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.
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.
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.
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.
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.
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.
In short, here’s what you should do if you encounter weeds in your newly seeded lawn:
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.
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.
This may be the most important step. With the first watering, make sure that you apply enough water to wet the soil down to at least 6 to 8 inches. Apply the water gently so that you don’t wash the seed away or create puddles.
As your new lawn becomes established, you can start easing up on the water, depending on the weather. If you continue your everyday watering routine, you’re likely to overdo it and rot the young seedlings. Also, if the ground is too wet, you can inhibit root growth.
Caring for your new lawn after germination
Watch the color of the soil surface. As the soil dries, the surface becomes lighter in color. When you notice about half to two-thirds of the surface lightening up, it’s about time to water.
When you have a pretty even ground cover of new seedlings, try skipping a day of watering and see what happens. Watch the grass carefully. If the color starts to go from bright green to dull gray green, the grass needs water. You may have to water some quick-to-dry areas with a handheld hose.
You will need to mow the new lawn when it reaches 3 to 4 inches high, depending on the type of grass. Mow when the soil is on the dry side; otherwise, you might tear up the new turf. Make sure not to have your mower set too low. It is always good to mow grass at 3 inches or higher to prevent it from drying out. This is especially true for your new lawn.