When growing plants in pots, it’s sometimes necessary to increase drainage because some plants are sensitive to excessive moisture around their root zone, and stagnant water at the bottom of the pot can lead to root rot.
The old garden myth of putting a layer of rocks in the bottom of a pot to increase drainage has been thoroughly debunked by many university agriculture extension agencies, and if you want to see the technical explanation, please read my article – Should You Put Gravel or Rocks at the Bottom of Plant Pots for Drainage?
There are proven ways to increase drainage in pots which are taught in horticulture schools and used by plant production nurseries. In this article I’ll explain the science behind how we increase drainage in pots, and practical advice on which materials we can use for the purpose.
Why Plant Pots Don’t Drain Completely
Water naturally flows to its lowest point due to the force of gravity, and if we pour water into an empty pot, it all leaks out through the drainage holes in the bottom, as expected.
If we fill the pot with an absorbent material, such as a potting medium (potting mix, growing medium, soilless potting medium, whatever you choose to call it), and pour water into the pot, a lot of water will drain out, but some will be retained. A good quality potting medium will drain well but still have enough water retention to supply the plant’s water needs.
The reason that potting media (the plural form of potting medium) retain water is because they are absorbent, and can wick water upwards against the force of gravity, preventing it all from draining out. The more absorbent a material is, the greater its ability to wick water, the higher the water will rise upwards, and the more water will be retained.
What makes a material absorbent? In materials which contain very fine pores or very fine air spaces between their particles, water can wick upwards by capillary action.
Capillary action works by a combination of two forces:
- Cohesion, where water sticks to itself and pulls more water along.
- Adhesion, where water sticks to other surfaces.
When the upwards force of capillary action is greater than the downward force of gravity, it prevents gravity pushing all the water out, and some water is left behind at the bottom of the pot in the potting medium.
This residual water that doesn’t drain out is known as the perched water table, because it will just sit there and keep the potting medium waterlogged if plant roots don’t access the water and use it up.
The way the forces of gravity and capillary action oppose each other is shown in the diagram below.
The Science of Increasing Drainage in Potting Media
To improve drainage and reduce water retention, we need to reduce the wicking ability of the potting medium.
Wicking ability is the same thing as capillary action, and it’s driven by the the cohesion of water to itself, and the adhesion of water to other materials as discussed earlier.
Since we can’t reduce water’s ability to stick to itself (cohesion), the only other option we have to reduce wicking is to reduce the water’s ability to stick to its surroundings, the particles of the potting medium (adhesion)!
If we understand which properties of a growing medium maximise wicking and water retention, then we can use the opposite to reduce it!
What is required for maximum wicking in a potting medium?
- Maximum surface area for the water to adhere to. Smaller particles in a growing medium create a larger available surface area. As an example, imagine a cake, which has a given external surface area. If we cut the cake into slices, we still have the original external surface area, but now also have the additional exposed surface areas of the inside of the cake on the sides of each of the slices.
- Very narrow air spaces small enough for water to be able to bridge. Smaller particles in a growing medium pack down closer together, creating narrower spaces between themselves. As an example, a jar filled with very large marbles will have very large gaps (air spaces) between them as they don’t sit too close together on account of their size. If the same jar was filled with small marbles, they will fit together much more tightly, leaving only extremely narrow spaces between each other.
How does the size of particles and the spaces between them translate to potting media?
Water can only bridge small gaps between potting medium particles due to its own weight. Increasing the size of the particles in the growing medium also increases the size of the air spaces between them, giving water less to hold on to, which reduces its ability to wick upwards.
This is the simple secret to drainage – by increasing the aeration (air spaces) throughout a material, the small gaps and large surface areas that water can cling to are reduced, so the water drains out more easily.
The only way to increase drainage in a potting medium is to change its composition, which change its physical properties, turning it into a faster-draining potting medium.
The diagram below shows how the size of particles in a potting medium affect drainage.
- The first pot is filled with a potting medium in a pot, and has a perched water table which sits quite high.
