Some pre-emergent herbicides can safely be used during seeding and usually come mixed with a seed starter. These products have the active ingredient Siduron – also known as Tupersan – that works by suppressing weed seeds while improving root development of the new grass. The fertilizer and pre-emergent herbicide mix is applied with a drop or rotary spreader using a rate of 2 1/2 pounds per 1,000 square feet. The spreader setting and actual application depends on the brand of starter fertilizer plus weed control you use, and you should always follow the instructions found on the label.
You can control weeds in newly planted grass seed and seedlings without the use of herbicides. Manually pulling the weeds by hand when they first appear keeps them from producing seeds and prevents the problematic plants from spreading, according to the University of California Statewide Integrated Pest Management Program website. They suggest keeping the newly planted grass weed free with proper mowing, irrigation and fertilization. Since newly planted turfgrass has short roots, keep the root zone moist by watering the soil lightly. However, avoid over saturating the soil. After the turfgrass has become established, promote deep and healthy root growth by watering infrequently but deeply.
Herbicides and Seeding
Weed management should be completed before seeding the lawn with a non-selective herbicide seven to 14 days before you till the soil. A second application of the herbicide may be required to kill any weeds you missed during the first treatment. Wait another seven days until tilling the soil if a second application is used.
Remember that all herbicides are different and the exact time you must wait to apply weed killers to newly planted grass will vary from one product to another. Also, some herbicides cannot be applied to certain species of turfgrass. For best results, always refer to the herbicide bottle’s label.
A general rule of thumb is to wait at least until you have mowed the new grass four times before using any standard postemergent broadleaf herbicide. A standard pre-emergent herbicide should not be applied until at least three to four months after seeding the area.
Dandelion, clover, spurge, and numerous other broadleaf weeds are stimulated as well. If you should have just a few weeds here and there, pull them by hand. However, if you are overrun, fall is an excellent time to control those broadleaf weeds. Perennial broadleaf weeds are busy sucking up much-needed nutrients to store for overwintering. An application of Gordon’s Trimec or Speedzone can be applied 28 days after grass seed germination. A fall application of Gordon’s Trimec with Uncle’s Stikit, spreader sticker or a granular application of Loveland Weed and Feed will be quickly absorbed by the broadleaf weeds and is sure to kill to the roots.
Each year during the fall over seeding season, we get a lot of folks that come in to the Grass Pad and say, “I followed your fall lawn renovation program; I used your weed-free grass seed. Why do I have weeds in my yard?” Our response is “Well, it means you really did it right!”
Disturbing the soil (verticutting, aerating, dethatching or raking) exposes soil and any pre-existing weed seeds that lay dormant in the soil. These weed seeds could be from last year or several years ago, suspended under the soil, too deep to germinate waiting for their opportunity to spring into life.
Controlling Broadleaf Weeds after Seeding
When properly preparing your lawn for fall overseeding, you created a pathway for grass seed to reach the soil. If grass seed didn’t reach the soil, it would not grow. Raking, verticutting, dethatching, or core aerating are all methods to ensure seed to soil contact.
Grassy weed seeds appreciate the same fertilizer and additional watering from your fall renovation program and flourish. Don’t panic – annual grassy weeds will be slow to germinate as soil temperatures cool in fall. Mother Nature will take care of these annual weeds at first frost. Using Prevent, crabgrass preventer in mid-April will eliminate their return.