Increasing the time (duration) plants are exposed to light can be used to compensate for low light intensity, as long as the plant’s flowering cycle is not sensitive to day length. Increased light duration allows the plant to make sufficient food to survive and grow. However, plants require some period of darkness to properly develop and should be exposed to light for no more than 16 hours per day. Excessive light is as harmful as too little.. When a plant gets too much direct light, the leaves become pale, sometimes burn, turn brown and die. Therefore, protect plants from too much direct sunlight during summer months.

Supplemental Light:

Additional lighting can be supplied with either incandescent or fluorescent lights. Incandescent lights produce a great deal of heat and do not use electricity very efficiently. If artificial light is the only source of light for growing plants, the quality of light or wavelength, must be considered. Plants require mostly blue and red light for photosynthesis, but for flowering, infrared light is also needed. Incandescent lights produce mostly red and some infrared light, but very little blue light. Fluorescent lights vary according to the amount of phosphorus used by the manufacturer. Cool-white lights produce mostly blue light and are low in red light; they are cool enough to position quite close to plants. Foliage plants grow well under cool-white fluorescent lights, while blooming plants require extra infrared light. This can be supplied by incandescent lights or special horticultural fluorescent lights.


Most plants tolerate normal temperature fluctuations. In general, foliage plants grow best between 70 degrees and 80 degrees F. during the day and between 60 degrees to 68 degrees F. at night. Most flowering plants prefer the same daytime temperature range, but grow best when nighttime temperatures range from 55 degrees to 60 degrees F. Lower nighttime temperatures help the plant: recover from moisture loss, intensify flower color and prolong flower life. Excessively low or high temperatures may cause: plant stress, inhibit growth, or promote a spindly appearance and foliage damage or drop. Cool nighttime temperatures are actually more desirable for plant growth than high temperatures. A good rule of thumb is to keep nighttime temperatures 10 to 15 degrees lower than daytime temperatures.


Atmospheric humidity is expressed as the percentage of moisture to air.This is important to plants in modifying moisture loss and temperatures. There are several ways to increase relative humidity around plants. A humidifier can be attached to the heating or ventilating system in the home or office. Also, gravel trays with a constant moisture level can be placed under pots or containers. As the moisture around the pebbles evaporates, the relative humidity in the vicinity of the plants is increased.


Another means of raising humidity is to group plants close together. Misting the foliage of plants is not generally recommended because of the increased potential for spreading diseases. If a mist is used, it should be applied early in the day so that leaves will dry before the onset of cooler nighttime temperatures.

For details on specific light & temperature requirements see Selected Foliage and Flowering Plants

Light affects us all in different ways. Some people want to raise their arms and greet the morning rays with gleeful shouts. Others turn into vampires and hide under the bed covers. These people usually need coffee before they’ll so much as peek out from under the sheets.

Regardless how humans may react when showered with sunshine, it may be helpful for the serious gardener to know: how does light affect plant growth? The answer could mean the difference between plants that dance and plants that crawl back into the dirt like the undead.

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Why Do Plants Need Light?

You might recall your old science classes in grade school discussing photosynthesis when answering the question, “How does sunlight affect plant growth?” Perhaps it looked something like this: 6H2O + 6CO2 —-> C6H12O6 + 6O2.

Well, that’s a lot of numbers and letters.

Perhaps this description is a little easier to comprehend if you have forgotten all that chemical equation stuff: plants use light, water, and carbon dioxide to make sugar, which is converted to ATP (the stuff that fuels all living things) by cellular respiration.

Chlorophyll absorbs the sun’s energy. Carbon dioxide enters the leaves through tiny pores. The roots draw up water from the soil. The energy from the light is what chops up the water molecules like your favorite horror movie villain. This horrific act gives us the oxygen we need to breathe so I guess it isn’t all that horrifying. The carbon dioxide befriends the abandoned hydrogen to make the plant’s fuel.

Who’d have thought the horror villain in this photosynthesis tale is actually the good guy?

What Kind of Light Do Plants Need?

Not all light is exactly the same. You may be wondering about indoor light systems and how they compare to natural sunlight. Here’s a quick explanation of what light plants actually need and use.

Light Spectrum

Most people are familiar with the breakdown of light into colors as displayed by a rainbow after a storm. Light from the sun refracting through raindrops allows us the rare and beautiful glimpse of Roy G. Biv. The spectrum includes these colors as well as many other types of wavelengths, like cosmic rays and gamma rays.

