Can I Just Cover the Weeds With Mulch to Kill Them?

Mulching is layering organic or inorganic material around plants or over grass and weeds. It is both decorative and beneficial to plants and soil. While a mulch layer can be placed directly over unwanted grass or weeds, it is not the most effective method of killing existing weeds or controlling new ones. There are several additional techniques in addition to laying the mulch.

To avoid weeds emerging through the mulch, a pre-mulching technique is recommended. This is as simple as applying an herbicide to the area that will be mulched. For complete eradication of weeds, solarization is a better choice. This process entails laying clear plastic over the area to be mulched, securing it with stakes or something heavy on the corners, completely limiting airflow. The plastic layer then traps solar radiation and raises the temperature underneath, killing established plants and thwarting any new seedlings attempting to emerge. When the plastic is removed, within four to six weeks, all plants in the affected area should be dead.

To completely block out sunlight and prohibit weed growth or regrowth, consider how deep the mulch should be laid. Fine-textured mulches, such as compost or sawdust, effectively blocks out sunlight with 2 to 3 inches of depth. However, coarser mulches with more gaps between pieces, such as wood nuggets or bark, require more depth to be effective, usually 4 to 6 inches.

This technique gives several benefits. While killing weeds and prohibiting new growth, sheet mulching layers break down to feed the ground, creating a richer soil. While a layer of landscaping fabric assists in stopping weeds, it is not foolproof. Many weeds break through the fibrous cloth. Another method is using newspaper sheets, which break down in time benefiting the soil. Choose a nonwindy day and spread opened newspaper sheets out in an overlapping pattern using at least 10 sheets per layer on the area to be mulched. Spray the newspapers with a hose to wet them thoroughly, and then spread mulch over the top of the wet newspapers.

While some methods are better than others, weeds and grass still appear at times and need to be hand-pulled or hoed. Postemergent commercial herbicides can be used. However, for those choosing organic methods, spraying young plants with rubbing alcohol, undiluted vinegar, or a mix of the two and dishwashing liquid is also effective.