Updated: Sep 30, 2020
Battling weeds in your yard is an ongoing fight, especially if you have bare, patchy areas in your lawn -- weed seeds quickly germinate in the sunny, exposed soil, preventing your turf from populating the space. You cannot smother weeds with a new grass establishment, whether from seed, sprigs or sod, since both species will compete for soil resources. Grass clippings spread on weeds as a mulch, however, may curb weed seeds from germinating. Practice consistent maintenance on your established lawn, such as proper mowing, to help deter unwanted plants.
One of the best ways to smother weeds is to keep your soil healthy for thick grass growth. In general, your grass needs a soil pH value range between 6 and 7. With a slightly acidic range, some weed seeds cannot survive in these conditions. Proper fertilizing approximately three times each year keeps the soil prime for grass growth as well -- sunlight cannot reach the soil where weed seeds germinate if the grass shades the soil. A fertilizer with a 3-1-2 ratio of nitrogen, phosphorus and potassium is sufficient for most grass species.
Some weed species are bound to invade your turf, so it is critical to remove these sprouts as soon as you find them. Neglecting even one weed causes stress to your grass as it steadily grows. Annual weeds do not create extensive root systems -- hand-pulling them directly from the soil typically removes all traces of the plant. But you need to carefully remove any portion of perennial weeds from the ground. Leftover root parts broken off from the main weed easily proliferate into new pesky plants. You may need to repeatedly hand-weed the turf of these perennials, especially if they had a chance to spread seed.
Grass Clipping Mulching
Applying a 2- to 3-inch grass clipping layer across your weeds does not typically smother them -- weed foliage still has ample sunlight and soil resources to grow steadily. This smothering mulch technique, however, does help you prevent weed seed germination. Seeds that do not have access to sunlight cannot properly develop into hardy seedlings.
Watering and Mowing
Although watering may seem to encourage weed growth, a proper irrigation strategy helps the grass grow tall to smother weed seeds -- removing critical sunlight warmth prevents seed germination. Water your turf in the morning so that the roots have a full day of sunlight and growth to continue spreading their roots deep into the ground. Healthy root establishment creates tall and dense grass blades to smother weeds. In addition, cutting your lawn to a minimum height of 2 inches helps to minimize weed growth. If you cut your lawn too short, sunlight penetrates the soil and helps weed seeds germinate. With sharp lawnmower blades, you create a crisp cut across all the blades to keep them healthy for future, vigorous growth.
Herbicides and New Lawns
Grass struggling to grow from a new establishment cannot smother weeds. In fact, it may be necessary to use herbicides on weeds so that your grass has a chance to populate the planted area. Herbicides such as phenoxy acid and benzoic acid effectively kill weeds while leaving your established grass unharmed. But your new grass needs time to establish itself before any herbicides are applied -- you could possibly kill off your new grass with too much chemical exposure. After your sixth time mowing the new seedlings, apply herbicide directly to the weeds. Your grass has enough strength to withstand the chemical application at this point, if hand pulling the weeds does not suffice.