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Gravel at the Bottom of a Pot Helps Drainage

We all know that excess water kills more plants than drought and therefore the common advice to help solve this problem is to put some gravel or pot shards in the bottom of the pot. Then add your soil over this drainage material.  This makes complete sense. Water will surely drain through the soil, and then quickly go through the gravel and out the bottom.

Even though this seems to make sense, it is a myth!

Gravel helps drainage

Gravel does not helps drainage in a pot

 

When water reaches the interface between two different types of soil – it stops moving. It is hard to believe, but in the above example, the water will accumulate in the soil and stay there. The gravel will be dry, and the soil will be saturated.

Let’s have a closer look at this. When water is added to the soil, it is absorbed by the soil particles. It fills the small holes between things like clay particles, and gets absorbed by the humus (organic mater) in the soil. As more water is added it starts filling the air spaces between soil particles. This water flows fairly freely to the bottom of the soil and then stops moving. Add more water and you reach a point where the soil can’t hold more water, but it still does not flow into the gravel. If more water is added the soil can’t hold any more and excess water runs into the gravel.

Plant roots  need both air and water. The ideal soil has about 25% water and 25% air in it. If the air is replaced with water the roots of most plants die.

In addition to creating a soil with less air, the gravel will also reduce the amount of soil in the pot. That is not good for roots either.

In the next post we will look at situations in the garden that have the same problem as described above.

Still Not Convinced?

Try this experiment. Take a kitchen sponge and set it on the table in a vertical position. Slowly pour some water onto the top. The water will make its way to the bottom of the sponge and sit there. Very little will leak out onto the table unless you pour too much into the top. You can take the sponge and turn it over and the water will again move down the sponge due to gravity, and stop before it runs onto the table. In this experiment, the sponge is acting similar to soil and the table represents the gravel.

Don’t put drainage in your pots.

References:

1) Photo Source: Kyknoord

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Robert Pavlis
Editor of GardenMyths.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

I hope you find Garden Myths an educational site that helps you understand your garden better.

6 Responses to 'Gravel at the Bottom of a Pot Helps Drainage'

  1. Myrbär says:

    A little late on the ball but as your posts are timeless I’ll sneak in a comment anyways.
    About the same phenomenon except horizontally is used when applying mortar on a brick wall as a finish on a buildings exterior.
    If a finer gradient of sand is used for each layer applied on the construction, finest gradient in the outmost layer, moisture will be sucked from the inside and more readily evaporate. The rain on the other hand has a harder time traveling through each layer due to capillariy law.
    For visualization one could maybe think of holding two kitchen funnels under the tap in your kitchen. It is easiest to pour through them both in one order only, from large opening through small to the others large end.
    Or another kitchen analogy but not exactly relating. Try pouring water trough an old school colander, the ones made of solid metal with tiny cast holes and usually enameled. Now turn it the other way around and almost nothing makes it through as water passes by the small holes as surface tension bridges the gaps.

    TL;DR everything is physics

  2. Brian Tremback says:

    The table analogy is a bit confusing. What occurs in that situation is the same as in soil overlying an impermeable layer like bedrock – water builds up in a layer of saturated soil whose thickness depends on the amount of water added and the ability of groundwater to move away from the point of addition. That’s not the equivalent of a pot with soil overlying gravel.

    Where a finer textured soil overlies a coarser one, water will build up along the interface until a thin layer of the finer material holds enough water to make the leap from finer capillary space to coarser capillary space of the underlying soil. If there are no capillaries, the lower boundary of the finer material will have to come to full saturation. As soon as it does so, water will begin to drain out.

    Going back to the sponge analogy, immerse a sponge in water and purge the air from it. Lift it out of the water, holding it by one end. Watch it drain. Under the force of gravity water will drip out. Only the lower edge will be close to saturation.

    The difference between a pot without gravel and one with gravel is that without, the almost saturated zone occurs only near the drainage hole because that’s the only place where there is a disruption in the soil’s capillaries. When gravel is placed in the bottom of the pot, the nearly saturated zone occurs along the entire interface between the soil and the gravel. How bad this is depends on the plant and how sensitive its roots are to excess moisture. Because the gravel allows air to circulate, there will probably not be an anaerobic zone formed, only one in which roots, like those of cacti, would be more susceptible to fungus because of longer lasting moisture.

  3. Nic says:

    Are you serious? Did you do any trials and publish your results in a scientific journal? Nic Combrink, Stellenbosch

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