by National Gardening Association Editors
What Mulches Do
If you use a woody material, such as bark, fresh sawdust or wood chips as a mulch, it's a good idea to add some extra nitrogen fertilizer to the soil. Otherwise the soil microorganisms, which need nitrogen for their own growth, will temporarily tie up the nitrogen your plants need until the mulch is broken down. Adding extra fertilizer supplies enough nitrogen for both plants and microorganisms.
In the South and Southwest, gardeners often use thick mulches of hay, straw or pine needles to protect tomato roots from too much heat during the hot summer months, as well as to help retain moisture in the soil. Some gardeners also pile up soil five or six inches around the stems of their plants to insulate the roots more and promote some extra growth. This technique is called "hilling."
Using black plastic as mulch is good in the South and Southwest only early in the season when the plants appreciate a little extra heat. If used in summer months, however, the roots would literally cook. If you use black plastic, you can cover it with a heavy organic mulch that will keep soil temperature down when the weather heats up.
Some gardeners in hot areas use a commercially available mulch with an aluminum foil surface that reflects the sun and keeps the soil cool. Of course, it also keeps weeds down and conserves soil moisture.
If you're going to use black plastic, put it down at planting time when the soil is moist. (The three-foot-wide plastic is probably best.) Secure it firmly around all the edges. The better contact the plastic makes with the soil, the more heat and moisture you trap. Then, to put in a transplant, simply punch or cut a hole or a slit in the plastic. Additional small holes here and there to let rainwater soak through are a good idea, too. IRT-plastic lets in more of the sun's warming rays, but still prevents weeds from germinating.
Staked and trellised plants usually benefit from a mulch to save moisture. More exposed to sun and wind than unstaked plants, they lose more water through their leaves. It takes extra effort to provide them with an ample and even supply of moisture, but in dry climates, it's worth it.
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