Q: I am trying to control weeds and zoysiagrass in ‘Blue Rug’ juniper. What should I use?
A: A quick answer is that you can simply dig them out but you’d better wear leather gloves and long pants if you don’t want to be stuck by the prickly juniper needles. There are three chemical avenues you can take to control weeds in groundcover beds.
The first is to apply a pre-emergent (click for sources) weed preventer twice each year: mid-March and mid-September. The pre-emergent will prevent weed seeds from sprouting in your juniper patch.
Your second task is to control perennial grassy weeds like zoysiagrass, bermudagrass and bahiagrass. This can be done with a selective post-emergent grass killer such as Vantage (click for sources) . The Vantage label lists specific plants on which it can be used. Chinese juniper and shore juniper are listed on the Vantage label but Juniperus horizontalis ‘Blue Rug’ is not. You can try the product on a small area first to see if it hurts your juniper while controlling the grassy weeds.
The third step is controlling broadleaf weeds. If you have tough perennial broad-leaf weeds like honeysuckle, brambles or dandelions amongst the junipers you can paint the weed leaves (do not spray) with glyphosate (click for sources) using a foam paintbrush.