ARS LANDSCAPE MATERIALS AND SUPPLY
If the main objective is soil improvement, consider an organic mulch that gradually breaks down. If the area is used primarily for annual flowers, it often is more practical to use temporary organic mulch, such as composted leaves or grass clippings, which can be turned under each fall. Inorganic mulches such as gravel or stone are a good choice where there are permanent plants in a landscape such as trees, shrubs or perennials.
Mulches can be used to prevent soil erosion by wind and rain. In windy areas, gravel or rock mulch may be preferred over lightweight organic mulches. Any mulch that reduces the impact of raindrops will help reduce water erosion, just make sure the soil surface is entirely covered with mulch. Black plastic (polyethylene) is not recommended as mulch in landscape areas.
Black plastic is impermeable therefore no oxygen exchange can occur in the soil. Lack of oxygen to the roots significantly reduces plant growth. Black plastic also prevents water penetration. If a weed barrier is needed, use landscape fabrics instead to allow water and oxygen exchange.
When to Apply Mulches
Mulches used to enhance appearance and control weeds may be applied at any time. If the mulch will be used to protect fall transplants by keeping soil temperatures above freezing longer into the fall (permitting better root growth), apply soon after transplanting. Reapply or refresh organic mulches after they have begun to break down.
If the mulch is meant to reduce frost heave and delay spring growth, apply after the ground has frozen. This type of mulch often is used to protect small bulbs such as scilla and crocus and to prevent early emergence.
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