Mulch, organic or inorganic materials laid down around plants in a garden bed, gives a more finished look to the landscape, prevents erosion, retains moisture in the soil and helps control the spread and growth of weeds. Gardeners often wonder if they must remove weeds before applying mulch. Will mulch suffocate weeds, preventing the need to weed?
How Mulch Prevents Weeds
Mulch of any type works to prevent the growth of weeds by preventing light from reaching weed seeds and roots, says the Washington State University Extension. Whether the mulch is manmade black plastic or natural wood chips, mulch covers garden soil, shading anything beneath it. In areas where gardeners want desirable plants to reseed, mulch may prevent those plants from doing so. Mulch controls all seeds, not just weed seeds.
What Mulch Can’t Do
Mulch of any type cannot prevent the growth of all weeds. Even seemingly impenetrable black plastic allows some light to reach the soil, and most weeds easily break through the plastic once they begin growing. Additionally, mulch cannot kill weeds that are already growing. Even covering the weeds with mulch will not smother them out.
How to Prepare a Garden
Because mulch can't kill weeds, it's important to remove weeds before applying mulch. Either hand-pull weeds after a rainfall or irrigation, dig up weeds with a spade or other tool or spray the weeds with herbicide. If the latter method is chosen, follow the manufacturer's directions carefully. In addition, mulch shouldn't be applied until the weeds are completely dead.
Once weeds are removed and mulch is applied, weed growth is reduced by as much as 90 percent, according to the California Integrated Waste Management Board. Mulching also makes weeding easier. The trick, however, is applying mulch to the correct depth. The University of Florida IFAS Extension recommends bulky mulches like bark chips be no more than 3 inches deep, whereas coarse pine needles can be up to 4 inches deep. Fine mulches like shredded leaves or grass clippings should never be more than 2 inches deep.