Answer: Grassy weeds can be controlled with products that kill grass but not shrubs and other broadleaf plants, such as these with the ingredients fluazifop and sethoxydim. If the weeds are not grasses you can apply glyphosate, Roundup has a variety of products available at The Home Depot. For long-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 rocks back on top of the fabric.
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