Organic mulch materials include grain straw, fresh or old hay, fresh-cut forage or cover crops, chipped brush, wood shavings, tree leaves, cotton gin waste, rice or buckwheat hulls, and other crop residues. Hay and straw are among the most widely used organic mulches in organic horticulture. Cover crops can be grown to maturity (flowering), mechanically killed, and left on the soil surface to provide an in-situ organic mulch for no-till planting. Leaf mold (decomposed tree leaves), compost, and aged manure have also been used as organic mulches, although their crumbly texture may not provide as effective a barrier to weed seedlings as other materials.
A few growers use compost as mulch, although the quantities required for effective weed suppression may not be economically feasible. In a study in Virginia, 1½–2 inches of leaf mold compost (50–90 tons per acre) did not suppress weeds quite as well as 4 inches (
Figure 2. This hay was cut too late in its development, and carried mature seeds. As a result, forage grasses (fine–textured seedlings) are growing in this cucumber bed. Additional weeds have emerged from the soil's weed seed bank because the mulch application was not sufficiently heavy to cover the soil surface completely. Photo credit: Mark Schonbeck, Virginia Association for Biological Farming.
Figure 9. Mulching with compost (a) A municipal compost, based primarily on tree leaves, was applied at 50 tons per acre in this trial. (b) By midsummer, considerably more weeds emerged through the compost than through a 4–inch (
Some grass hay is produced with the use of weed control products that contain highly persistent active ingredients, including clopyralid, aminopyralid, picloram, and aminocyclopyrachlor, all of which are highly toxic to broadleaf plants. Hay from fields treated with any of these materials can cause severe damage to tomato family, cucurbit family, and other vegetable crops around which the hay is applied as mulch (Plaksin and Bynum, 2007). Symptoms include curling and twisting of leaves and petioles (leaf stalks), and stunted growth, which can lead to crop failure or plant death. Subsequent vegetable or broadleaf cover crop plantings may continue to show symptoms for a year or more after initial contamination, and the field may lose eligibility for organic certification until herbicide residues have disappeared.
Weeds can’t survive without moisture. In areas with little or no summer rain, drip irrigation or soaker hoses help prevent weed seeds from sprouting by depriving them of water. These systems deliver water to the root zone of plants at the soil level. The soil surface and area surrounding the plants stays relatively dry. In contrast, overhead sprinkler systems spray water over the entire soil surface and supply both garden plants and weeds with water.
Check the label to determine if it is safe for use around the kinds of landscape plants you have and effective against the weeds normally present.
Think it’s an overstatement to call it the war against weeds? Here’s what you’re up against.
How to Mulch Over Weeds
True Temper Hardware
Camp Hill, PA 17011
You can also use landscape fabrics to control weeds under decks and in pathways (spread over the excavated soil base before you add gravel or sand). A 3×50-ft. roll of landscape fabric, such as the Typar shown below, costs about $10. The fabric is also available in 36-in. die-cut circles (about $3 each) for installing at the base of trees.
2250 Agate Ct.
Simi Valley, CA 93065
Request the free “Drip Watering Made Easy” guide.
As with most types of prevention, discouraging weed seeds from sprouting requires some extra time now so you can save a lot of time later.