Broadleafed plants such as dandelions, chickweed, plantain, and other common lawn weeds usually have the advantage over grasses in the hot, dry, sunny conditions of newly tilled soil prepared for the lawn seed. They are ecologically adapted to these conditions. Herbicides are commonly used to kill weed seeds both prior to sowing lawn grass seed and after the grass seed has germinated. However, if this practice is controversial in your neighborhood or you are looking for non-toxic alternative methods for preventing weed growth when you install your lawn, there are some things you can do.
Start by rototilling the area you will be planting. Spread a two-inch layer of composted manure (or composted sewage sludge if it is available and acceptable to you) over the seedbed and rototill again to incorporate the organic amendment into the soil. The material is applied at this stage because it too, may contain weed seeds.
Next, irrigate the soil until the top two inches is moist. This top layer of soil is where most problem seeds will be found. Let the seedbed lie fallow for a week or two, until it becomes covered with a green "fuzz" of weed seedlings. If the area is large, rototill the soil again to kill the seedlings. Position the rototiller blades at their highest setting so they till as shallowly as possible. Try to till only an inch deep or so, to avoid bringing up more weed seeds from deeper in the soil. If the number of germinated seedlings was very high, indicating a large supply of weed seeds in the soil, irrigate again to germinate remaining seeds, then till them under a second time.
In small lawn areas, or in exceptionally weedy soil, the weed seedlings can be killed using a flamer, as an alternative to rototilling. A flamer is a portable propane torch that emits a barely visible blue flame that is passed over the weed, causing the plants to wilt and die within 24 hours. Just pass the flame over the weed briefly, there is no need to char the plant. Flamers can be rented from some rental shops or purchased at farm and ranch supply stores, and some garden suppliers. They have been used in Europe for garden weed control for many years. After flaming, irrigate a second time, and flame the second crop of weed seedlings one to two weeks later. Use the flamer carefully to avoid fire hazards or injury.
After the second tilling or flaming, complete soil preparation by raking in any additional nutrients that may be needed, removing any rocks and clods. Level the soil, sow grass seed, rake it in lightly, apply ½ inch of organic mulch, and irrigate again to moisten the lawn seed. Be sure to select a variety of grass that is adapted to your soil type and growing conditions. That will ensure it is as competitive and vigorous as possible. Purchase only high-quality, weed-free seed.
Many annual weeds will germinate among the new grass plants, even after your efforts. Most will be killed or crowded out after several mowings. Mow these weeds they can produce seed, because the seeds can remain viable in the soil to plague you for many years. To maintain your lawn with minimal use of herbicides, and to help grasses develop deep, competitive root systems, see the topic sheets in this series.
Be advised, however, that any deep-rooted or persistent perennial weeds present on the site, such as Canada thistle, field bindweed, quackgrass, etc., will not controlled by these methods because new, independent plants can be formed from vegetative root buds, sometimes growing many feet deep in the ground. If these weeds are a problem in your area, you may wish to make one initial application of a systemic herbicide that will translocate down into the root systems, such as glyphosate (e.g. Roundup Concentrate) applied according to label instructions (more than the label calls for is not more effective). If you choose this option, apply it either in fall when rosettes of those weeds are carrying carbohydrates down into the roots, carrying the herbicide with them, or when they are actively growing in spring. A single initial application of glyphosate can be used to remove all growing vegetation, including old grass, and annual and perennial weeds. Follow with the methods listed above to eliminate germinating weed seeds, which will not be affected by the glyphosate.
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