Killing tough woody type weeds
I have a large side yard that was overgrown with fast growing weeds that were as thick as small tree branches. I had an excavator come in and dig up the weeds and roots before leveling the remaining dirt/gravel mix. We want to grow grass in that area but are concerned the tough to kill weeds will spring up from below. We intend on putting about 4" of topsoil down and then Scotts Turf seeds. How can we keep the weeds from coming back without killing the grass we want to grow? Should I use something like Crossbow on the dirt and then put the topsoil on top of that or will the grass die when it's roots hit the layer of Crossbow spray and dirt/gravel? Thanks for your help!
It may be too late for the Crossbow (or other herbicide) since it only works on growing plants. It will not stay in the ground to kill new weed shoots or weed seeds. You could wait through the fall and use herbicide on any weeds that come up, then plant grass in the spring. Or use turf, which forms a dense mat that makes it more difficult for weeds to penetrate.
Otherwise, plant your grass seeds as you suggested. Weeds and grass seed will germinate, but after 3 or 4 mowings, you can use a broadleaf weed killer containing 2,4D, such as Weed & Feed, to promote healthy grass growth and kill off broadleaf weeds.
The following OSU publication will help you in selecting appropriate grass seed for your situation and follow up care.
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