Gravel at bottom of pot

[Grim Tuesday]

I have a nice, ceramic, perfectly sized pot that unfortunately only has one drainage hole. I was thinking of putting gravel at the bottom of it to effectively make a lot of drainage holes (that eventually all go to the main one at the bottom) but I encountered this puzzling article by the University of Nebraska Extension. I have a really hard time understanding what they mean by it. Does anyone here have experience (positive or negative) with gravel at the bottom of a pot? I'm thinking like 1/2 inch river rocks, not sand.

Here is the article:
The Hard Truth about Rocks at the Bottom of Planting Containers | Nebraska Extension

[Louis_W]

Are you using it for orchids? Like with bark? If so I dont think this will apply at all.

[Roberta]

First of all, remember that they're talking about plants that grow in dirt ... so there's the issue of soil washing out of the pot. That's not an issue for orchids - your mix should be well-drained anyway. How big is your drainage hole?

Another concern if you're using a nice ceramic pot for your orchids, think ahead a couple of years (repotting time) to what's going to happen if it gets really happy and roots go wild, sticking to the sides of the pot. You could need to make the choice between the pot and the roots, sometimes with stuck-on roots the only way to get the plant out of the pot without harming roots is to break the pot. I suggest that you use a cheap plastic pot, or terracotta pot and then nest it in the decorative pot for aesthetics. But gravel at the bottom isn't going to hurt anything - you don't want the orchid sitting in water, no matter how you pot it. You want the water to flow through the medium (pulling air into the root zone) and draining out, no matter how you decide to "organize" it, doesn't matter whether it is one hole or many it's the drainage that is important.

[Ray]

Here’s the facts:

Water in the media column is held by 2 factors: absorption and surface tension that holds water between the particles. The absorbed water is pretty independent, particle by particle, but the interstitial water is affected by the weight of the water above it. The taller the column, the more the weight above any point, and that overwhelms the surface tension, pushing some of the water out the bottom, with end result being less water contained.

If you add a “drainage layer”, you shorten the column, reducing the weight of the water column, allowing more of it to stay in the medium.

You can prove it to yourself using an ordinary sponge. Soak it and hold it flat until it drains. Then stand it on an end and more water will drain out.

[Dimples]

The thing about having gravel at the bottom of the pot is, that regardless of people’s experience, be it good or bad, the article is correct and that means everyone’s plants could have been better without the gravel. The science is sound. When using a fine particulate planting material, putting a traditional drainage layer in the bottom actually makes drainage worse because it reduces the total amount of aerated soil in the pot.

If you’re using particles over 1/16-1/8th inch (it’s right around there) then it doesn’t matter because gravity will win over adhesion, and extra water will run off regardless of particle size distribution in the pot. So, orchid bark and similar stuff don’t operate under the conditions described in the article.

[Ray]

Originally Posted by Dimples

If you’re using particles over 1/16-1/8th inch (it’s right around there) then it doesn’t matter because gravity will win over adhesion, and extra water will run off regardless of particle size distribution in the pot. So, orchid bark and similar stuff don’t operate under the conditions described in the article.

I'll partially agree with that.

The same physics applies no matter what, but it's a matter of degree. However, the particle shape and size distribution of the potting medium plays a huge role in the degree of bridging water.

If the particle size is relatively large and approaching spherical, the percentage of bridging water is low. The smaller the particles and/or the more irregular in shape they are, and/or the greater the particle size distribution, the more of it will be retained.

I did an experiment with 1 cm Grodan rock wool cubes to test that.

[Dimples]

Ray - yes, there will still be some bridged water between particles, even when the average particle size in a mix is large. That aspect of any growing medium isn’t positively influenced by a drainage layer being present (or absent) in a pot, which is why drainage layers are counterproductive until the average particle size in a mix increases to the point where the entire soil column is decently aerated. Otherwise all that’s achieved by adding a drainage layer is a reduction of the available aerated soil/mix because the perched water table now sits higher in the pot.

Now, adding a ballast to reduce the volume of mix present at the bottom of the pot can help minimize the total volume of fully saturated mix at the bottom, but it still doesn’t directly influence the actual drainage of the mix.

[Subrosa]

I use rocks at the bottom of my Catasetum pots, but only for ballast so they don't blow over. Since they're sitting in trays of water, drainage isn't really an issue.

[Grim Tuesday]

Would I be correct in characterizing the results of this discussion as: in the case of large enough gravel, and relatively large, non-highly-wicking medium (e.g. bark) then then we are outside of a regime where gravel makes drainage worse?

[Dimples]

Originally Posted by Grim Tuesday

Would I be correct in characterizing the results of this discussion as: in the case of large enough gravel, and relatively large, non-highly-wicking medium (e.g. bark) then then we are outside of a regime where gravel makes drainage worse?

Yes