- For comparison,the second pot is filled only with a soil amendment material, such as perlite, which has very large particles, retains very little water, and therefore has an extremely low perched water table.
- If we amend the potting medium by mixing the larger particles of the amendment material all the way through it, as shown in the third pot, we create our own custom-made potting medium mixture which has greater aeration due to the larger air spaces between particles, which causes it to drain faster than the original potting medium, and have a lower perched water table as a result.
Let me re-emphasize that point. By amending the potting medium, we are in fact creating a totally different, faster-draining type of growing medium.
So, forget about putting a layer of rocks or gravel in the bottom of pots, beneath the potting medium, that does nothing except reduce the pot volume and push the perched water table upwards where you don’t want it, as that can encourage root rot.
If a potting medium is not modified, it will behave in exactly the same way as it always has, and it will drain as it did previously, that should be logical. Placing something underneath the potting medium does not change its physical properties, which is the factor that determines drainage.
To quote the University of Illinois Extension article on the subject “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.”
Three Soil Amendment Materials for Improving Drainage in Pots
In this section we’ll discuss the various amendment materials which can be used to increase drainage in potting media, as well as their advantages and disadvantages. By knowing how they perform, it’s much easier to make an informed decision as to which is best to use for a particular purpose.
Perlite is a white, lightweight, highly porous material which is produced by rapidly heating volcanic silicate rock to high temperatures above 870°C (1,600°F) which causes the water in the perlite to be converted to gas that causes the heat-softened mineral to expand like popcorn, by 4–20 times its original volume.
Expanded perlite is very light, with a bulk density of about 30 – 150 kg/m³ (2 – 9 lb/ft³) depending on the grade. It comes in different grades ranging in size from 3 – 6mm (1/8 – 1/4”) in diameter.
It’s used extensively in potting media, greenhouse growing media, nursery propagation applications and as a hydroponic growing medium. Fine grades of perlite are available which can more easily fill very small containers for use in seedling plug production.
- Improves aeration and drainage.
- Non‐toxic, sterile, odourless.
- Chemically inert, pH neutral with a pH of 7.0 – 7.5, no pH buffering capacity, contains no mineral nutrients, almost no CEC (cation exchange coefficient) so it cannot hold nutrients.
- Will not compact over time.
- Low water-holding capacity, water is only retained on surface and in empty spaces between particles. Has “closed cell” pore structure, so pores don’t absorb or hold water.
- Lightweight, reduces the weight of the potting medium while increasing drainage, unlike coarse sand which improves drainage but increases weight and creates less aeration.
- Can be steam-heated to sterilize.
- Moderate cost.
- Has a tendency to float to the top of the potting medium during watering.
- Very dusty when dry, dust is harmful if inhaled. Avoid health risk by wearing a dust mask when handling the product.
- Must be moistened before mixing into other ingredients to keep dust down.
- May contain levels of fluoride that may be toxic to fluoride-sensitive plants. Avoid fluoride toxicity problems by keeping the pH above 6, and by not using fluoride-containing commercial phosphate fertilizers (such as superphosphate, diammonium phosphate, ammonium nitrophosphate), which you shouldn’t be using in the first place because they’re synthetic, not organic-certified and really bad for your soil!
- Can release toxic levels of aluminium into solution when the pH is low, avoid this problem by keeping the pH above 6.
Vermiculite is a lightweight, highly porous material, consisting of glossy flakes that vary in colour from dark gray to sandy brown, which are produced by heating chips of the layered mineral mica to high temperatures of around 800 – 1100 °C (1,472 – 2,012°F), which causes the laminated, plate-like structure to expand, much like an accordion, creating a highly porous lattice structure with good aeration and water-retention properties.
Expanded vermiculite is very light, with a bulk density of 64 – 160 kg/m³ (4 – 10 lb/ft³), depending on the grade, and it usually comes in four different grades, where #1 is the coarsest and #4 the finest in terms of particle size.
It’s used to increase moisture and nutrient retention in potting media. The finer grades are used for seed germination and topdress seedling flats, while the coarser grades are used in potting media.
