Regional Gardening Reports :: National Gardening Association

In the Garden:
Southwestern Deserts
February, 2006
Regional Report

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These unhealthy roots growing on the surface are a common drawback of using plastic mulch.

Beware Plastic Mulch!

I spent my weekend groveling with gravel. My friend recently purchased a home with a huge yard and wanted my expert advice. Okay, he wanted my free labor. Whatever.

Unfortunately, the previous owners had made one of the most common mistakes in desert landscaping. They spread black plastic on top of the soil and then layered decomposed granite mulch on top of it. I know this practice was common a decade or two ago when the problems with plastic weren't understood. It didn't stop me from muttering about their folly while I picked gravel out of my knees.

Plastic Misconceptions
Plastic was initially thought to be a good idea as a weed barrier. In reality, it doesn't keep the weeds out in the long term because seeds blow in, park themselves in the gravel and germinate when conditions are favorable. But that's a minor sin in comparison to what plastic does to landscape plants and soil over the long term. Plastic impedes water penetration deep into the soil where the roots need it. Instead, it traps moisture at the very top layer of soil. Plant roots stay up at the surface, trying to poke through the plastic to soak up a sip of water.

When roots don't grow deeply, they can't anchor the plant. Trees are prone to blow over in strong winds. Soil temperatures in summer heat up like an oven in the top inch or two of soil, and roots basically cook. Rainfall can't soak into the ground to replenish the moisture bank in the soil. Plants just can't grow with vigor in such conditions. Then their owners complain that "desert plants don't look good." You wouldn't look good either if you had to sit outside for months on end without a decent drink of water!

When I said the plastic had to go before we planted anything, my friend gave me one of those "you've got to be kidding me" looks. A yard full of gravel weighs a ton. Maybe two tons. I doubt that I convinced him, but sorry-looking plants in his yard probably did. So, we raked back gravel in a large oval around a dead plant, chopped through the plastic with a shovel, and pulled it out.

The plastic had inhibited air and water transfer, turning the top layer of soil into a hard crust with white salt deposits. Anemic-looking plant roots from the defunct plant crawled along the surface right where I said they'd be. Aha! My expert advice was needed after all!

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