I have six year old established blackberry bushes that are becoming overgrown with various weeds. What type of weed control can be used that will not harm bushes?
Weed Control for Established Blackberry Bush
Certified GKH Gardening Expert
If you are looking for a chemical solution, a broad leaf specific herbicide combined with a grass specific herbicide will take care of most weeds but will not hurt the blackberries.
For a more organic solution, a thick layer of mulch laid down (at least 3") will help suppress most weeds in the patch.