Can I spray weed killer on recently seeded lawn? I don’t want the weeds to take over before my new lawn gets established.
Lawns seeded in spring tend to get weedy, which is one reason why autumn is the recommended time for planting cool-season grasses (such as Kentucky bluegrass and tall fescue). Before applying an herbicide, wait until new turf grows enough that it requires mowing three times. Young grass plants are sensitive to weed killers and may be damaged if the herbicide is sprayed too soon.
Don’t apply weed killer to new grass until you have mowed it three times, advises North Dakota State University Master Gardener Extension. The University of California Integrated Pest Management Program suggests pulling weeds by hand as they appear. This will help prevent a larger invasion of weeds. In addition to manual removal, proper care will control weeds in both new grass and established lawns. The roots of newly planted grass are short for the first few weeks and require only a light watering to keep the top 2 to 3 inches of soil moist. Once established, water deeply but less frequently to encourage the roots to grow deeper, which will lead to a healthier lawn.
Herbicides applied before the undesirable plant germinates are called pre-emergent weed killers. These herbicides kill sprouting weed seeds and weeds newly germinated, but won’t kill existing weeds. Pre-emergent herbicides leave a residue in the ground that stays active for an extended period. Post-emergent herbicides kill established weeds. These types of herbicides control various lawn weeds such as dandelions. Herbicides are described as either translocated or contact. Weeds absorb translocated herbicide — when it is applied to their foliage — which interferes with metabolism. This will kill both above-ground and below-ground portions of the weed. Contact herbicides only kill the portions of weed they come in contact with and the weeds will often grow again.
Weeds consume the nutrients and moisture that new grass desperately need for proper growth. These undesirable plants will also cause the new grass to appear messy and unruly. Weed killers are readily available to control various species of weeds. Unfortunately, weed killers can cause more harm than good on new grass.
Weed Killers in New Grass
New grass is more susceptible to damage from herbicides than an established lawn. It is weaker, more delicate and cannot tolerate harsh chemicals designed to prevent and kill undesirable plants. Pre-emergent herbicides will interfere with any remaining grass seeds that haven’t germinated yet or have just started sprouting. Post-emergent herbicides can severely damage new grass that isn’t established. Unless the weed killer is designed for use at the time of seeding, do not use the herbicide on new grass.
It is best to get rid of any weeds before planting sod or grass seed, but use a post-emergent herbicide, not a pre-emergent weed killer. Pre-emergent herbicides leave active residues in the soil for several weeks or months, which can damage new grass. Post-emergent herbicides containing glyphosate will kill the weeds and won’t leave residue behind. Weeds springing up is generally a sign that something is wrong with the lawn. A well-maintained lawn generally won’t have problems with weeds. Over-watering, poor drainage, nitrogen deficiency and mowing the lawn too short can all lead to weed growth.
Amanda Flanigan began writing professionally in 2007. Flanigan has written for various publications, including WV Living and American Craft Council, and has published several eBooks on craft and garden-related subjects. Flanigan completed two writing courses at Pierpont Community and Technical College.
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.
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.
Before you plant grass seed, you should always prepare the area by removing any weeds that may be growing in the location. Even with careful preparation of the planting site, weeds can still develop among the newly planted grass seed. Weed killers, however, can harm grass seeds and seedlings if applied too early or improperly.
Herbicides and Seeding
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.
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.
Marylee Gowans has written about gardening for both online and print publications. She attended the University of Akron, graduating with a Master of Fine Arts in creative writing. In 2009, she received master gardener certification from the Master Gardeners of Summit County, Ohio.