Well, we’ve already dabbled in the horror genre. I suppose it’s appropriate we’ve stumbled into the science-fiction realm now. It won’t be long until I’ll be telling you how to grow plants on the moon.

Phytosynthetically Active Radiation

A new question may be forming in your mind now that we’re talking rainbows: does the color of light affect plant growth? It does indeed.

We use nanometers to measure wavelengths. Plants use different ranges of nanometers for different growth phases. The useful range for gardeners to know is referred to as this mouthful of a phrase, Phytosynthetically Active Radiation. Measured from 400 to 700 nanometers, this range encompasses all those colors we adore.

However, PAR is not used all at once. The purple and blue light wavelengths, 400 to 490 nanometers, stimulate the vegetative growth phase while the yellow-orange-red wavelengths are used for flowering and fruiting. You emerald lovers probably already know that the plant doesn’t use the green wavelength and reflects it back to our eyes.

So when choosing colors of light to use on your plants, skip on the green lights and aim for either the shorter, faster wavelengths of purple and blue or the longer, slower wavelengths of yellow, orange, and red.

All over the world you will find native plants that thrive in their unique conditions. The Beast’s enchanted rose might have withered long before Beauty arrived to save the castle if he lived in, say, Death Valley. How well a plant grows depends on three factors: wavelength, duration, and intensity.


We’ve already touched on the wavelengths that plants like. What about the ones that they abhor? Different nanometers of ultraviolet rays may do nothing at all for your plants but some ranges can be extremely detrimental.


How long a plant is in the sun will affect its growth. Alaskan gardeners can grow gigantic pumpkins that need a crane to lift them onto weight scales due to their crazy amounts of sunlight in the summertime. Other plants might not like such a long sunbath and end up seeking shade back under the soil.


Intensity refers to how strong the light is and goes hand-in-hand with duration. If the light is too strong, the plant will scorch like a vampire who forgot the key to his coffin after a late night of partying. If it’s not strong enough, your little vampire won’t even come out and play.

Indoor Grow Lights vs. Sunlight: Which Wins?

The effect of indoor light on your little green babies depends on the type of bulbs you choose. While sunlight will always have a natural edge to quality, providing everything the plant needs to grow, full-spectrum bulbs are almost equal to the sun’s herculean task of feeding your fronds.

For more information on grow lights, from the standard fluorescent light to the sci-fi-esque LED and plasma lights, check out my in-depth guide to indoor grow lights here.

Whether you are a morning person or a night-crawling creature, your plants have their own light needs that must be met. Now that you understand how your plants use light to make their food, you can feel confident in choosing your own light sources that suit your needs as well as your garden’s. And if your setup really does look like something out of a Star Trek episode, send me pictures!

In the meantime, any questions and comments are welcome. Share this article with your friends and make your light wishes come true together. Thanks for reading!

The Green Thumbs Behind This Article:
Kevin Espiritu
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Measuring Plant Growth with Sunlight


Now that we have witnessed the growth of a seed to plant and can better understand the role of sunlight in the growing process, it is important to discuss a few ideas:

  1. Germination occurred in all three growing environments. The process started when the seeds were provided with soil and constant water supply. The water started metabolic (growth) activities within the seeds that produced enough energy for plant growth. When the shoots emerged, photosynthesis began. (It is interesting to think that germination can occur in almost complete darkness.)
  2. Plants are called autotrophs; meaning that they create their own food source. To make food, plants need carbon dioxide, water, and sunlight; this process is called photosynthesis.
  3. Photosynthesis is the process by which green plants make their own food. Photosynthesis happens when a plant absorbs carbon dioxide, nutrients, and water through the holes found in the roots (branches, stem, flowers, leaves, etc.) of the plant. The light energy (from the sun) then triggers a chemical reaction that breaks down the carbon dioxide and water molecules. This process creates a sugar called glucose and also produces oxygen. The glucose is then broken down by organelles called chloroplast and provide the energy needed to grow and repair plants. Each chloroplast contains a green chemical called chlorophyll which gives the leaves a green color. Both the “full sun” and “some sun” plants were able to allow for photosynthesis due to sunlight exposure.
    Additional note: Photosynthesis provides food for the plant and also releases oxygen into the atmosphere for humans to breathe.
  4. If a plant gets limited sunlight, the photosynthesis process slows down and the plant begins to grow upward and stretch their stems to reach for the sunlight (this process is called etiolation). It is easy to see this process in both the plants that received partial and limited/no sun. These basil plants grew to have longer stems and reached out towards the sunlight energy.
  5. Plants deprived of light will lose their color and die. The shoots exposed to “limited/no” sunlight had a yellow/white color due to the fact that photosynthesis could not occur. The lack of sunlight stunted photosynthesis and therefore the sprouts were not able to produce the chlorophyll needed to create a green color.

