Should You Put Gravel or Rocks at the Bottom of Plant Pots for Drainage?
Several long-held gardening myths claim that the best way to improve drainage in plant pots is to add gravel, but how true is this? Is it possible that the practice actually harms plants more than it helps?
It is important to improve drainage in pots because most plants hate having ‘wet feet’ – otherwise known as waterlogged roots – because they can die of root rot.
It is a standard feature of plant pots, planters, tubs, and containers designed to hold plants that they contain drainage holes to allow water to drain out freely and to avoid any accumulation at the bottom of the pot.
If holes in pots allow drainage, then why should drainage be increased? In part, it is because the potting medium in which the plant grows is designed to hold water, at least to some extent.
How Did The Tradition of Putting Gravel At the Bottom of Pots Originate?
In order for gravel to increase drainage, the pot has to have inadequate drainage, either due to not having enough drainage holes, or by having blocked drainage holes.
As speculation on my part, I suspect that the reason gardeners traditionally used gravel in the bottom of large pots may be due to the fact that these pots were traditionally made of clay rather than plastic, and they had only one large drainage hole in their base. There was a traditional method (and still is) to prevent potting mix from spilling out of these pots by placing a very loosely fitting stone over the hole. A blocked hole would result in stagnant water in the pot that would slow down the drainage, leaving the pot waterlogged. Terracotta pots with glazes are even worse, as they do not keep moisture from penetrating their sides and stay wet for longer periods of time.
If terracotta pots are layered with gravel at the bottom, it creates a small reservoir for excess water that would otherwise drain out of the pot on its own if there was no gravel there for it to collect in if the drainage hole becomes blocked. This is my educated guess, and like most traditions, we eventually forget why something was done in the first place, and we just continue to do it the same way out of habit.
Plastic pots are designed with drainage holes to allow excess water to drain quickly, and many come with a domed base with drainage holes at each edge in order to increase drainage. Let’s dig into the science to determine what’s best for plants.
Potting Mediums, Striking the Perfect Balance
The roots of plants rot when too much water is applied; when not enough water is applied, plants dry out. For plants to thrive, you need a good mix (potting medium) that is capable of drying plants out whilst retaining a sufficient amount of moisture.
The material in potting mix must be able to absorb and retain moisture, much like a sponge does. Any decent quality potting mix must include that material. Any potting medium’s wicking or absorbent characteristics are crucial to understanding how water behaves in pots.
The Science of Plant Pots and Perched Water Tables
Gravity causes water to run to the lowest point, and all water will run out of a container with drainage holes in the base unless there is another mechanism to hold it there.
It is the same for absorbent materials, such as wet sponges standing upright or wet towels hanging on a line. There will be a downward movement of the water, and some of it will drip away. In most cases, the top of a wet sponge or bath towel will dry first, while the bottom portions of the sponge or towel will remain damp longest.
The behavior of potting mediums when wet is the same as that of other absorbent materials.
We can check out some basic physics by looking at two opposing natural forces present in a wet medium in a pot.
- Gravity causes the water to be drawn downward and drained away through the drainage holes.
- The capillary action retains the water and causes it to saturate the potting medium by exerting an upward pull.
The limitations of both forces, however, are as follows:
- Water can only be wicked up by capillary action to a relatively limited height above gravity, no higher.
- As a result of the gravitational force acting against the capillary action, the water will only experience a limited downward pull.
A perched water table is formed at some point in time due to the balance between these two opposing forces, and when this happens a layer of water-saturated pot medium sticks to the bottom of the pot and cannot drain. The water literally sticks there since it cannot drain.
It’s important to understand that the perched water table does not drain. The water stays in the grooves unless a plant root draws it up, or if it evaporates when the potting mix dries out, in which case the plant won’t survive!
You should also be aware that all pots filled with any kind of growing medium, potting mix, or potting medium, whatever the name, have a perched water table.
A pot’s size and shape don’t matter. No matter how tall and narrow or how wide and deep the pot is, or how big or small it is, the perched water table will always be the same height as long as the growing medium/potting mix is the same.
The perched water table height will differ among different growing media, the more absorbent growing media will have higher levels, and the less absorbent ones will have lower levels.
Understanding Capillary Action
You can read more about the science in this section if you are interested, otherwise, please move on to the next section. I enjoy teaching from the beginning, as I feel this allows us to get to the core of the matter much more easily.
There is no need to explain gravity; it’s a constant force on this planet that pulls everything down.
In liquids, capillary action is caused by cohesive and adhesive forces.
- Cohesive forces are those that bind molecules together of the same type. A molecule of water, for example, is able to cling together.
