What to put in bottom of planter for drainage?

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Rocks in pots? What a crock!

Should you line your pots with gravel, stones or old broken china to help water drainage? This month, The Geeky Gardener stops the rot and puts some physics in your pots.

Crock pot: There’s conflicting advice about how best to make sure your pots drain well (Source: cjp/iStockphoto)

Pots, pots, pots. Gardeners are potty about pots, be they terracotta, plastic, self-watering or not. They fit into small spaces, they’re easy to move into the shade when the temperature rises and they love a good summer soaking.

But there’s conflicting advice about how best to make sure your pots drain well. Do you line your pot with gravel, sand or old broken china? And do you need a physics degree to work out the difference?

Think of water in a well-drained pot like a good house guest. Good house guests arrive and stay a while but know not to outstay their welcome. In a well-drained pot, the water hangs around in the potting mix long enough to do its job, then trickles out through the drainage hole at the bottom.

But badly drained pot water can cause problems, just like the house guest who refuses to leave. Slow moving water waterlogs the roots, prevents air from circulating in the root zone and provides the sort of conditions that soil-borne diseases love. Roots can rot, and your much-loved roses, rocket or rhododendrons can die.

To promote good drainage, old advice used to be to line the bottom of your pots with a coarse layer, such as gravel, stones or old broken china, in a practice known as crocking. Crocking was supposed to encourage water to pass down from the potting mix into the gaps in the coarse layer below and out through the drainage hole.

It sounds so plausible doesn’t it? Maybe that’s why so many people still do it.

Yet soil scientists have known for years that crocking doesn’t help drainage. In fact, it can hinder it.

According to Associate Professor Linda Chalker-Scott, an urban horticulturist at Washington State University, crocking can lead to waterlogging, exactly the problem you wanted to avoid in the first place.

“Nearly 100 years ago, soil scientists demonstrated that water does not move easily from layers of finer textured material to layers of more coarse textured,” she writes in a review. “Since then, similar studies have produced the same results.”

Rather than water freely trickling down across the boundary between the layers, water ‘resists’ crossing it. That’s because the pull of the water upwards in the finer layer offsets any gravitational pull downwards. Instead, water builds up in the finer level above and is only released into the coarser layer below when it’s saturated, like a full sponge that can’t hold any more water.

Chalker-Scott points to another study showing that more moisture stays in the upper level if there’s gravel, rather than sand, underneath it. The take-home message is that the coarser the underlying material, the more difficult it is for water to move down across the interface and out through the drainage hole.

“Imagine what happens in a container lined with pot shards!” she writes.

Kevin Handreck, former CSIRO soil scientist and author of Gardening Down-Under and Good Gardens with Less Water, also believes that crocking increases the risk of damaging your plants by overwatering.

But he says crocking does have a place in pots without drainage holes, particularly terrariums, those indoor gardens popular in the 60s and 70s making a comeback today. Here, he says, the gaps between the gravel can hold extra water that would otherwise waterlog your potting mix.

The upshot is that in most cases, research has shown that crocking your pots is a load of, well, crock. Except, that is, if you want to channel the 60s and 70s with your very own terrarium, man.

More linksGarden Professors blog (science-based gardening information)
Fact sheet: containers (ABC Gardening Australia)

About the authorAnna Evangeli writes about the science of gardening at her award winning blog The Geeky Gardener. She is a former news editor at ABC Science and science correspondent for ABC Gardening Australia magazine, and has degrees in biochemistry and journalism.

Tags: earth-sciences, physics

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Published 10 February 2014

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Using Styrofoam In Containers – Does Styrofoam Help With Drainage

Whether set on a patio, porch, in the garden, or on each side of an entryway, stunning container designs make a statement. Containers are available in a wide array of colors shapes and sizes. Large urns and tall decorative glazed pots are especially popular these days. While decorative pots like this add to the beautiful dramatic appearance of container gardens, they have some drawbacks.

