Tall plant pots can look quite elegant indoors or out in the garden, their shapes serving as visually appealing aesthetic design elements. One of the downsides though to using such deep pots is the large volume of growing medium (potting mix) that it takes to fill them!
What are the problems with filling tall pots to their full capacity with potting mix?
- The large volumes of potting mix needed to fill multiple large, tall pots can get rather expensive.
- Tall pots are usually quite heavy to begin with and become much heavier when filled with potting mix, and even heavier when watered.
- When potting mix breaks down over time, it collapses, dropping in height below the top of the pot, causing the plant to sink lower down into the pot. This is far more pronounced in taller pots due to the greater vertical height of the growing medium relative to the size of the pot.
- The potting mix at the bottom of overly deep pots stays far too wet for a very long time, creating a perched water table that doesn’t dry out easily, which can cause root rot in plants, especially if the plants are shallow-rooted, and the roots are unable to reach down into the waterlogged lower levels of the growing medium to take up the excess water.
To avoid these issues in tall pots, in cases where deeply rooted plants or trees are not being grown, many gardeners fill the bottom section of overly tall pots with other materials to fill up some of the unneeded space. This raises the question, what should we fill the bottoms of tall pots with?
What Are the Best Materials to Fill the Bottom of Tall Pots With?
When filling space in the bottom of overly deep pots, especially when they’re very tall and narrow, there’s always the risk of making them top heavy and unstable when using very light materials underneath, so that’s an important consideration in certain situations.
Here are the five best materials to use:
- An inverted terracotta pot can be placed in the bottom of the taller pot, it adds some weight, is porous, and its tapered shape permits water to escape. Use the regular height terracotta pots, not the low and wide ‘squat pots’.
- Standard plastic nursery pots can be double or triple stacked (one inside the other) to increase their load bearing capacity. Place the inverted stacked pots at the bottom of the tall pot.
- Fill the pot to the required level with scoria (a lightweight porous volcanic rock) and cover it with two layers of geotextile fabric (or if it’s not available, shade cloth will also work) to prevent the potting mix falling down into it.
- Fill the pot to the required level with coarse gravel and use it in the same way as the scoria described above. Gravel is heavier than scoria though and may make the pot far too heavy to move! The coarser gravel has larger air spaces between particles than the finer grades, which improves drainage and makes the total volume a bit lighter.
- A length of slotted ag-pipe (agricultural drainage pipe) which is around 10cm (4″) wide can be folded and placed in the bottom of a tall pot. Either point the open ends down, or seal them by tying pieces of geotextile fabric or shade cloth material over the ends.
What Materials Are Not Suitable to Fill the Bottom of Tall Pots With?
There is a wide range of materials that are often used to fill excess space in the bottom of tall pots, but a many of these are unsuitable for the following reasons:
- Unstable, and prone to packing down over time, causing the potting medium to sink down. This includes materials such as plastic bottles and food containers, Styrofoam, florists’ foam, flexible foam such as pool noodles, etc.
- Organic and biodegradable, breaking down over time, causing the potting medium to sink down. This includes materials such as leaves, sticks and branches, pinecones, newspaper, cardboard, etc.
- Extremely heavy, making the pot difficult to move and prone to breakage. This includes materials such as fine gravel, small pebbles, intact or broken ceramic tiles, broken pottery, bricks, sand, etc.
Also, don’t use crushed aluminium (aluminum) cans, these are a cheap and readily available material to use, but they’re a really bad idea, as this metal is considered toxic to plants. Having aluminium metal in contact with the soil or growing medium can react with it, releasing aluminium compounds, which can lead to aluminium toxicity in plants. A concentration of 2-5 ppm in the soil is considered toxic to many plants.
Aluminum toxicity may induce nutrient imbalances and restrict the plant’s uptake of macronutrients calcium, magnesium phosphorus, potassium, and the micronutrients (trace elements) manganese, iron, and zinc.
The symptoms of aluminum toxicity are:
- Primarily root growth inhibition, which adversely affects plant growth and development, and ultimately reduces yields in productive plants.
- A set of symptoms that resemble those of phosphorous deficiency, overall stunting, with small, dark green leaves, late maturity, purpling of stems, leaves, and leaf veins, yellowing and death of leaf tips.
In some cases, aluminium toxicity may induce calcium (Ca) deficiency or produce reduced Ca transport problems, leading to symptoms such as curling or rolling of young leaves and death of growing points or petioles (leaf stalk).