Overall, this experiment depicts just how important the sun is to the survival of plants and also humans (oxygen supply). Without proper sunlight, plant growth would stop due to the lack of photosynthesis and all of the other components needed for healthy plant growth. Once again, it is easy to see just how important the sun, a renewable resource, is to both plants and mankind.

Understanding how plants use sunlight

Professor Gabriela S. Schlau-Cohen (center) and graduate students Raymundo Moya (left) and Wei Jia Chen worked with collaborators at the University of Verona, Italy, to develop a new understanding of the mechanisms by which plants reject excess energy they absorb from sunlight so it doesn’t harm key proteins. The insights gained could one day lead to critically needed increases in yields of biomass and crops. Credit: Stuart Darsch

Plants rely on the energy in sunlight to produce the nutrients they need. But sometimes they absorb more energy than they can use, and that excess can damage critical proteins. To protect themselves, they convert the excess energy into heat and send it back out. Under some conditions, they may reject as much as 70 percent of all the solar energy they absorb.

“If plants didn’t waste so much of the sun’s energy unnecessarily, they could be producing more biomass,” says Gabriela S. Schlau-Cohen, the Cabot Career Development Assistant Professor of Chemistry. Indeed, scientists estimate that algae could grow as much as 30 percent more material for use as biofuel. More importantly, the world could increase crop yields—a change needed to prevent the significant shortfall between agricultural output and demand for food expected by 2050.

The challenge has been to figure out exactly how the photoprotection system in plants works at the molecular level, in the first 250 picoseconds of the photosynthesis process. (A picosecond is a trillionth of a second.)

“If we could understand how absorbed energy is converted to heat, we might be able to rewire that process to optimize the overall production of biomass and crops,” says Schlau-Cohen. “We could control that switch to make plants less hesitant to shut off the protection. They could still be protected to some extent, and even if a few individuals died, there’d be an increase in the productivity of the remaining population.”

First steps of photosynthesis

Critical to the first steps of photosynthesis are proteins called light-harvesting complexes, or LHCs. When sunlight strikes a leaf, each photon (particle of light) delivers energy that excites an LHC. That excitation passes from one LHC to another until it reaches a so-called reaction center, where it drives chemical reactions that split water into oxygen gas, which is released, and positively charged particles called protons, which remain. The protons activate the production of an enzyme that drives the formation of energy-rich carbohydrates needed to fuel the plant’s metabolism.

The left and middle figures illustrate fluorescence behavior of Vio-enriched and Zea-enriched LHCSR proteins These figures show probability distributions of fluorescence intensity and lifetime from experiments with hundreds of individual LHCSR proteins enriched with either Vio carotenoids (left) or Zea carotenoids (middle). The right figure illustrates fluorescence response to pH changes. This figure shows the response of Vio-enriched proteins when subjected to a lower pH than in the left figure, thus an increase in proton concentration replicating conditions in bright sunlight. Credit: Massachusetts Institute of Technology

But in bright sunlight, protons may form more quickly than the enzyme can use them, and the accumulating protons signal that excess energy is being absorbed and may damage critical components of the plant’s molecular machinery. So some plants have a special type of LHC—called a light-harvesting complex stress-related, or LHCSR—whose job is to intervene. If proton buildup indicates that too much sunlight is being harvested, the LHCSR flips the switch, and some of the energy is dissipated as heat.

It’s a highly effective form of sunscreen for plants—but the LHCSR is reluctant to switch off that quenching setting. When the sun is shining brightly, the LHCSR has quenching turned on. When a passing cloud or flock of birds blocks the sun, it could switch it off and soak up all the available sunlight. But instead, the LHCSR leaves it on—just in case the sun suddenly comes back. As a result, plants reject a lot of energy that they could be using to build more plant material.

