Does Gravel in Pots Increase Drainage – The Latest Science

Logic would have you believe that adding gravel or pot shards in the bottom of a pot or container will increase drainage and this has been a common practice for a long time. More recently some gardening experts and scientists have declared this to be a myth, claiming that gravel does not increase drainage when placed in the bottom of a pot. That is the current thinking.

New science has now been published that debunks the claimed myth and concludes that drainage material in the bottom of a pot or container does increase drainage.

Original Thinking About Drainage

Before going any further it is important to define what drainage means from a gardener’s perspective. It turns out that defining this is not so simple. I asked our Facebook Group, Garden Fundamentals this question and got several answers. Part of the confusion lies in the fact that the term is used differently when discussing real soil in the ground compared to soil in pots and containers.

When talking about garden soil outside we think about drainage in terms of how fast water runs away. The “soil perc test” consists of digging a hole, filling it with water, and timing how quickly the water runs away. Good draining soil is one where water runs away quickly.

In pots, however, time is not the issue. In pots, we are more interested in “how wet” the soil is after excess water runs out of the pot. A poorly draining soil is one where the remaining soil is very wet after watering. A well-draining soil is drier.

Since pots are the focus of this discussion, I’ll define drainage as the amount of water held in the soil after watering. Therefore good drainage results in soil that holds less water.

Historically, gardeners put large inert material in the bottom of pots to increase drainage. This usually consisted of clay pot chards or rocks as shown in the above picture.

This seems very intuitive. These large materials don’t hold water very well and excess water should simply run through them, resulting in drier soil above the drainage material. It was believed that drainage material increased drainage.

Note: this post is all about gravel below the soil and not about gravel mixed into the soil.

The Myth: Gravel Does Not Increase Drainage

A number of years ago the idea that gravel increased drainage was declared a myth, and this claim has become more widely known.

To understand the myth it is important to understand the idea of a Perched Water Table which I have discussed fully in What is a Perched Water Table.

As water is added to a pot of soil, it creates a saturated area near the bottom of the soil, known as the perched water table. The myth claims that adding drainage material in the bottom of the pot does not change the perched water table. Instead, it simply moves this saturated layer higher in the pot.

Adding a drainage layer creates three layers: a drainage layer, a saturated soil layer (i.e. perched water table), and an unsaturated soil layer.

Since the drainage material does not eliminate the perched water table there is no value in using it. Many people have tried to demonstrated this with sponges, but these have never been a satisfactory explanation to me.

Movement of Water

Some of the scientific supporting evidence for debunking the gravel myth was the movement of water between two different layers.

What happens when you layer one type of soil above another? You might expect that water easily moves from one layer to another but that is not what happens. Water has difficulty moving from one soil to another when the particle sizes of the soils are different. This was shown many years ago in a simple experiment. A container had a lower level of sand (large particles), and an upper level of silt loam (smaller particles). Water was dripped onto the top. You can see in the picture that water (the black areas) moved down by gravity until it reached the sand layer, image (a). At that point, water started moving sideways instead of moving down (b). As more water was added, the silt loam became saturated and capillary forces could no longer overcome gravity and the water started moving into the sand layer (c).

The movement of water in this experiment is no different than water moving in a pot. Water in a pot full of soil does not easily leave the pot because the soil and air have different particle sizes. This experiment also illustrates why you should never layer different types of soil in the garden. Always mix them together.

Still Not Convinced?

If you still doubt the existence of a perched water table, try the simple experiment in this video. My method to demonstrate this is better than the sponge demoes you might have seen.

New Science Debunks the Claimed Myth: Gravel Does Increase Drainage

The myth-busting discussed in the previous section was based on a theory that incorporated the above diagram showing water moving between silt and sand, as well as other known facts about soil and how water moves in it. It seemed like a reasonable explanation.

However, The theory had never been tested in a lab. That has now changed.

A recent study by Avery Rowe measured water levels in three different potting media, both with and without a variety of drainage materials. This study clearly shows that drainage material does increase drainage. It debunks the claimed myth!

The author has produced a summary of the scientific study on his website, https://tradescantia.uk.

Experimental Setup

1. Fill the container with the drainage layer and a measured amount of potting mix, then weigh the whole container.
2. Temporarily seal the drainage hole, fill the container with water to the brim, and weigh it again.
3. Remove the seal and let the excess water run out.
4. Weigh the drained container one last time.

The same procedure was repeated with only drainage material to get the weight of water collected in the drainage layer. This value was then used to calculate the weight of the water in the soil.

Three soil types were tested.

• CPB is a mix of 60% coir, 20% perlite (Vitax 2–6 mm), and 20% pine bark (Melcourt Propagating Bark 2–7 mm) by volume.
• CV is a mix of 70% coir (Coir Products Coco Peat) and 30% vermiculite (Westland Gro-Sure) by volume.
• JI is a loam-based John Innes “No. 1” compost (Bathgate) (i.e. potting media).

A range of drainage materials were used to evaluate different particle sizes and porosity. This material was also layered at two depths: 30 and 60 mm.

• Gravel (Melcourt Horticultural Gravel 4–10 mm)
• Leca (LECA Lightweight Aggregate 4–10 mm)
• Grit (LECA Lightweight Aggregate 2–4 mm)
• Sand (Dehner Aquarienkies 1–2 mm)

Results

The original research paper reports results in terms of water-holding capacities. I have taken the raw data from the paper and calculated the amount of water held in the soil layer.

This table shows the volume of water, in milliliters, in the soil layer as well as the % change between only soil (i.e. no drainage layer) and soil with a drainage layer.

Material	CPB Soil (water in ml)	% change	CV Soil (water in ml)	% change	JL Soil (water in ml)	% change
Only soil	304		381		335
soil + 30 mm gravel	167	-45	334	-12	297	-11
soil + 60 mm gravel	188	-38	234	-39	297	-11
soil + 30 mm Leca	145	-52	313	-18	321	-4
soil + 60 mm Leca	137	-55	256	-33	309	-8
soil + 30 mm grit	156	-49	317	-17	258	-23
soil + 60 mm grit	111	-63	219	-43	282	-16
soil + 30 mm sand	158	-48	250	-34	256	-24
soil + 60 mm sand	106	-65	208	-45	215	-36

CPB soil is more porous than CV soil, which is more porous than JL soil. The drainage layer had the greatest effect on the most porous soil.

The author has commented privately that based on a visual examination of the containers the above values for soil above drainage layers are probably on the high side. Refer to Model B in the study for a detailed discussion of this.

It should be noted that the process of adding water was not the typical method used by gardeners. To improve the consistency of the process the container was completely filled and then drained. Gardeners are more likely to pour water into the pot and allow excess to drain as they are pouring. This can water unevenly and add air bubbles, both of which were eliminated in the current study. It is not clear how this difference in watering would affect the data.

Conclusion

The results in the above table clearly show that the soil holds less water when a drainage layer is used. This is true of all three soil types and all of the drainage materials tested.

Drainage layers were more effective with coarser potting mixes, and thicker layers were more effective than thinner ones. Overall a 60mm layer of coarse sand was the most effective.

These results debunk the above-described myth. Drainage material in the bottom of a cup will increase drainage.

This testing did not measure the existence of a perched water table however the concept of the perched water table is still valid, but drainage material may alter its depth.

The study also did not test the effect of a drainage layer on plant growth. Plants may or may not grow better with a drainage layer. Remember that plant growth is also affected by the porosity of the potting soil and selecting the best one may be more important than the drainage layer.

Should you use a drainage layer? In most cases, you do not need it.