Stones as drainage?
Stones have been used for centuries for the drainage of pots, but this practice has been questioned in recent times.
The use of stones or rocks to improve drainage is a practice that has existed for a long time, but it has been a subject of much controversy in recent years. With the intention of knowing if it is necessary or not, we analyze its effectiveness from several points.
The use of stones to improve the drainage of containers is a practice that has existed for centuries and that is still used today for containers such as pots, garden planters, and hanging baskets. The purpose is that by filling these containers with stones, the water can run quickly through them and drain.
Although many gardeners state that this practice is not effective, there are others who consider it an essential element to ensure proper drainage in their containers without having to resort to another method such as drilling holes in the base of the container. So who’s right? To know more about it we must analyze several aspects:
Do We Have to Use Stones in the Bottom of the Pots?
For decades it has been recommended to use stones at the bottom of pots to improve drainage and air circulation.
It would seem that one of the main reasons why this advice is given is because stones are heavy, and weight is supposed to help drainage. However, when the weight is placed under the pot, this actually prevents water from draining out through the bottom of the pot.
In fact, if you put a small rock on top of a pot filled with water and then try to drain out some water through the bottom hole, you will see that you cannot do it! Therefore, using stones at the bottom of pots is certainly not necessary for improving drainage or air circulation.
Does Using Stones or Gravel Improve Drainage?
Contrary to what some recommend, numerous studies have shown that adding stone or other coarse material to the bottom of pots does not improve drainage at all. Rather it deteriorates it.
The reason for this is that the amount of free drainage space in a pot is directly proportional to the surface area of its bottom and the porosity of its material.
For example, a pot with a surface area of 80 cm² and made of clay with a porosity of 45% will have approximately 20% free drainage space regardless of whether there is coarse material at the bottom or not. If one were to add 5 mm cubic stone (porosity 0%) on top of that same clay, one would effectively be reducing the porosity to 40%, resulting in only 10% free drainage space.
Now, let us suppose that our above 80 cm² pot has coarse material added to its bottom. As long as we keep the total surface area constant, we are effectively decreasing the free drainage space by adding an impermeable layer (the stone) while retaining an already relatively impermeable material. The result is that we get a smaller free drainage space than what was available before adding the stone.
If you check plants in natural environments, such as forests or meadows, you will see that there are no stones under their pots!
Associate Professor Linda Chalker-Scott of Washington State University calls it “the myth that refuses to die.”
Does It Improve Circulation?
It seems logical to add stones to the bottom of pots, as water passes through coarse-textured material faster than fine material. This sounds good in theory.
The problem is that water does not move easily from layers of finer textured materials to layers of coarser textured materials. In other words, it is very difficult for water to pass from the earth (fine matter) to the stone (coarse matter).
To test the truth of this claim, a sponge can be placed on top of a pile of gravel. Then pour the water into the sponge. Does gravel make the sponge drain faster? No, the sponge fills up and won’t leak until it can’t hold another drop.
Stones Worsen Drainage
Stones at the bottom of pots make drainage worse. Why? Because it leaves less room for the roots. The soil in the pot becomes saturated before the water leaves it and enters the gravel. This situation could cause the roots to rot.
Also, the stone adds unnecessary weight to the pots. It seems that the only benefit of the stone is that it prevents soil from escaping through the drain hole.
The reason why stones can prevent soil from escaping through a drain hole is that they can block or restrict airflow, which is needed to pull out moisture in wet soil.
To be able to pull out moisture in wet soil, air or oxygen must be able to flow freely. Soils with good drainage do not have a problem with excess water because water can flow freely and easily out of it into gravel or a saucer under a pot.
When stones are placed on the bottom of pots, they block or restrict airflow, which causes saturated soil and saturated air (in other words, air filled with water vapor). This prevents water from freely moving out of wet soil and keeps it from evaporating into dry air; instead, it allows water to accumulate in the bottom of the pot without moving anywhere. When this happens, humidity levels rise in an enclosed space.
- Using stones in pots is bad according to experts.
- Only use pots with drainage holes and a quality substrate.
- Keep doing what has given you results.