Mulch is an essential component of Xeriscaping in a semi-arid climate like ours.
Mulching minimizes evaporation from the soil surface. This is particularly important in a new garden where there is a lot of space between small plants.
Mulch helps keep soil temperatures low. This protects plant roots from getting overheated, and it reduces transpiration. Transpiration is “perspiration for plants,” the water plants lose through their leaves to heat.
Mulch also reduces weed growth, helps control erosion and adds a finished look to the garden.
Mulch can be inorganic or organic, mineral or vegetable. Organic mulch is vegetable material. It is typically wood-based: cedar chips, deco bark, chipper chips and pole peelings. Organic mulch decomposes slowly. This improves soil texture, but it means replacing the mulch periodically.
Fresh organic mulch has a reputation for removing nitrogen from the soil as it decomposes. This happens, but the nitrogen loss is generally insignificant. If you are concerned about nitrogen loss, add a high-nitrogen fertilizer when you apply the mulch.
Inorganic mulch is mineral rather than vegetable. It is usually stone: river rock, pea gravel, cobblestone or lava rock, for example. Inorganic mulch is virtually permanent. It retains and radiates heat, so it’s not the best choice for the sunny side of a house. It works best in formal settings where the rock will remain visible after the plants mature and is the appropriate mulch for alpine plants or plants from arid locations. Organic mulch holds too much water for these drier plants.
Mulch should be applied directly on top of the soil in a 3- to 4-
inch layer for a tree or shrub bed. Herbaceous perennials or annuals need only 1-2 inches. In either case, you can apply inorganic mulch over a weed-barrier fabric. Nonporous plastic will suppress any plant growth below the mulch. Porous plastics should be used under the mulch if you want plants and soil organisms to live underneath it. Don’t use nonporous plastic barriers under organic mulch, or you will prevent seepage of nutrients into the soil.
The city and county of Denver recycles Christmas trees every year, turning them into mulch. This mulch is free for the taking each spring. For information, visit denvergov.org/recycle.
David Winger is a water-conservation specialist with Denver Water.
