Potting Mix and Drainage

J.E. Shields jshields104@insightbb.com
Wed, 21 Feb 2007 06:14:26 PST
In the Daylily list, Susan Bergeron of Canada reported on an experiment she 
did testing usefulness of coarse drainage materials in a pot. These are her 
words below, slightly rearranged to correct a typo she tells me she made:

Last week we discussed the suggestion that drainage material in the bottom 
of plant pots doesn't help, and may actually impede, drainage of the medium 
above it. The issue was raised by Dr. Gillman at the MWS [Midwinter Daylily 
Symposium], and this is also what I was taught when I took soil science, so 
I said that I believed him to be correct.

I tried to do an experiment to confirm it for anyone still skeptical, which 
actually didn't prove it! I said I'd try again when I had the materials to 
do it properly, and I also realized the way I'd done it the first time was 
somewhat flawed. For anyone interested in the methodology of attempt 2, 
keep reading. For anyone interested only in the result, skip to the end of 
this email.

I took some clear, plant pot shaped, containers which also had measurements 
on them at 1, 2 and 3 cups. I made drainage holes in the bottom. I filled 
Container 1 to the top with Miracle Gro potting mix only. I firmed the mix 
until the level was the same in both containers, i.e. to the 3 cup 
level.   I then filled Container 2 to the one cup level with non-absorbent 
polystyrene packing material as drainage, then topped it up to the top with 
Miracle Gro potting mix. (This is what I realized I should have done in the 
first attempt because the media would be firmed around a plant being 
planted, and would also have compacted around a plant that had been in the 
pot for some time).

So, we now have two containers, one full of potting mix to the 3 cup level, 
and one with one cup of drainage material and two cups of potting mix. I 
placed each container over a plastic container to catch the drainage water 
individually. Then I placed a coffee filter over each container so that the 
water was distributed evenly and didn't disturb the medium (another 
omission I made the first time). I then poured 440 ml of water into each 

When they had finished draining in an hour, I measured the amount of water 
that had drained out of each container. 85 ml had drained from Container 1 
(three cups of potting mix), and 160 ml had drained from Container 2 (two 
cups of potting mix over one cup of coarse drainage material). I deducted 
these amounts from the original 440 ml and was left with:

Container 1: 440 - 85 = 355 ml still in the container
Container 2: 440 - 160 = 280 ml still in the container

Since there were three cups of potting mix in Container 1, 355ml divided by 
3 equals 118ml retained water per cup. Container 2 (the one with drainage) 
contained two cups of potting mix, therefore 280ml divided by 2 equals 
140ml retained water per cup.

Therefore the container with the coarse drainage material at the bottom 
retained more water per unit volume of potting mix than did the container 
without coarse material at the bottom. Thus, according to this experiment, 
Dr. Gillman (and my soil science teacher!) would appear to be correct that 
drainage material in the bottom of a plant pot does not help drainage and 
can actually increase the amount of water held by each cup of potting mix.

Just to make sure, I tried this again slightly differently, with one 
container having two cups of potting mix, and one having two cups of 
potting mix over a cup of drainage material, i.e. exactly the same amount 
of potting mix per container. I didn't wait an hour this time but measured 
the drained water as soon as it had more or less slowed down to an 
occasional drip. The result was the same. The potting mix alone retained 
105 ml of water per cup and the container with drainage material retained 
135 ml of water per cup of potting mix.

Therefore, in these two experiments, coarse "drainage" material in the 
bottom of the pot resulted in the potting mix above it staying wetter per 
cup of medium than in a container of potting mix alone.

Now to go clean up the kitchen...............!

Sue Bergeron
Ontario, Canada

You really have to read the above carefully. Susan did a careful 
experiment, and demonstrated to us what many experts have told us: Adding 
rocks, pottery chips, or polystyrene foam "peanuts" will not improve the 
drainage around the roots of a potted plant.

Just thought folks might find this interesting.

Jim Shields
in chilly, foggy central Indiana (USA), where there is still snow on the 
ground after two days of mild weather

P.S. Reproduced with Susan's permission

PPS.  440 ml is probably just a little less than 2 cups. A liquid ounce is 
around 30 ml.

Jim Shields             USDA Zone 5             Shields Gardens, Ltd.
P.O. Box 92             WWW:   http://www.shieldsgardens.com/
Westfield, Indiana 46074, USA
Tel. ++1-317-867-3344    or     toll-free 1-866-449-3344 in USA

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