How Long After Using Round-Up Grass Killer Can New Grass Be Planted?

Glyphosate

• The active ingredient in Roundup is glyphosate, an herbicide developed by the Monsanto Company and now in widespread use. Glyphosate is only effective on actively growing plants. The plants absorb the herbicide through direct leaf contact and transport it to growing cells. Glyphosate interferes with the cell's division, killing the plant. Dormant plants do not move the absorbed herbicide to the growing cells and remain unaffected.

Weed Death

• On occasion, glyphosate does not kill persistent weeds and grasses. Although the herbicide damages the plant, many unwanted plants return after surviving the first glyphosate application. Cooler temperatures, unexpected rains and deeply rooted weeds may contribute to the failure. Respray Roundup on surviving plants to ensure the unwanted vegetation is dead before planting new grasses.

Herbicide Persistence

• Herbicides, like glyphosate, remain in the soil for varying periods of time and may harm new, tender grass. The manufacturer indicates that lawn grasses can be planted three days after application of Roundup and Iowa State University states that the glyphosate bonds to molecules in the soil upon contact and becomes inactive. However, Rhode Island Cooperative Extension suggests waiting 10 to 14 days after the application to plant seed or lay sod. Glyphosate may also remain active in the treated plants themselves, since they are unable to metabolize the herbicide.