Why Floral Foam Should Not Be Used in the Bottom of Pots
Florist foam should never be used at the bottom of a pot, because it can either become too wet, or dry out and become unwettable. It’s also designed to be biodegradable and break down,which will caus ethe potting mix to sink down in the pot.
The following information is from the Oasis floral foam website:
“Hydrating the foam – Allow foam to fully saturate, approximately 2 minutes. Do not exceed 20 minutes. Soaked floral foam should be used within 1 to 2 weeks. Once wet floral foam has dried, it cannot be re-wet. Floral Foam can last up to 12 months when stored in a dry dark place. It can turn brown rather quickly if left in sunlight or damp conditions.”
Why Polystyrene (Styrofoam) Should Not Be Used in the Bottom of Pots
Polystyrene (Styrofoam) is a technically expanded polystyrene foam, as regular polystyrene is a common hard plastic used for CD cases, on many appliances, and in many other uses.
Polystyrene is a polymer of the styrene monomer, meaning styrene molecules are linked together to make polystyrene.
Going into the technicalities of polystyrene foam toxicity is beyond the scope of this article, but we can point out that Styrofoam (expanded polystyrene foam) under certain conditions leaches out styrene, a neurotoxic chemical which is also classified as a possible human carcinogen by the EPA and by the International Agency for Research on Cancer (IARC), it’s a compound that causes cancer in animals.
The risk with Styrofoam is when it’s used in food containers, where it leaches out toxic styrene when in contact with hot, acidic or oily food or alcohol, causing a human health risk. Never microwave food in Styrofoam containers or eat oily food from them either, and don’t ever drink tea or coffee, fruit juice or alcoholic drinks from Styrofoam cups! Alcohol dissolves polystyrene foam to release toxic styrene.
It’s much safer when used as a worm farm container or vegetable growing container due to the lower temperatures involved. The high bacterial activity in a worm farm or garden tub would most likely break down any styrene released, here’s an extract from a scientific publication on bacterial degradation of styrene:
From the book “Microbial Degradation of Xenobiotics, Environmental Science and Engineering” by editor Shree Nath Singh, Springer Science & Business Media, 2011 ISBN 3642237894, 9783642237898 is the chapter “Microbial Styrene Degradation: From Basics to Biotechnology” by Dirk Tischler, Stefan R. Kaschabek, from which I have quoted the following summary:
“Abstract – Styrene, the simplest representative of alkenylbenzenes, is one of the most important monomers produced by the chemical industry today. The compound shows a distinct toxicological behavior and is classified as a possible carcinogen due to its metabolism in human via a highly mutagenic epoxide. Considerable amounts of styrene are released by emissions and effluents during production and usage in polymer manufacture and by deposition of industrial wastes. A high chemical reactivity as well as its natural occurrence are reasons for the ubiquitous presence of styrene-catabolic activities among microorganisms. Rapid breakdown of styrene occurs in soils and aquifers under aerobic as well as under anaerobic conditions. As a consequence, styrene can be classified as readily biodegradable. Furthermore, a high volatility and susceptibility to photooxidation prevent bioaccumulation of styrene.”
Styrofoam is highly persistent, doesn’t break down much at all chemically, but may fall apart physically and flake into smaller pieces, which would cause the level of the potting mix in a tall pot to sink down dramatically.
If polystyrene does degrade back into the toxic chemical styrene that it’s made from, it will eventually be broken down by soil microorganisms, but before that happens, sufficient soil concentrations of this mutagenic compound will be available for uptake by plant roots, which can’t be good for the health of any plant!
- Microbial Degradation of Xenobiotics, Environmental Science and Engineering” by editor Shree Nath Singh, Springer Science & Business Media, 2011 ISBN 3642237894, 9783642237898
- OASIS® Floral Foam https://www.oasisfloral.co.uk/floral-foam
- Cropaia, “Aluminum Toxicity in Plants”, https://cropaia.com/blog/aluminum-toxicity-in-plants/
- Rahman, R., Upadhyaya, H. Aluminium Toxicity and Its Tolerance in Plant: A Review. J. Plant Biol. 64, 101–121 (2021). https://doi.org/10.1007/s12374-020-09280-4
- Sathyaseelan, Neenu & K S, Karthika. (2019). Aluminium toxicity in soil and plants. 2. 15-19.