BAKING SODA a safe way to keep weeds & grasses from growing; Sprinkle handfuls of baking soda onto the concrete & simply sweep it into the cracks. The added sodium will make it much less hospitable to dandelions & their friends--use spring & fall.
WHO KNEW? 5 pound bags of Baking soda. Simply pour or sweep a thick layer of baking soda into sidewalk and patio cracks. The baking soda will kill any small weeds that are already there, and prevent new ones from sprouting"
Natural Weed Killers - 11 Ways to Kill Weeds Without Poison