Grass and weeds frequently invade mulch beds, transforming a tidy landscape into a frustrating maintenance task. This problem occurs because the conditions within a mulch bed are ideal for germination and growth. Wind often carries seeds from nearby lawns or fields, depositing them directly onto the mulch surface, while aggressive lawn grasses use underground runners, called rhizomes, to creep directly into the soft, moist soil beneath the mulch. Addressing this issue requires a multi-pronged approach that targets existing growth while setting up long-term barriers to prevent future invasions.
Manual and Smothering Techniques
Physical removal is the safest method for eliminating grass growing close to desirable ornamental plants. Hand-pulling is most effective when the soil is damp, making it easier to remove the entire root system without breaking it off at the crown. This complete removal is especially important for perennial grasses, such as Bermuda grass, which can regrow aggressively from even small root fragments left behind.
Tools like a sharp trowel or weeding hoe can help sever and lift the roots of young grass seedlings. For larger areas or dense patches of grass, sheet mulching can smother the unwanted growth. This involves laying down overlapping layers of cardboard or several sheets of plain newspaper directly over the grass.
The paper barrier blocks sunlight, halting photosynthesis and starving the grass of energy. This layer must then be covered with a fresh application of organic mulch to hold it in place and improve aesthetics. This method takes several weeks to fully kill the grass underneath, but it is an excellent way to prepare a new bed or renovate an existing one without disturbing the soil structure.
Selecting the Right Chemical Solution
When manual removal is impractical, chemical herbicides offer a targeted method for grass control within mulch beds. Herbicides fall into two main categories: non-selective and selective. Non-selective herbicides, which often contain glyphosate, kill virtually any plant they contact by disrupting a specific enzyme pathway necessary for growth.
These products are highly effective for killing grass, but they must be applied with caution to avoid drift onto desirable shrubs or flowers. Spot-treating is the safest application method, often using a trigger sprayer or foam applicator to ensure the chemical only touches the grass blades. For the highest degree of precision, a small paintbrush can be used to coat the leaves of the grass without risk of spray drift, allowing the plant to absorb the herbicide and transport it down to the root system.
Selective herbicides are generally less useful for controlling lawn grass that has spread into a bed because they are formulated to target specific plant types, such as broadleaf weeds, without harming turfgrass. Always read the label to confirm the product’s safety around existing ornamentals, as some chemicals can leach through the mulch and harm the roots of nearby plants.
Utilizing Natural and Homemade Grass Killers
Gardeners often use non-commercial solutions, such as high-concentration vinegars, as alternative grass killers. Horticultural vinegar contains acetic acid in concentrations of 20% to 30%, significantly higher than household vinegar’s typical 5%. This potent solution acts as a contact herbicide, quickly burning and dehydrating the grass foliage. However, high-concentration acetic acid is corrosive and requires the use of personal protective equipment, including gloves and eye protection, to prevent skin and eye burns.
Another popular homemade method is pouring boiling water directly onto the grass, which works by causing thermal shock and destroying the plant’s cell structure. Like vinegar, boiling water is a contact killer, meaning it often fails to eliminate the deep root system of perennial grasses, leading to regrowth.
Salt mixtures are also sometimes suggested, but they carry the greatest risk to the overall health of the soil. Salt dehydrates and kills plant life by disrupting the osmotic balance, but the sodium residue remains in the soil long after the grass is gone. Excessive use of salt can permanently sterilize the soil, making it unsuitable for future plantings by inhibiting water uptake.
Strategies for Long-Term Prevention
Once the existing grass has been removed, maintaining a proper mulch layer is the primary defense against future invasions. A mulch depth of two to four inches is ideal for blocking sunlight, which prevents the germination of most wind-blown seeds. Mulch that is too thin (less than two inches) will allow sunlight to penetrate, while a layer deeper than four inches can smother the roots of desirable plants.
Installing a physical barrier, or edging, is another effective preventative measure, especially against creeping lawn grasses that spread via rhizomes. Materials like metal, stone, or brick edging should be installed with a vertical depth of at least three to four inches below the soil line to physically stop underground runners from entering the bed. Regular maintenance, such as an annual re-cutting of the edge, helps to maintain this physical separation.
Pre-emergent herbicides can also be used as a preventative measure, but they must be applied only after the grass has been removed. These granular products create a chemical barrier that inhibits the germination of new seeds. However, they are ineffective against existing, established grass and do not stop creeping rhizomes. Pre-emergent products must be reapplied every few months to maintain the barrier.