When filled with potting medium, large pots can be extremely heavy and unmovable. Many glazed decorative pots may also lack proper drainage holes or do not drain well due to all the potting mix. Not to mention, purchasing enough potting soil to fill large pots can become quite expensive. So what’s a gardener to do? Read on to learn more about using Styrofoam for container filler.

Using Styrofoam in Containers

In the past, it was recommended that broken pieces of clay pots, rocks, wood chips or Styrofoam packing peanuts be placed in the bottom of pots as filler and to improve drainage. However, research has shown that clay pots, rocks and wood chips may actually cause the pots to drain slower. They can also add weight to the container. Styrofoam is lightweight but does Styrofoam help with drainage?

For decades, container gardeners have used Styrofoam for drainage. It was long lasting, improved drainage, did not add weight to the pot and made an effective filler for deep pots. However, because landfills are overfilled with non-biodegradable products, many Styrofoam packing products are now made to dissolve in time. It is not recommended to use Styrofoam peanuts for potted plants now, because they may break down in water and soil, leaving you with sunken in containers.

If you find yourself with a large amount of Styrofoam from product packing and question: “Should I line potted plants with Styrofoam,” there is a way to test the Styrofoam. Soaking these packing peanuts or broken bits of Styrofoam in a tub of water for several days can help you determine if the type you have breaks down or not. If pieces begin to dissolve in the water, do not use them in the bottom of pots.

Does Styrofoam Help With Drainage?

Another problem gardeners have had when using Styrofoam in containers is that deep plant roots may grow down into the Styrofoam. In pots with little to no drainage, the area of Styrofoam may be waterlogged and cause these plant roots to rot or die.

Styrofoam also contains no nutrients for plant roots to absorb. Too much water and lack of nutrients can cause beautiful container designs to suddenly wilt and die.

It is actually recommended that large containers be planted in the “container in a container” method, where an inexpensive plastic pot is planted with the plants, then set atop filler (like Styrofoam) in the large decorative container. With this method, container designs can easily be changed out each season, plant roots are contained within the potting mix and, if Styrofoam filler does break down in time, it can be easily fixed.

Successful Container Gardens

Choosing a Container for Planting

Drainage Is Critical to Plant Health

A hole at the bottom of the container is critical. It allows water in the soil to drain freely so adequate air is available for the roots. While various kinds of plants have differing drainage needs, few can tolerate sitting in stagnate water. Healthy roots mean healthier plants. So be sure there are holes for drainage. Shoreline plants love wet soil, so if the pot does not drain, consider using them. See the water gardening section for more information.

If the pot does not come with a hole in it, figure out a way to make a hole. One way is to drill one. Some decorative resin or plastic pots have pre-punched holes at the bottom for easy removal. Generally, very little soil falls through the hole. To keep soil from falling through large drainage holes some folks use a coffee filter paper over the hole, though this is not necessary. Small holes in the bottom of the pot allow the water to drain out and very little soil media is lost.

Sometimes a plant is already planted in a pot with no drainage at all. The best solution in this case is to take it to a sink, water it, then after a few minutes turn it on its side for a minute or two to let excess water drain out.


Beware of pots with permanently attached saucers where emptying the overflow is difficult or impossible. I learned this the hard way when my basil leaves turned black because the roots were waterlogged – the result of a permanently attached saucer. Drainage of excess water is vital for the health of plant roots. It is much better to use a pot with a detachable saucer. Be sure to empty the excess water so soils have an opportunity to dry out somewhat.

Double potting

Slipping a container inside a slightly larger one is called double potting. When double potting is used, the plants grow in a pot liner. Often this is a plain plastic pot. This allows you to slip the pot liner in or out of the decorative container without disturbing the plant roots. Check to be sure that the plants in the pot liner never stand in water (unless you have aquatic plants) If water accumulates in the bottom of the larger container, remove the inside pot and drain the water from the outside pot. Place gravel in the bottom of the outer pot if the decorative pot is deep enough. A little excess water can accumulate in the gravel without the plant roots having to stand in water.

Double potting techniques may be used:

  • to overcome the problem of no drainage hole in an attractive decorative pot;
  • to quickly change seasonal displays;
  • to combine plants with different environmental requirements;
  • to minimize extreme soil temperature fluctuations;
  • to keep desirable but invasive plants from spreading.