Only horticultural-grade vermiculite should be used for gardening purposes, the type sold at garden centres, and not the grade sold for construction or industrial purposes.
- Improves aeration and drainage.
- Non‐toxic, sterile, odourless.
- Excellent pH buffering capacity.
- Fairly high CEC (cation exchange coefficient (2 – 2.5meq/100cc), so it can hold soil nutrients and slowly release them.
- Contains some potassium, magnesium and calcium that slowly becomes available to plants.
- Highly absorbent, with a very high water holding capacity, can hold water, nutrients, and air, unlike perlite.
- Easily compressible, should not be compacted or pressed, especially when wet, as this will destroy its structure and reduces its ability to hold water and air.
- Less durable than coarse sand and perlite.
- The finer grades which are used to fill seedling plug trays have particles which are too small to hold much air or water for developing roots.
- The pH can vary from slightly to very alkaline, depending where it is mined. Most vermiculite from the US has a pH between 6.3 -7.8, which is neutral to slightly alkaline, whereas vermiculite from Africa can be quite alkaline, around pH 9.
Vermiculite and Asbestos Contamination
There were health concerns around vermiculite in the 1990’s over contamination with fibrous tremolite asbestos. Here’s the history. The vermiculite mine near Libby, Montana was the largest and oldest vermiculite mine in the US, and was started in the 1920s. It was producing more than half the worldwide supply of vermiculite from 1925 to 1990, which was found to be contaminated with asbestos and asbestos-like fibres. Mining operations were stopped at this site in 1990 in response to asbestos contamination.
Due to this incident and the attention it drew, which resulted in a massive lawsuit against the mining company, most mines these days closely monitor their operations to avoid problems with asbestos contamination in commercially available expanded vermiculite. As a safety precaution, it is advisable to keep vermiculite moist while using it in order to minimize dust, and to wear a dust mask to avoid inhaling any dust from the material.
The Differences Between Vermiculite and Perlite
Vermiculite and perlite are used interchangeably for certain applications, as they both provide increased pore space due to the size of their particles, improving drainage while minimizing the weight of the soil.
They do differ however in certain properties, which will determine whether one is used in preference to the other.
- Vermiculite is composed of an expanded, plate-like structure with air spaces between layers which allow it to absorb water.
- Perlite has a “closed cell” structure without open pores, and cannot absorb water.
Cation-exchange coefficient (CEC)
- Vermiculite can bind and slowly release positively-charged nutrients such as potassium magnesium and calcium.
- Perlite cannot not bind nutrients.
- Vermiculite has a slightly alkaline to very alkaline pH, and has a pH buffering capacity, which stabilises the pH.
- Perlite is neutral in pH, and has no buffering capacity.
- Vermiculite particles are soft and compressible, and will compact easily with repeated digging, losing their beneficial properties.
- Perlite particles are hard and brittle, and cannot be compressed, will tolerate repeated digging, with some particles being broken into smaller pieces.
3. Coarse Sand
Sand is one of the basic components of soil, and is composed mainly of small particles of of silica (silicon dioxide) in the form of quartz. Grains of sand are solid particles which do not absorb water.
Unlike other amendment materials, sand is extremely heavy, with a bulk density of 1,520-1,680 kg/m³ (95-105 lb/ft³). It comes in a range of grades ranging from 0.05mm to 2.0mm in diameter.
Coarse sand, which has a large particle size, is used as an amendment for potting media, and commonly used in greenhouse and nursery propagation mixes. It’s added to improve drainage and increase the weight of the potting mixes, acting as a ballast to help prevent potted plants being blown over by winds in outdoor plant nurseries, and is most often found in very fast draining potting mixes used for cacti and succulent plants. Another common use of coarse sand is to top dress lawns, it’s used both on existing lawns, and when laying instant lawns, especially buffalo varieties.
Only washed or horticultural-grade sand of medium to coarse grade (0.25 – 2 mm) should be used in potting mixes. Deep-mined, white mountain sands are used for this purpose because they’re mostly silicon dioxide with very little silt, clay or other contaminants.
