Weeding before planting grass seed helps prevents weeds from outgrowing the new grass. Annual weeds grow, flower and die within one growing season, leaving seeds behind in soil, and perennial weeds return every year, sprouting from vigorous roots. Preparing a lawn area before sowing grass seed involves removing annual and perennial weeds and their roots, and encouraging weed seeds to sprout so they can also be removed.
Digging up Weeds
Digging up weeds is a labor-intensive but nonchemical method of weeding before sowing grass seed. A garden fork is the best tool for digging up weeds because it digs deeply but is less likely to slice through roots than a spade. Cutting weed roots is a bad idea because small pieces of perennial weed roots can grow into new plants.
Push a garden fork into the soil at one corner of the lawn area, and lever the fork upward to lift the weed roots out of the soil. Try to remove the roots whole, or pick out out of the soil any root pieces that break off. Remove the weed roots from the whole patch, working backwards to avoid treading on soil you've dug.
Digging up weeds removes weed plants but not weed seeds.
Pushing and pulling a sharp hoe at a shallow depth in the soil cuts through weed stems and controls or weakens the plants. To be effective, you must irrigate and hoe the lawn area two or three times over about six weeks. Irrigating the soil encourages weed seeds to sprout.
Water the lawn area with a garden hose fitted with a spray attachment until the soil is moist to a depth of 7 or 8 inches. Two or three weeks later, before the weed seedlings have four leaves, hoe the soil with a sharp-bladed hoe, cutting through the weed stems and roots about 1 inch below the soil surface. Irrigate and hoe the area again two or three times.
Hoeing weeds when the soil is dry provides the best results.
Tilling the Soil
Perennial weeds can often survive one experience of tilling, but tilling the soil three or four times often effectively controls perennial and annual weeds and their seeds. Tilling with a mechanical rotary tiller takes the backache out of controlling weeds, brings weed seeds to the soil surface, where they sprout, but tilling also leaves plenty of small pieces of root in the soil. When the soil is tilled again, however, this weakens perennial weeds and controls weed seedlings.
Put on long pants, a long-sleeved shirt, leather gloves, boots with steel toecaps and safety goggles before using a rotary tiller. Run the tiller up and down the lawn area until all the soil has been tilled. Till the soil again two weeks later, and twice more at two week intervals before planting grass seed.
Irrigating the soil after tilling it encourages weed roots and seeds to sprout, ready to be removed by tilling again.
Solarizing the Soil
Soil solarization involves covering the ground in plastic in summer so that the soil heats up under the hot summer sun, controlling weeds and weed seeds. The process takes four to six weeks and is most effective during the hottest part of the year and in areas that experience hot summers.
Solarization doesn't control all weeds. Two weeds that aren't controlled by solarization are field bindweed (Convolvulus arvenis), which is hardy in U.S. Department of Agriculture plant hardiness zones 3 through 9, and bermudagrass (Cynodon spp.), which is hardy in USDA zones 7 through 10. Solarization does control field bindweed and bermudagrass seeds.
Before solarizing the lawn area, hoe the existing weeds to remove the top growth. Irrigate the soil so that it's moist to a depth of 7 to 8 inches, and spread a sheet of clear polythene 1 to 2 mm thick over the soil surface. Use the thicker polythene in windy areas. Bury the edges of the polythene in shallow trenches to prevent the wind from lifting it, and leave it for at least four weeks but no longer than six or seven weeks. After removing the sheet, the soil can be made ready for sowing.
Non-selective herbicides such as glyphosate effectively control weeds before sowing a new lawn.
Put on protective clothing, safety goggles and gloves, and spray a ready-to-use 2 percent glyphosate herbicide over the lawn area on a dry, still day. Glyphosate is a systemic herbicide that kills weeds by traveling to their roots, and it can take up to two weeks to work. If there are green weeds remaining after two weeks, spray the area again. Remove the dead weeds by digging them up or tilling them into the soil.
Glyphosate harms most plants, including ornamental varieties you may wish to keep. Don't spray glyphosate near valued plants, or you can protect them with cardboard before you spray.