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The Myth of Drainage Material in Container Plantings:

Regarding instructions to put gravel at the bottom of plant containers to help with drainage, it has been demonstrated that this doesn't do what you would think it does. See the quote below from an article published by Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor, Puyallup Research and Extension Center, Washington State University. The name of the article is "The Myth of Drainage Material in Container Plantings:
'Add a layer of gravel or other coarse material in the bottom of containers to improve drainage' " and found at http://http://puyallup.wsu.edu/~Linda%20Chalker-Scott/Horticultural%20Myths_files/Myths/Container%20drainage.pdf

 

"Nearly 100 years ago, soil scientists demonstrated that water does not move easily from layers of finer 

textured materials to layers of more coarse textured. Since then, similar studies have produced the same
results. Additionally, one study found that more moisture was retained in the soil underlain by gravel
than that underlain by sand. Therefore, the coarser the underlying material, the more difficult it is for
water to move across the interface. Imagine what happens in a container lined with pot shards!"

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Posted 2014-01-15T23:40:51+0000  by Matilda47 Matilda47
 

Hey Matilda47. 

 

I have always said that putting rocks in a pot is a waste, whenever roots could reach it, as a quality potting soil would be better for the plant to retain nutrients. In larger, deeper pots, with larger drainage holes, rocks can benefit by helping channel the water out without washing the potting soil out just so long as roots could not reach the rocks.

 

Sometimes people add rocks, simply to keep from having to spend too much on expensive potting soil. This is not the right reason for adding rocks. Smaller pots should never have rocks added as potting soil is better for roots.

 

And your link is not working, as I would like to read the article.

 

 

Posted 2014-01-16T17:02:51+0000  by Ingar_HD_ATL

I'm not sure why it didn't link, but I've pulled up the article and copied the URL: http://puyallup.wsu.edu/~linda%20chalker-scott/Horticultural%20Myths_files/Myths/Container%20drainage.pdf

 

Just in case that doesn't work, I've copied and pasted the article here:

 

Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor,
Puyallup Research and Extension Center, Washington State University

The Myth of Drainage Material in Container Plantings:
"Add a layer of gravel or other coarse material in the bottom of containers to improve drainage"

The Myth
This is just one of those myths that refuses to die, regardless of solid scientific evidence to the contrary!
Nearly every book or web site on container gardening recommends placing coarse material at the bottom
of containers for drainage. The materials most often recommended for this practice are sand, gravel,
pebbles, and pot shards. Other ‘benefits’ often mentioned include preventing creatures from entering
through the drain holes, and stabilizing the container.
Some of these recommendations are quite specific and scientific sounding. Consider this advice from a
1960’s book on container plants: “Adequate drainage is secured by covering the hole in the bottom of the
pot with a piece of broken flowerpot, concave side down; this in turn is covered with a layer (1/2" to 1"
deep) of flowerpot chips. On top of this, a 1/4" to 3/8" layer of coarse organic material, such as flaky leaf
mold, is placed.” The advice seems to make perfect sense, and it’s presented so precisely. After all, we
know that plants need good drainage so their roots receive adequate oxygen, and we also know that water
passes through coarsely textured material faster than it does fine material. So what’s not to like?
The Reality
Nearly 100 years ago, soil scientists demonstrated that water does not move easily from layers of finer
textured materials to layers of more coarse textured. Since then, similar studies have produced the same
results. Additionally, one study found that more moisture was retained in the soil underlain by gravel
than that underlain by sand. Therefore, the coarser the underlying material, the more difficult it is for
water to move across the interface. Imagine what happens in a container lined with pot shards!
Some of my previous columns have mentioned soil interfaces and their inhibition of water movement.
We can see the same phenomenon occurring here: gravitational water will not move from a finely soil
texture into a coarser material until the finer soil is saturated. Since the stated goal for using coarse
material in the bottoms of containers is to "keep soil from getting water logged,” it is ironic that adding
this material will induce the very state it is intended to prevent.

The Bottom Line:
• Planting containers must have drainage holes for root aeration.
• “Drainage material” added to containers will only hinder water movement.
• Use good topsoil throughout in perennial container plantings for optimal water conditions and
soil structure.

For more information, please visit Dr. Chalker-Scott’s web page at http://www.theinformedgardener.com

 

 

Posted 2014-01-18T23:03:25+0000  by Matilda47

 

Hi Matilda,

 

I’m Jen, from The Home Depot.  Thank you for posting that information.

 

When I went to school, they drilled the “myth” into our heads.  They wanted us to educate the public, NOT to use stones etc. in the bottom of the pot for drainage.

 

I must say, it is still difficult to convince people not to use rocks for drainage.  I think that it is something that has been known for a long time and used in people’s homes when planting.  It is sometimes difficult to encourage people to make the “change”.

Posted 2014-01-21T14:32:44+0000  by Jen_HD_BOS
 
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