Skip the gravel inside the bottom of individual or pot liners

It is a myth that a layer of gravel (inside the bottom of an individual pot) beneath the soil improves container drainage. Instead of extra water draining immediately into the gravel, the water “perches” or gathers in the soil just above the gravel. The water gathers until no air space is left. Once all the available soil air space fills up, then excess water drains into the gravel below. So gravel in the bottom does little to keep soil above it from being saturated by overwatering.

Damp gravel placed in a saucer underneath the pot may help by increasing the humidity in the immediate area of the plants as the water evaporates from the gravel surfaces.

Self-watering pots

Self-watering pots use various methods to effectively draw water from a bottom reservoir into the soil without causing the soil to become too wet. Water may be drawn up into the soil by capillary action (or wicking) through small soil columns, rope wicks, or the use of moisture sensors. Self-watering containers are especially useful for weekend cottages and people who do not have time to check water needs daily.

Consistently available water is great for vegetables, and tropical houseplants. Imagine a dozen stalks of sweet corn producing ears on your patio in a self -watering container like the Earth Box™. Plants that need to dry out like thick-leaved cactus and succulents do not usually warrant the extra cost of a self-watering container.

For more information, use a web search engine using the keywords “self-watering planter” or “self-watering container” or “earth box.”

Decorative pots and wraps without drainage holes. Sometimes you fall in love with a wonderful container that has no drainage hole. This would be the perfect time to find a pot liner to fit inside so both you and the plants are happy. In addition to decorative pots, decorative foil or plastic pot wraps are a form of double potting. The wrap keeps water from leaking out where it is not wanted. To protect the plant from becoming prone to root rot, pierce a hole in the bottom of the wrapper or foil. Then place the container on a saucer. Or, take the container to a sink, remove the wrapper, and then water. Let the water drain freely out the holes in the bottom of the pot. After the pot finishes draining, replace the wrapper.

Changing seasonal displays. Double potting makes changing out seasonal plants a breeze. Fresh plants are easily rotated in and tired ones out of a large landscape container holding multiple plants. Double potting makes it possible to sink individual potted plants into the landscape (or remove them) without disturbing the roots.

Combining plants with differing requirements. Plants that have different soil drainage (aeration) requirements can be combined in a landscape planter if they are in their own separate pots. This allows different watering practices for each plant. On the other hand, if there is not enough light, two sets of plants can be used. As individual plants begin to decline, rotate them back to a higher-light nursery. Replace them with healthy plants. It is less work to choose plants with similar environmental needs or ones well adapted to conditions available. However, this is not always possible.

Reducing fluctuation of soil temperature. Roots are generally more susceptible to cold damage than stems and leaves. When overwintering an otherwise hardy plant, sink the potted plant into the ground. Then mulch over the soil with wood chips, soil or other material to help it survive winter. The soil temperature underground does not fluctuate like container soil which is exposed to wind and extremes of temperature.

Restraining invasive plants. Some plants are attractive, but spread invasively by underground roots. They invade space that does not belong to them. For example, most people love mints for their fragrance and culinary uses, but they can spread aggressively in the garden. Sink a large pot into the ground. Then place the invasive mint in a slightly smaller pot liner. This limits the spread of the roots.

More Information on Choosing a Container

  • Container Material Choices
  • Drainage Is Critical to Plant Health
  • Considering Size and Shape

Talk to any gardener, and they will probably share at least one tip with you about how they use Styrofoam in their garden. Whether they have a flower bed, a veggie garden or even a container garden, there is a good chance they make use of Styrofoam in one way or another. And if they don’t, they probably still have a few Styrofoam gardening tips they learned from someone else that they would be more than happy to share with you.

Styrofoam has been used in gardens in different ways for nearly as long as the material has existed. While there are some scientific facts surrounding the use of this material in gardens, there is also a good bit of fiction. Let’s examine some of the most common myths about gardening with Styrofoam to separate the truth from the old wives’ tales.

