Tuesday, December 9, 2008

Landscape - What Makes a Good Mulch

Mulching is a common landscape practice that can reduce weed competition, conserve moisture, improve soils over time, and improve the appearance of landscapes. What makes a good mulch? Here are some of my thoughts on the subject.
  • Organic mulches like bark are much more beneficial than inorganic mulches such as stone even though they require maintenance and additions on a regular basis. That is because they add organic matter to the soil over time and improve soil conditions.
  • Organic materials that are high in lignin versus cellulose are preferred because they break down more slowly and do not immobilize nitrogen as readily. High lignin materials include bark, pine needles, ground root mulches, straw, salt marsh hay, and some leaves
  • High cellulose materials will cause nitrogen deficiencies in plants by using N from the soil in decomposition. These materials need to be aged or partially composted before use as mulches. This includes wood chips, sawdust, some ground yard waste (depends on source material), and paper waste.
  • Mulches should contain large particles and limited amounts of fines. This allows for water and air penetration and for quick drying out. For mulches to work, they must be able to dry out quickly after rainfall or watering. Otherwise, they start acting like soils and will allow weed seeds to grow.
  • Mulches should not easily crust over. Again, the best way to avoid crusting is to limit fines in the mulch.
  • Mulches should not have dramatic effects on soil chemistry or contain toxic substances. Some plant materials have natural resins or chemical compounds that can harm plants around them. Others may reduce or increase soil pH quickly which can lead to plant performance problems.
  • How mulch is stored and handled also is a concern. Mulches stored in large piles that is not turned regularly to aerate it can develop anaerobic spots in the middle that start to ferment and produce acids. When this acid mulch is put around plants, it can damage or kill them.

Gordon Johnson, Extension Horticulture Agent, UD, Kent County.

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