Homeowners and professional arborists depend on mulch to discourage weeds from growing, conserve moisture during drought periods, and allow better use of water by controlling runoff and increasing water-holding capacity of light, sandy soils. Mulches help maintain a uniform soil temperature. A 3 - 4 inch layer of mulch can add to the aesthetic value of a garden while protecting the base of trees from being injured by equipment like lawn mowers. Mulch rings also decrease competition from lawn grasses that rob trees of valuable nutrients and moisture.

Many organic materials can be used as a mulch, like bark mulches, wood chips, and pine needles. Mulch can be applied almost any time of year when trees and shrubs are being planted. The best time to apply mulch in established bed areas is mid-spring when the soil temperature has warmed up enough for sufficient root growth. If applied earlier, the mulch will keep the soil temperature lower and root growth could be delayed. 

Mulches should be applied 2 - 4 inches in depth over relatively clean, weed-free soils. Identify and remove weeds before the mulch is applied and keep mulch pulled 12 inches back from the tree trunk. Most arborists consider organic mulches as the most compatible with trees, but there are several inorganic materials used as mulches, including weed barriers. 

Black plastic is sometimes used to discourage weeds, however, it interferes with the normal oxygen and water supply to the tree’s roots. When the plastic is used, a very shallow root system is created and during drought periods the plants may not withstand stress, so it's recommended to not use black plastic around trees. 

There are several landscape fabric mulches available that will function the same as plastic but allow for normal water and oxygen exchange. Sometimes called geotextiles or weed barriers, they are placed on bare soil around trees and shrubs with mulches used on top. There are many brands and types of materials from which to choose. They have proven to be helpful in discouraging weeds and conserving moisture.