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.
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.
Weed Killers in New Grass
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.
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.
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.
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.
Herbicides and Seeding
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.
In short, here’s what you should do if you encounter weeds in your newly seeded lawn:
Although this can be frustrating and we can appreciate that a quick solution will be desired, the good news about these types of weeds is that they are largely shallow rooting and should come out with the first mow at the 6-8 week mark after sowing. If they don’t, they should be easy to pull out of the turf.
If you find that the weeds are recurring past the 6-8-week mark, you may wish to consider using a selective herbicide to spot spray your weeds. Some weed killers such as glyphosate (Roundup) kill more than just weeds, so it is important to not apply these as if they are not done precisely, they can kill your grass. Shop bought selective weed killers will recommend when to apply their product and how often and you should read the instructions thoroughly and adhere to them.
However, you can get rid of these weeds just as fast as they have appeared.
The important thing to remember when new weeds appear in your newly sown lawn is not to act hastily – do not apply a Feed, Weed and Moss Killer type product of any kind on a newly sown lawn.
If you find that when the lawn is at least six months old and has been taken over by weeds or moss, you can use a Feed, Weed and Moss Killer product.