Avoid using calcareous sands (created from coral, shell fragments and skeletal remains of marine organisms) as these are high in calcium carbonate which is the same thing as limestone, or garden lime, and has a very high (alkaline) pH value. Also, don’t use sands sourced from the ocean as they are saline, they contain sea salt which is harmful to plants.
Which Grades of Sand Can be Used in Potting Media?
It is important to note that sand only improves drainage and aeration by providing increased pore space due to the size of its particles, so it only works when its particles are larger than those of the medium it is amending.
Soil is composed of sand, silt and clay, and the reason why sandy soils drain so well is because the largest particles in the soil are sand particles. Silt particles are smaller than sand, and clay particles are the smallest.
That said, sand should not be mixed with clay soils. The Puyallup Research and Extension Center at Washington State University warns that adding some sand to clay soil will not improve it, as the fine clay particles will simply fill the spaces between the sand particles. This will result in a heavier, denser soil with less total pore space than either the sandy or the clay soil alone. We are told that soil must consist of nearly 50% sand by volume for it to behave like a sandy soil.
As we can see from the table below, the very finest sand particles are 0.05 mm in diameter, the same size as silt particles. Compared to silt and clay particles in soil, even the smallest sand particles are fairly large.
Soil particles in order of increasing size
Clay …………….…….. <0.002 mm
Silt ………………..…… 0.002 – 0.05 mm
Sand ……………….…. 0.05 – 2.0 mm
Very fine sand ….…. 0.05 – 0.10 mm
Fine sand ……………. 0.10 – 0.25mm
Medium sand ….….. 0.25 – 0.5 mm
Coarse sand ……..…. 0.5 – 1.0 mm
Very coarse sand .… 1.0 – 2.0 mm
For more information on soil types, please see the article Soil Texture and Types of Soils
Typically, only medium to coarse grades of sand (0.25 – 2 mm) are used to amend potting media as the particles are large enough to provide optimum improvements to the media texture.
Cactus and succulent growing mixes tend to be the fastest-draining growing media available for pots and containers. These mixes can contain between 10-30% coarse sand, crushed quartz or other crushed rock, and and are very heavy for that reason.
- Improves aeration and drainage.
- Non‐toxic, sterile, odourless.
- Chemically inert, pH neutral.
- Will not compact over time.
- Less dusty than perlite and vermiculite.
- Very low water-holding capacity, water is only in empty spaces between particles.
- Cheapest inorganic amendment material.
- Heaviest amendment material.
- Improves drainage like perlite does, but creates less aeration than while increasing weight of potting medium.
Other Amendment Materials Which Can Be Used to Improve Drainage in Pots
All the inorganic (non-carbon based, not derived from living matter) soil amendments discussed so far all increase the pore size between media particles, creating larger air spaces which decrease water holding capacity and improve drainage and aeration.
There are many other inorganic materials that can be used for the same purpose, such as:
Pumice – extremely porous igneous volcanic rock naturally expanded by gases in the molten rock. A rock-foam of volcanic glass with so much air in its structure that it floats on water.
Scoria – very porous igneous volcanic rock naturally expanded by gases in the molten rock. A porous basaltic lava with very small vesicles (pores) less than 1mm, smaller than those in pumice, and with thicker walls, making it more dense so it sinks in water. The most common variety of scoria (lava rock) used in landscaping is red in colour, even though it can be black or dark brown. The red scoria actually starts out black, but the iron content in the rock is oxidised (chemically rusted) during volcanic eruption which turns it red. The name scoria comes from the Greek word for rust, which is σκωρία, skōria.
Pea-gravel – small, smooth, rounded pebbles up to the size of a large pea, shaped naturally by exposure to running water, or through a tumbling process for polishing. Aquarium gravel is usually smaller in size, and both are often used for top-dressing the soil in water-plant pots in ponds and water gardens to avoid clouding the water.
