This has been a hot topic for a long time, but there have never been any formal experiments on the effects of drainage layers. I just had a piece of research published which found that drainage layers usually reduce water retention, and almost never make it worse. In short: they do work to improve drainage.

I hope it's okay for me to share the link to the research study (I don't make money from this, I just want people to have good information!)... https://doi.org/10.1371/journa...

Some of us are a little leery of opening unfamiliar links and the end of yours is cut off so I can't see what the publication (?) is, but I'd like to know more. Could explain what you mean by drainage layers? In the ground, in a pot, composed of...?

Google 'Washington State University soil drainage videos' and watch some from 1959. Formal experiments in a University lab. I think the question is answered.

I clicked the link. It's a little different subject than the Washington studies about movement of water in soil; this is specifically about how (much) water is retained in potting media in pots with and without drainage material (gravel, sand, etc) in the bottom of the pot.

Since many of us here at NGA have embraced the current thinking that drainage material in the bottoms of plant pots is either unnecessary or even counterproductive, this study, if accurate, might suggest some reconsideration. However, as I read it, this study is specific to overall water retention, not necessarily the level of the perched water, and there are a lot of variables, but it's late and my brain doesn't want to wrap around this right now. For now, I'll just continue using my gritty potting mixes that drain freely on their own so I don't need to worry about excess retained water in my potted plants.

In any event, congratulations Avery, for doing this study and getting your work published.

NMoasis said: Some of us are a little leery of opening unfamiliar links and the end of yours is cut off so I can't see what the publication (?) is, but I'd like to know more. Could explain what you mean by drainage layers? In the ground, in a pot, composed of...?

The link is a DOI which goes directly the scientific publication in the journal Plos One. You can also search for the title of the study to find it directly, "Effect of drainage layers on water retention of potting media in containers."

Drainage layers are shallow layers of gravel (or similar coarse substrate) added below the soil in plant pots. This approach is often recommended by gardeners as a way to improve drainage or reduce water retention. But in recent years some experts have argued, based on principles of water movement, that these layers will actually make drainage worse by retaining more water.

CalPolygardener said: Google 'Washington State University soil drainage videos' and watch some from 1959. Formal experiments in a University lab. I think the question is answered.

The study I posted refers to those videos, and also to the decades of more thorough and advanced research results since then! The question is not as "answered" as you think - water draining out of a plant pot with a layer of gravel at the bottom is a very different physical situation to the narrow containers of field soil being gradually saturated in those videos.

NMoasis said: I clicked the link. It's a little different subject than the Washington studies about movement of water in soil; this is specifically about how (much) water is retained in potting media in pots with and without drainage material (gravel, sand, etc) in the bottom of the pot.

Since many of us here at NGA have embraced the current thinking that drainage material in the bottoms of plant pots is either unnecessary or even counterproductive, this study, if accurate, might suggest some reconsideration. However, as I read it, this study is specific to overall water retention, not necessarily the level of the perched water, and there are a lot of variables, but it's late and my brain doesn't want to wrap around this right now. For now, I'll just continue using my gritty potting mixes that drain freely on their own so I don't need to worry about excess retained water in my potted plants.

In any event, congratulations Avery, for doing this study and getting your work published.

Thank you! It's true, the research only measured the amount of water retained, and didn't quantify *where* that was was distributed in the container. We can reasonably predict that less water retention = a lower perched water table, but it's not proven. I hope that the results of my research at least encourage further study into how water moves through plant pots. Even if - like you - I wouldn't particularly bother recommending drainage layers when a good potting mix on its own does the job just fine most of the time!