An evolutionary success

Much research has focused on the quenching mechanism that regulates the flow of energy within a leaf to prevent damage. Optimized by 3.5 billion years of evolution, its capabilities are impressive. First, it can deal with wildly varying energy inputs. In a single day, the sun’s intensity can increase and decrease by a factor of 100 or even 1,000. And it can react to changes that occur slowly over time—say, at sunrise—and those that happen in just seconds, for example, due to a passing cloud.

Researchers agree that one key to quenching is a pigment within the LHCSR—called a carotenoid—that can take two forms: violaxanthin (Vio) and zeaxanthin (Zea). They’ve observed that LHCSR samples are dominated by Vio molecules under low-light conditions and Zea molecules under high-light conditions. Conversion from Vio to Zea would change various electronic properties of the carotenoids, which could explain the activation of quenching. However, it doesn’t happen quickly enough to respond to a passing cloud. That type of fast change could be a direct response to the buildup of protons, which causes a difference in pH from one region of the LHCSR to another.

Clarifying those photoprotection mechanisms experimentally has proved difficult. Examining the behavior of samples containing thousands of proteins doesn’t provide insights into the molecular-level behavior because various quenching mechanisms occur simultaneously and on different time scales—and in some cases, so quickly that they’re difficult or impossible to observe experimentally.

This specially designed microscope is capable of detecting fluorescence from single LHCSR proteins attached to a glass coverslip. Credit: Stuart Darsch

Testing the behavior of proteins one at a time

Schlau-Cohen and her MIT chemistry colleagues, postdoc Toru Kondo and graduate student Wei Jia Chen, decided to take another tack. Focusing on the LHCSR found in green algae and moss, they examined what was different about the way that stress-related proteins rich in Vio and those rich in Zea respond to light—and they did it one protein at a time.

According to Schlau-Cohen, their approach was made possible by the work of her collaborator Roberto Bassi and his colleagues Alberta Pinnola and Luca Dall’Osto at the University of Verona, in Italy. In earlier research, they had figured out how to purify the individual proteins known to play key roles in quenching. They thus were able to provide samples of individual LHCSRs, some enriched with Vio carotenoids and some with Zea carotenoids.

To test the response to light exposure, Schlau-Cohen’s team uses a laser to shine picosecond light pulses onto a single LHCSR. Using a highly sensitive microscope, they can then detect the fluorescence emitted in response. If the LHCSR is in quench-on mode, it will turn much of the incoming energy into heat and expel it. Little or no energy will be left to be reemitted as fluorescence. But if the LHCSR is in quench-off mode, all of the incoming light will come out as fluorescence.

“So we’re not measuring the quenching directly,” says Schlau-Cohen. “We’re using decreases in fluorescence as a signature of quenching. As the fluorescence goes down, the quenching goes up.”

Using that technique, the MIT researchers examined the two proposed quenching mechanisms: the conversion of Vio to Zea and a direct response to a high proton concentration.

To address the first mechanism, they characterized the response of the Vio-rich and Zea-rich LHCSRs to the pulsed laser light using two measures: the intensity of the fluorescence (based on how many photons they detect in one millisecond) and its lifetime (based on the arrival time of the individual photons).

More information: John I. Ogren et al. Impact of the lipid bilayer on energy transfer kinetics in the photosynthetic protein LH2, Chemical Science (2018). DOI: 10.1039/C7SC04814A

Toru Kondo et al. Single-molecule spectroscopy of LHCSR1 protein dynamics identifies two distinct states responsible for multi-timescale photosynthetic photoprotection, Nature Chemistry (2017). DOI: 10.1038/nchem.2818

Journal information: Chemical Science , Nature Chemistry Provided by Massachusetts Institute of Technology

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation and teaching.

Citation: Understanding how plants use sunlight (2018, December 5) retrieved 1 February 2020 from https://phys.org/news/2018-12-sunlight.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.

Plant Grow Lights: Which Kind to Choose

You love your house, but the lighting is all wrong for keeping houseplants happy. Luckily, there are many indoor grow lights that are affordable, easily accessible, and make your plants as happy as they would be on a sunny windowsill. Here’s how to pick the best type of grow lights for your plants and your home.

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HID Plant Grow Lights

The brightest plant grow lights are high-intensity discharge (HID) lights. They can be installed anywhere in your home, garage, or greenhouse to supplement existing light, and they can serve as the sole source of light for your plants.

