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Place drainage holes in pots--not rocks!

Posted 3/2/2016

photo of pots

For a long time, gardening experts advised people to place rocks or broken pottery in the bottom of their planting pots. The idea was that doing so would improve the drainage and therefore, the health of the plants in them.

Now, we are trying to change that message, since placing stuff in the pot can cause much more harm than any good it might have done.

The idea behind that practice was that the rocks sitting inside the pot would slow down the loss of soil and water through the drainage holes. While that can work, after a fashion, an even better idea is placing a moist paper towel over the drainage holes. That works more efficiently, allowing water to drain away. Soil will not become compacted in a pot, as long as there is a way for excess water to escape.

When a pot is full of soil, plant and water, it can become heavy, particularly if it happens to be made from glazed clay. The addition of rocks makes it even more so. Not only that, but adding rocks decreases the amount of room for roots to grow. One inch of rock in an 8” x 8” deep pot gives the plant about 12 percent less growing room!

The increased weight and decreased space are not the only problems when there are rocks in the pot. They can actually block drainage holes completely, defeating their purpose. The bottom of the pot will accumulate mud, something very few potted plants can tolerate.

Another idea we should eliminate is that rocks at the bottom could replace drainage holes entirely, as if somehow they would add enough space at the bottom for the roots to grow. People will often try this when they have a decorative container that would look wonderful with a plant growing inside. Sadly, this is the opposite of what happens.

When the plant is watered, the potting soil (never soil from the garden outside) will flow down to the bottom and stay there. That mixture of water and soil will be airless and muddy. The plant within will survive, even thrive for a while, but eventually the airless environment will ultimately cause it to weaken. Oddly, it will probably begin to look as if it were dry, since roots that cannot obtain air will actually stop working. They cannot perform their essential functions. Although the plant is drowning, it appears to be the reverse.

Beautiful containers without holes should definitely hold plants, but they should hold a plant that is inside of another, plain, pot. Here is where rocks can be useful. The decorative pot can have a layer of rocks, and the plain pot can sit on them. The paper towel at the bottom of the inner pot stops water from pushing soil out, but water will drain into the rock. As that water evaporates, it forms an area of increased humidity, which is essential for many of the potted plants we want to have thrive around our homes.

Email or call Angela O’Callaghan, Social Horticulture Specialist at 702-257-5581.

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