Mulch, a protective layer placed over the soil, is an effective tool in landscape management. When applied correctly, this layer can kill existing grass and weeds while preventing new ones from growing.
How Mulch Suppresses Plant Growth
Mulch primarily suppresses the growth of unwanted plants by blocking light, which is necessary for photosynthesis. Without access to sunlight, the grass and weed plants cannot produce the energy required to grow and sustain themselves, leading to their eventual death and decomposition. This light deprivation is the most effective mechanism for controlling vegetation already established in an area.
A secondary mechanism involves the moderation of soil temperature and moisture. A deep layer of mulch insulates the soil, keeping it cooler in the summer and warmer in the winter. This temperature regulation and the retention of moisture can reduce the germination rate of many weed seeds that require specific environmental cues to sprout.
The physical weight and density of a thick mulch layer also act as a physical barrier. This barrier prevents delicate new weed seedlings from pushing through to the surface. Furthermore, by covering the soil, the mulch prevents weed seeds carried by the wind or animals from settling into the soil where they might germinate.
Applying Mulch to Eradicate Existing Growth
Using mulch to kill established grass and weeds requires a strategic application process often referred to as sheet mulching. Preparation involves cutting the existing grass or weeds down to the lowest possible level. While not strictly necessary, removing large perennial weeds, like dandelions, first prevents them from potentially pushing through the layers.
A barrier layer must be applied directly over the prepared ground to ensure complete light blockage and suffocation of the underlying vegetation. Flattened cardboard boxes are highly effective for this purpose, as they are biodegradable and offer a robust light-blocking layer. If using newspaper instead of cardboard, it is recommended to use at least ten sheets thick to achieve the same level of suppression.
The barrier material must be laid down so that the edges overlap generously, preventing any gaps that weeds could exploit to find sunlight. Once the cardboard or newspaper is soaked with water to help it conform to the ground, a substantial layer of organic mulch is applied on top. This final layer must be applied at a thickness of four to six inches to be effective as a killing agent.
This significant depth serves two purposes: it ensures complete light exclusion, and it provides enough mass to hold the underlying cardboard barrier in place. It is important to maintain a safe distance, typically several inches, between the mulch and the base of desired trees and plants. Piling mulch directly against trunks, creating a “mulch volcano,” can trap excessive moisture, leading to rot and creating an entry point for pests.
Choosing the Right Mulch Material for Weed Control
Mulch materials are broadly categorized into organic and inorganic types. Organic mulches, such as wood chips, shredded bark, and straw, decompose over time, benefiting soil health by adding organic matter and nutrients. However, materials like straw or grass clippings can sometimes introduce new weed seeds if they are not properly sourced.
Inorganic mulches, including landscape fabric, plastic sheeting, and gravel, provide a powerful, long-lasting physical barrier. Since these materials do not decompose, they offer superior, low-maintenance weed control over many years, though they do not contribute to soil fertility. Plastic sheeting and fabric are particularly effective at blocking light, but they must be covered with an aesthetic layer of organic material or gravel.
When choosing an organic material, finer-textured mulches, like shredded bark, tend to be more effective at blocking light than coarse materials because they settle into a denser layer. Ultimately, the ability of the mulch to kill existing growth depends primarily on the applied depth and the use of an underlying barrier, rather than the material type itself.