- Adhesive forces are those that attract molecules of different types. Other materials can cling to molecules of water, for instance.
By definition, a capillary action occurs when a fluid tends to rise (or rise in the case of mercury) in a narrow tube (capillary tube) because of the relative strength of adhesive and cohesive forces.
In order to understand how this works, we need to understand the nature of water.
It has one side that contains a negatively charged ion and the other a positively charged ion. This makes water (H2O) a polar molecule. During the formation of the hydrogen and oxygen atoms, the V-shape of the molecule induces a partial positive charge on the hydrogen side and a partial negative charge on the oxygen side.
Polar molecules act as magnets, their (+) positive charge atoms and (-) negative charge atoms are attracted to one another.
It is due to hydrogen bonds formed by the positive and negative sides of water molecules that water molecules stick together, forming the strong cohesive forces between water molecules that explain why water clings to itself.
When water molecules come into contact with materials that are even more polar (have a stronger electrical charge) than water, strong adhesive forces will allow them to adhere to those materials as the attraction will be greater than that which holds water molecules together.
The upward movement of liquid against gravity, known as capillary action, is caused by:
- There may be a force of attraction between water molecules and another material above the water’s surface that does not already have water attached to it (adhesion), which makes the water molecules ascend a little.
- Hydrogen bonds between water molecules cause them to become attracted to one another (cohesion), causing them to become self-adhesive.
The capillary effect can also be described as a combination of adhesive and cohesive forces exhibited by water.
After we understand why water moves upward and creates perched water tables in growing media, we can now revisit an earlier question with a more scientific viewpoint.
The Effect of Placing Gravel at The Bottom of a Pot on the Perched Water Table
Does the placement of the sponge make a difference if it is on top of the gravel in the sink or upright in the sink? Understanding how the forces of adhesion and cohesion within liquids create capillary action, leading to the formation of a perched water table at the bottom of an absorbent medium, we can see that this won’t affect these forces at all.
As the downward force is due to gravity, which we cannot increase, a lower layer of another material will not alter the adhesive forces between the growing medium and water molecules, nor will it change the cohesive hydrogen bonds between water molecules.
So what is the effect of adding gravel at the bottom of a pot below the growing medium?
This will result in a decrease in potting medium volume, as well as a rise of perched water tables in the pots
There are two potentially serious problems associated with adding gravel to a pot’s bottom:
- The increased level of saturated water near the plant roots can really increase the risk of root rot, due to the roots staying wetter for longer periods of time.
- The reduction in growing medium volume will decrease root space and root volume, as well as available moisture, thereby reducing the plant’s drought tolerance and potential growth size.
The addition of gravel or rocks to a pot has no benefit when viewed from a scientific first-principles perspective!
The Correct Way to Increase Drainage in Pots and Containers
As long as you use the same potting medium in any pot, the water table will always remain the same height. This is because the perched water table is determined by the wicking ability of the potting medium, since gravity never changes.
Increase drainage by adding materials throughout the entire potting medium and by reducing capillary action. This will increase the space between the roots and increase the air space in the mix.
The potting mixes in containers of some plants must drain extremely well. Orchids, for example, are epiphytes (not soil-growing plants, but plants that obtain moisture and nutrients from the air and rain and grow on top of other plants), and a lot grow in trees. Cymbidium orchids are usually grown using a composted pine bark mix, which is mainly composed of coarse 20mm (3/4″) pine bark pieces. The bark pieces barely retain moisture, so there is no perched water table in this mixture. It contains enormous air spaces and drains extremely well.
Potting mixes designed for cactuses and succulents typically consist of coarse organic ingredients to allow some moisture to be retained and lots of gritty ingredients, such as crushed quartz or other crushed rock, that act as a sand-like soil and allow almost straight-up water flow.
In the same way that popcorn is made more porous by heating, perlite and vermiculte are materials used as soil amendments and are made porous by expanding with heat. They can be used to improve the drainage and aeration of potting mixes because of the large gaps they have within them. The purpose of perlite is primarily drainage, while vermiculite holds some moisture and keeps nutrients in place as well. Aeration will be improved, drainage improved, and the perched water level lowered when these amendments are mixed directly into the potting mix.
In hydroponics, perlite is also used as a potting medium, as are ‘clay balls’, which are actually clay coated pumice balls that are very porous and weigh very little. Growing media with large air spaces inside and between their particles drain extremely well while retaining enough moisture to keep plant roots moist.
In horticulture, it is common practice to modify the composition of the potting medium to increase the amount of air space within it in order to increase drainage in pots and containers, and not to alter the space beneath the pots themselves.