These bulbs pass electricity through a glass or ceramic tube containing a mixture of gases. The blend of gases determines the color of the light given off by each type of lamp. HID lights are twice as efficient as fluorescent lamps; one 400-watt HID lamp emits as much light as 800 watts of fluorescent tubing. All HID lights can run on regular 120-volt household current but they require special fixtures with ballasts.

Two Types of HID Lights

There are two categories of HID lamps: metal halide (MH) and high-pressure sodium (HPS). Both emit a much more intense light than fluorescent bulbs, which also pass electricity through a gas-filled tube.

MH bulbs emit light that’s strongest at the blue end of the spectrum. It’s a stark, cool white light that produces compact, leafy growth. Because the light does not distort the colors of the plants and people it illuminates, this type of plant grow light is a good choice for a light display in a living area.

Agrosun gold halide bulbs are color-corrected to give off more red/orange light than regular metal halides. This helps boost flowering in addition to supporting compact foliar growth. Halide bulbs should be replaced about once a year.

HPS bulbs last slightly longer; they should be replaced every 18 months. They emit light strongly at the red/orange end of the spectrum, which promotes flowering. However, HPS lighting may also produce leggy growth unless used together with daylight or a metal halide system.

If your goal is lots of bloom, use high-pressure sodium lamps, but be advised: Their light has a red/orange cast that distorts the colors of everything they illuminate. This plant grow light is not flattering in a living room; everyone looks slightly jaundiced.

Buy it: AeroGarden Grow Light Panel, $85

Test Garden Tip: You can use both high-pressure sodium and metal halide bulbs in a single location, but a metal halide bulb cannot be used in a high-pressure sodium fixture, and vice versa. HPS ballasts include an igniter and MH ballasts do not. If you have multiple fixtures, consider a combination of HPS and MH systems. If you have only one fixture, you can use a conversion bulb, using metal halide to promote foliage growth, then switching to a conversion high-pressure sodium bulb to encourage flowering.

High-Intensity Fluorescent Grow Lights

High-intensity fluorescent grow light bulbs are also an excellent choice. Fixtures resemble those of HID bulbs, but they are less expensive, and cool and warm bulbs are available that fit in the same ballast. Choose according to which light is more appealing to your eye.

Related: How to Build a Hydroponic Garden

Fluorescent Grow Lights

Traditional fluorescent tubes are the most economical choice if you’re going to use grow lights for indoor plants. They can be used in inexpensive shop light fixtures or multitier growing carts.

The light produced by fluorescent tubes is much less intense than the other choices, so you are more limited in what you can grow. If you’re just trying to supplement natural light rather than replace it, fluorescent light may be a good option.

Fluorescent tubes come in cool, warm, or full-spectrum. Light from cool tubes has a blue cast, while warm tubes emit a pink/white light. Full-spectrum tubes closely approximate the color of natural daylight. Full-spectrum LED grow light bulbs are a bit more expensive but many growers consider them worth the price since the color of the light doesn’t distort the color of your plants.

Less light is emitted from the ends of the fluorescent tubes than from the center. Plants with lower light needs should be placed under the 3 inches of the tube at either end of the fixture.

Fluorescent tubes should be replaced every 18 months if they are being used approximately 16 hours per day.

Buy it: Hydrofarm Jump Start 4 Foot Grow Light Kit, $80

How to Determine Plant Grow Light Wattage

Once you’ve decided which kind of plant grow light you want, it’s time to decide how big a bulb you need for the space you have.

Our Grow Light Wattage Formula

First, determine how much space you need to illuminate. As a rule, you want 20 to 40 watts per square foot. Divide the wattage of your bulb by 20 (such as 1,000 ÷ 20 = 50), then divide the wattage of your bulb by 40 (1000 ÷ 40 = 25).

The answer gives you the extremes of your light intensity range. With one 1,000-watt system, you can light between 25 and 50 square feet of interior landscape, depending on the plants and their light requirements.

Adjust your setup as you observe how well your plants grow, and increase or decrease the intensity of the light accordingly. This can be done by shifting the placement of your plants or light fixture so they are closer together or farther apart, but not by changing the bulb in your lamp to a bulb with more watts.

Each lamp is designed for a specific wattage and a 400-watt bulb cannot operate safely in a 250-watt system.

Related: 27 Easy Houseplants to Grow

  • By Ellen Zachos

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