In the Garden:
New England
December, 2011
Regional Report

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Container-grown plants do best if their pots have drainage holes and are filled with a uniform soil mix.

Send This Garden Myth down the Drain

Who knows how garden myths get started? Sometimes a piece of advice that seems on the surface like it ought to make sense gets passed from gardener to gardener over the years. It becomes gospel without ever having been scientifically tested to see if it is, indeed, true.

For many years gardeners were instructed to paint over the wood of freshly made pruning cuts to seal them and prevent rot. And generations of gardeners did just that. It seemed to make sense, like the idea of covering a wound with a bandage. Not until researcher Alex Shigo studied the effects of wound dressings scientifically in the 1970s did we learn that not only do the dressings not help, they actually promote decay in some cases.

A similar case can be made for the recommendation to put a layer of gravel at the bottom of a plant pot or container "to improve the drainage." Sometimes this method is suggested as a way to compensate for pots with no drainage hole. Again, this seems like it makes sense, the gravel providing a layer in which excess water can collect.

What it doesn't take into account is physics, which dictates that water does not move easily across the boundary between finer textured material--in this case, potting mix or soil-- and coarser-textured material like gravel. It isn't until the finer-textured potting mix becomes completely saturated that water begins to move across the interface into the coarser material. Rather than improving drainage by adding a layer of gravel, you are actually making it worse! So make sure all your growing containers have drainage holes and then fill them with a uniform soil mix for the best plant growth. A small square of screening laid over the drainage hole at the bottom of the pot will keep potting mix from leaking out and won't interfere with drainage.

What to do if you want to use a decorative container without a drainage hole? I have some lovely, large cachepots that I use for plants-- just not for actual planting. My plants are growing in pots with drainage holes that are slightly smaller than my decorative pots. I set some supports on the inside bottom of the outer pot-- I find that three or four, overturned, 3-inch plastic pots work well, then set the plant in its growing pot so it rests on the supports. This way, when I water, any excess water drains off into the decorative pot, with the planted pot well above so it's never standing in water. I leave the drained water to evaporate and increase humidity around the plant growing above.

Now if I could just figure out how to dispel the myth about the poodle in the microwave!


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