Myth #1: Styrofoam Improves Drainage in Containers

Whether you have a few flowers planted in pots or you have an entire vegetable garden in containers, you have probably heard that adding Styrofoam to the bottom of the container helps improve drainage. Gardeners have used foam packing peanuts for this purpose for decades, but does it really do any good?

The answer: Not really. In fact, if you are using a pot with little to no drainage, adding Styrofoam packing peanuts could do more harm than good. Deep plant roots can grow into the foam material, and without sufficient drainage, they can become waterlogged and rot or die.

Because the synthetic material used in traditional Styrofoam peanuts contains no nutrients, it provides no value to the growing plant. Bio-degradable packing peanuts are made from natural materials, but they will absorb water and break down, providing no benefits in terms of drainage.

Myth #2: Styrofoam Helps Keep Soil Loose and Aerated

If you have ever purchased potting soil that contained squishy white balls, you may have assumed that they were made from Styrofoam.

The answer: No. While these materials are used to prevent potting soil from retaining too much water and becoming too dense, they are not made from the foam material that they are commonly mistaken for.

The white, foam-like balls in your potting soil are there to help keep soil loose and aerated, but they are most commonly made from a naturally occurring volcanic glass known as perlite. When heated to 1,600 degrees Fahrenheit, the glass puffs up like popcorn, resulting in the Styrofoam-like appearance. While it does not absorb moisture, it holds it in tiny cavities around the outside. This makes water readily available to nearby plant roots.

Unlike Styrofoam, the perlite beads decompose naturally over time. As a result, they do not pose any threat to the environment. If you put Styrofoam in your garden, it will not decompose for millions of years. It may also take on moisture and become waterlogged, resulting in soil compression. It could even float away during heavy rains and make its way into a body of water. Styrofoam accounts for a significant portion of ocean pollution, and keeping it out of your garden is one way to avoid contributing to this ever-growing problem.

Myth #3: Styrofoam Cones Keep Roses Warm During the Winter

When cold weather approaches, gardeners grow concerned about keeping their plants — especially delicate ones like roses — safe and warm. Styrofoam cones marketed as “rose cones” have become popular in recent years, but do they really do anything to keep plants warmer when the temperature outside plummets?

The answer: Yes and no. Styrofoam acts as insulator, so temperature fluctuates faster outside of the box than inside. If the outside air suddenly warms up, the air under the cone will actually be colder. However, on most days, the air inside does stay slightly warmer. The Styrofoam also helps protect the plant from wind. It’s important to note, though, that it will not keep the air inside above freezing during extremely cold weather. And in many instances, the temperature difference inside the cone is not significant enough to really make a difference in terms of plant health.

The cones are easily damaged, especially by heavy snow. Fragments can break off and find their way into the environment. For all the benefit they may provide, they aren’t really worth the potential environmental risk.

Closing Thoughts

Styrofoam is an extremely versatile material that can be used for numerous applications. From packaging peanuts that protect merchandise while in transit to insulation in homes and more, it is useful in numerous situations. Because of its negative impact on the environment, however, your garden is one place it probably does not belong.

Browse Our Styrofoam Packing Peanuts email

Styrofoam as Perlite Substitute

Perlite is exellent for amending soil used for container grown plants and for starting seedlings in a soil or soiless medium. It insures better drainage and aeration which results in a larger, healthier root system.


I grow lots of plants in containers and particularly in concrete mixing tubs sunken into the ground. Buying enough Perlite for all my containers has become cost prohibitive. I have found a very good substitute.

Thick styro meat trays which have been thoroughly cleaned and scissored into half inch strips and then into about half inch squares are my substitute. An average meat tray will yield about 1 1/2 cups of these little squares. They are put into a blender with enough added water to float the squares near the top. Blending for about 1-2 minutes and then draining in a sieve will give you a nice Perlite substitute. It has the consistency of coarse sand, but very lightweight.

Note: Thin meat trays will not work. They produce an airy, snowflake like product which tends to blow away in the breeze and does not provide any real aeration benefits.

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