Answer: There are several approaches you could try. One is to use an herbicide. Glyphosate is a nonselective herbicide that will kill both grass and broadleaf weeds, but it would also damage the junipers. You could apply it by brush to the target weeds taking care to shield the junipers from contact with the herbicide. There are also some herbicides that kill grass only, that could be applied over the junipers to control the grass. You would need to talk to your county extension for specific recommendations on what to use.
Another method would be to smother the weeds by covering them with cardboard or a thick layer of dampened newspaper. You might be able to slide that in under the junipers. Then lay mulch on top so it looks tidy. Keep the mulch topped up and fluff it occasionally with a rake to disturb any weed seeds that may have blown in.
You could also try to prevent new weeds by using a pre-emergent herbicide, this kills weed seedlings as they germinate. Corn gluten is one product, apply it per the label directions. Note that this forms a barrier layer on the soil or mulch surface, so do not disturb the soil/mulch after you apply this, or make sure you apply this as the last step.
You may have to use a combination of methods. Good luck with the weeds!
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