Saw a kid dismally picking weeds out of a crack by a curb (obviously assigned by parent or other authority) and remembered a great FREE and ecologically sound tip for controlling those little bitty weeds that come up through cracks in pavement: boiling water. I used what was left in my canning pot to kill weeds in my rock path. This works well on driveways and curbsides too. Plus you get a really nice upward waft of cooking greens, one of my own favorite smells!