When used correctly before weeds sprout in early spring, pre-emergent herbicides are one effective way to prevent weeds coming up in mulch. They won’t, however, do anything for weeds that have already sprouted.
Weed control is one of the primary reasons for applying mulch, yet pesky weeds may persist, even through a carefully applied layer of bark chips or pine needles. This happens when weed seeds are buried in the soil or are distributed by birds or wind. What should you do if you’ve got weeds coming up in mulch in spite of your best intentions? Keep reading for a few helpful tips.
Manual Mulch Weed Control
Mulch acts as physical barrier against weeds, but it must block sunlight in order to be effective. If you notice weeds coming up in mulch, you may need to thicken the layer as blocking light generally requires at least 2 to 3 inches (5-7.6 cm.). Replenish mulch as it decomposes or blows away.
If you haven’t applied mulch yet, landscape fabric or weed barrier cloth is a safe way to block weeds while still allowing water to pass through to the soil. Unfortunately, landscape fabric isn’t a perfect solution because some determined weeds will push through the fabric, and those weeds will be extremely difficult to pull.
A Note about Glyphosate: You can use glyphosate to stop weeds in mulch, but this approach requires extreme care because glyphosate, a broad-spectrum herbicide, will kill any broad-leaved plant it touches, including your favorite perennials or shrubs. Apply glyphosate directly to weeds, using a paintbrush. Be extremely careful not to touch nearby plants. You can also protect plants by covering them with a cardboard box while you’re applying the herbicide. Don’t remove the box until the treated weeds have time to dry completely.
Figure 4. (a) Potato tuber yields are often enhanced by the cooler soil conditions under a straw mulch. (b) The straw was applied after the soil had warmed to optimal temperatures for eggplant, and is now helping the crop thrive during intense summer heat. A few weeds have emerged at this point, but are unlikely to affect yield in the vigorous, established eggplant crop. Photo credit: Mark Schonbeck, Virginia Association for Biological Farming.
These forest product mulches are most often used on perennial crops such as berries (Fig. 7) and ornamental perennials, many of which like a somewhat acidic soil rich in mycorrhizae and other beneficial fungi supported by these materials. They tend to be coarser and higher density than hay or straw, require higher tonnage per acre to suppress weeds, and may not be economical for most larger-scale applications. Other characteristics include:
Figure 8.(a) An intense rainstorm has washed a fine sawdust mulch away from newly planted blueberries. (b) The same storm damaged soil structure in un-mulched beds (right and background), whereas chipped brush held firm, protecting both soil and crop (left foreground). Several years after these photos were taken, the blueberry bushes mulched with sawdust remained visibly smaller than those in other mulches, as a result of N immobilization by the fine sawdust. Photo credit: Mark Schonbeck, Virginia Association for Biological Farming.
Sawdust is chemically similar to other wood products, but because it is so finely divided, it has the following disadvantages as a mulching material:
Organic mulches suppress weeds in several ways. First, they block seed germination stimuli by intercepting light, reducing soil temperature, and greatly dampening day–night temperature fluctuations. As a result, fewer weed seeds germinate under the mulch than in uncovered soil. Second, the mulch physically hinders emergence of those weeds that do germinate. If the mulch is thick enough to prevent light from reaching the trapped seedlings, they eventually die. Third, some mulch materials, such as grain straw and fresh-cut forages like sorghum-sudangrass, release natural substances that inhibit weed seedling growth for several weeks after application, a process known as allelopathy. Finally, organic mulch can enhance crop growth and competitiveness against weeds by conserving soil moisture and moderating soil temperature.
Hay from off-farm sources is a notorious source for new and serious weed species on a farm. Even in fields with good weed management, hay that has been cut too late in its development will carry seeds of the forage species themselves, which can be a nuisance if they come up in a vegetable crop. In addition to weed seed, the grower must be alert to the possibility of herbicide residues.
Straw, defined here as the stalks and other residues left after harvest of a mature grain, is similar to hay in texture, potential for soil protection and moisture conservation, weed suppression, and application methods. Straw differs from hay in that it: