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May 18, 2004

DESERT BLOOM: Too Much Water


A friend and I were talking about landscapes a little while ago, and I mentioned that plants do really poorly when they get overwatered. This friend is a biologist, but not, obviously, a plant biologist, because he'd always figured plants were somehow like gold fish, who'll continue to eat until they die. He thought plants'd take up water as long as it was provided.

But anyone who's grown a houseplant has heard at some point - "Plants don't like wet feet." While there might be purists who cringe at the statement, you cannot dispute its fundamental truth. The wet feet are roots that're submerged in too much water. A potted philodendron sitting in water will soon look mighty unhappy. This doesn't just happen to houseplants, either.

If you live in Southern Nevada, how often have you seen rainfall or irrigation water sitting in turgid pools on the surface of some yards because the soil's drainage is so poor? You might've already learned that landscape plants here often die from something akin to drowning. This can also happen when an unsuspecting gardener sees a wilting shrub and decides to irrigate, and irrigate, and irrigate. Until the plant is, finally, done for.

It's a fact that, when the roots of most plants sit for too long in water, they stop being able to use the water at all. Having too much water is almost the same as not enough. Some researchers think they've been able to determine what the biochemistry of that process is.

Not what most of us would think of as a great survival mechanism? It's not.

Plants that survive flooding have come up with ways to compensate, that's all. It makes me wonder, though, about true desert plants. They go through intense dry periods, then get all their water in a deluge. Maybe desert roots have something unique. We know that some elements of their metabolism are completely different from garden-variety, crop plants. Not a huge amount of research has been done on Mojave roots, as yet.

Roots are so funny. We don't see them, and usually we shouldn't see them since they're mainly designed to be covered up with soil. As a result, we scarcely think about them. We do pay attention when the roots of a tree are accused of heaving up a sidewalk, but in general, they're just down there, in the background, doing whatever it is they do.

Of course what they do is to bring all the necessary raw materials from the soil to the rest of the plant so growth can happen. Water, minerals, fertilizer, even carbon dioxide, everything gets carried into the plant through the roots. They literally anchor the plants to the earth, too, and when you think about the strength of winds that can blow 60 miles an hour, you realize that you want a healthy, robust root system attached to all the plants in your landscape.

So how do you do that? You've already gathered - you don't want them drowning in a muddy soup. That's clear, because roots need air. Some, but not too much. When they're flooded, they gasp for air. But when the soil gets too dry, then there's only air, no water, and of course the plant's going to desiccate, and die. One curious fact that becomes obvious when you think about it is that roots won't grow through dry soil. You want to make sure the soil is fertile enough to provide all the nutrients they need. Fertilizer, compost, companion plants, living mulches - any one or all of them can help. In some ways, the very things you think of for a healthy plant in general will help to build a healthy root system.

There's a final aspect of roots that I wanted to mention. One of the master gardeners brought in a plant to demonstrate how bad things can be - he called it a "15 gallon plant in a one gallon pot". And it was. The roots of this pathetic plant were so bound up that they were almost in a single giant knot. Not very efficient at pulling up water and nutrients from the soil. Roots are designed to spread out. They can only take being pot bound but so much.

So keep in mind that when you're putting money down on a plant, that there's more than is meeting the eye. You absolutely want a healthy-looking plant, with the right size, shape and color of leaves, no signs of insects or disease symptoms, but remember -there's another whole world inside the pot, because that's where the roots are.

For KNPR's Desert Bloom, this is Dr. Angela O'Callaghan of the University of Nevada Cooperative Extension.

See discussion rules.


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