I am sorry for this delayed reply to your gardening question. The spring rush has brought a deluge of questions and we are working hard to catch up!
Grassy weeds can be controlled with product that kill grass but not shrubs and other broadleaf plants such as the ingredients fluazifop and sethoxydim. If the weeds are not grasses you can apply glyphosate (Roundup is one brand) with a wiper applicator so you don't get it on the desirable plants but just the weeds. For longer term weed control you may want to remove the lava rock and lay a landscape weed block fabric down around the plants. Then put the lava rock back over the fabric.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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