(Image credit: Svetlana Khoroshilova)

Whether you are reseeding an entire lawn or simply sowing seeds in bare areas of existing turf, you likely have questions. One of the biggest questions may be regarding grass seed and weed killer. Should I plant grass seed or kill weeds first? Can you put weed killer and grass seed down together? Don’t worry -- we’ve got you covered. Read on to learn how to get rid of weeds in new grass. 

Plant Grass Now or Control Weeds First?

Whether to sow grass seeds or control the weeds prior to sowing is a big question, and one that depends on three different scenarios. For areas with broadleaf weeds and a few annual grasses sow now and control the weeds later. 

Broadleaf herbicides should not be sprayed onto newly seeded turf until it has been mowed at least twice which can first be done when the grass is three inches (8 cm) tall. 

If, however, you have an area that has lots of broadleaf weeds and crabgrass, it is better to eradicate the weeds prior to sowing. If you want to sow as soon as the next day, opt for an herbicide that allows you to do so.

Lastly, if the area is covered with perennial grasses such as quackgrass, the best plan is to eradicate these grasses first using a non-selective herbicide. This requires multiple applications and should begin weeks prior to sowing grass seed. Because these perennial grasses grow via rhizomes, at least two possibly three applications should be made. Keep in mind that the area will be unsightly for weeks to months. 

Is there a Weed Killer that Won’t Kill Grass Seed? 

On occasion a newly sown lawn will grow up with weeds at the same time. Then, of course, the grower wishes to kill the weeds but not the grass. Weeds may infiltrate the grass for a number of reasons, but did you know that one of these is that there were weed seeds in amongst the grass seed? 

It’s true. Grass seed comes in many varieties, but all must be labeled as to their makeup. This includes the types of grass seed but also the percentage of weed seed both noxious and otherwise by weight. The best quality grass seed will say “none” or “no noxious weed” on the label. The label “no noxious weed seed” does not preclude the possibility that other types of weed seeds may be intermixed with the grass seed. 

In any case, do not put grass seed and weed killer down at the same time. Most herbicides are for use on established turf and at the very least the area should go through two mowing cycles before applying. Then use a three-way herbicide in the fall to control weeds. 

Note: Any recommendations pertaining to the use of chemicals are for informational purposes only. Specific brand names or commercial products or services do not imply endorsement. Chemical control should only be used as a last resort, as organic approaches are safer and more environmentally friendly.

Amy Grant

Amy Grant has been gardening for 30 years and writing for 15. A professional chef and caterer, Amy's area of expertise is culinary gardening.