Clay balls – used extensively as a hydroponic growing medium, these are not actually balls of clay, but rather small pumice balls coated with a later or clay. Quite expensive to use as an amendment material.
The only concern with mixing rocks into a potting medium is the obstruction they create when digging, much like digging with a shovel into soil full of rocks. If the potting mix is not going to be frequently dug into then this shouldn’t be that much of an issue.
Some of these materials may also contain fine rock dust, which may need to be washed out to avoid filling the air spaces in the potting mix. This is easily done by putting the rocks into a pot with drainage holes, and using a jet of water under pressure to hose them down. If the drainage holes in the pot are too large, put a piece of shade cloth or flyscreen material inside the pot first to stop the rock washing out.
How to Test Drainage in Potting Mixes
Once a potting medium has been amended to improve drainage, it’s probably worthwhile testing it to see if the changes have really made any difference.
Here is simple test to determine how well a potting mix drains:
- Fill a pot with 1 litre (approx 1 US quart) of dry non-amended potting mix.
- Fill a second pot with 1 litre (approx 1 US quart) of dry amended potting mix.
- Pour 500ml (approx 2 US cups) of water into each pot of potting mix.
- Measure the amount of water that drains out after a few minutes and compare the two.
If any mix is draining adequately, then around half the water, or 250ml (approx 1 US cup) of water should drain out after a few minutes.
If the addition of the amendment to the potting medium has improved drainage and reduced water retention, then more water should drain out of the pot containing the amended medium.
How Different Pot Materials Can Affect Drainage
How can we improve drainage without changing the potting medium?
By changing the pot! The material from which a pot is constructed can make a huge difference to drainage.
Unglazed terracotta pots are porous and will wick water away from the potting medium, and are therefore are ideal for plants which prefer better drainage.
Plastic pots only lose water from their drainage holes at the bottom, and tend to retain more moisture, making them a great choice for plants which prefer more moisture.
- University of Illinois Extension, Urban Programs Resource Network – Successful Container Gardens, Choosing a Container for Planting – Drainage Is Critical to Plant Health https://web.extension.illinois.edu/containergardening/choosing_drainage.cfm
- Pennsylvania State University, College of Agricultural Sciences, PennState Extension – Homemade Potting Media, 2007 https://extension.psu.edu/homemade-potting-media
- The Texas A&M Agrilife Extension – Ornamental Production, Growing Media https://aggie-horticulture.tamu.edu/ornamental/greenhouse-management/growing-media/
- The Texas A&M Agrilife Extension – Ornamental Production, Media, Repotting & Containers https://aggie-horticulture.tamu.edu/ornamental/a-reference-guide-to-plant-care-handling-and-merchandising/media-repotting-containers/
- University of Connecticut, UConn Home & Garden Education Center – Potting Media, 2016 http://www.ladybug.uconn.edu/FactSheets/potting-media.php
- Arizona Cooperative Extension – Potting Media for Containers
- University of Connecticut, The Connecticut Cooperative Extension System, Soil Nutrient Analysis Laboratory – Packaged Potting Media by Dawn Pettinelli
- University of Arkansas, Division of Agriculture, Cooperative Extension Service – FSA6097, Greenhouse and Nursery Series, Growing Media for Container Production in a Greenhouse or Nursery Part I –Components and Mixes by James A. Robbins
- University of Tennessee, Institute of Agriculture, Agricultural Extension Service – PB1618, Growing Media for Greenhouse Production
- L.P. Ramteke, A.C. Sahayam, A. Ghosh, U. Rambabu, M.R.P. Reddy, K.M. Popat, B. Rebary, D. Kubavat, K.V. Marathe, P.K. Ghosh,
Study of fluoride content in some commercial phosphate fertilizers, Journal of Fluorine Chemistry, Volume 210, 2018, Pages 149-155,
ISSN 0022-1139, https://doi.org/10.1016/j.jfluchem.2018.03.018.
- Puyallup Research and Extension Center, Washington State University – The Myth of Soil Amendments Part II: “If you have a clay soil, add sand to improve its texture” by Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor