Mulch is any material, organic or not, placed over the surface of soil to conserve moisture, kill weed seedlings, modify soil temperatures, or make the garden look more attractive — or do all four at once. Mulch was traditionally thought to mean natural, organic materials, such as leaves, wood chips, and sand. Now, a multitude of plastic-based films or woven materials are available.

A common goal of mulching is to reduce weeding, so use inorganic mulches or seed-free organic mulches, such as the following:

• Grass clippings: Make sure that the clippings come from a weed- and pesticide-free lawn.

• Leaves: Make sure that they’re shredded or composted.

• Newspaper: It can be either shredded or flat.

• Pine needles: Use for acid-loving crops.

• Salt hay: This generally weed-free plant is from oceanside meadows.

• Shredded bark: Use around trees or other prominent areas because this type of mulch is the most attractive.

• Wood chips: Use fresh chips from a local arborist, or better, composted wood chips from a soil or amendment supplier.

Inorganic mulch, such as gravel, landscape fabric, sand, and stone, holds in moisture and stops weeds but doesn’t add fertility to the soil. Use this mulch around perennials, shrubs, or trees that are naturally adjusted to your soil and don’t require additional fertilizer.

Double your gardening pleasure by using fertile mulch, which controls weeds and provides small amounts of nutrients. All organic mulches made of plant material fit this group. Some organic mulches quickly rot (decompose) and dissolve nutrients into the soil; these are green, fresh, and not too woody. The mulches that quickly decompose are useful in annual flower and vegetable beds. When sprinkled with water, or in rainy-summer areas, organic mulch that decomposes fairly quickly also leaches some nutrients while sitting on top of the soil.

Other organic mulches are slow to decompose and release few nutrients; these are usually dry, woody, and very low in nitrogen. Bark mulches are slowest to decompose because bark is naturally rot-resistant. Use these for pathways, or around trees and shrubs. Chips of tree sapwood may be fresh or composted. The latter is preferable, if available. Fresh wood chips can make an excellent mulch, but you should apply a little extra nitrogen fertilizer over the mulch so that it doesn’t take all the nitrogen at your plants’ expense.

Fertile mulches that quickly decompose but have weed seeds include cow, rabbit, goat, sheep, and horse manure; hay; some poultry bedding; sewage sludge; and straw. Fertile mulches that also quickly decompose but have no weed seeds include clean grass clippings, leaves, and salt hay. Newspaper, shredded bark, and wood chips add little fertility to the soil and decompose slowly because they are high in carbon; they have no weed seeds.

All the mulches we mention are just part of a nearly infinite selection of local specialties. Rice hulls, cocoa shells, sugar cane refuse, ground corncobs, peanut shells, and grape pomace are a few you may encounter, depending where you live.

Woven plastic materials (called landscape fabric) act as a seedling barrier as well, but these effective materials are not attractive enough for some situations. Also, sunlight deteriorates these mulches, so covering them with a weed-free organic mulch to block the sun’s ultraviolet rays is a good idea.  ....read more