Nowadays, sidewalks are made by laying down long sections of cement, which are separated by a narrow space that is usually filled with a strip of wood. This space is there by design---it gives the cement room to expand during hot weather. However, when the strip of wood between the cement sections rot away, soil, weeds and grass begin to sprout from this area. This can be unsightly, and you will need to kill the grass and weeds growing in the sidewalk cracks.
Prepare the glyphosate spray according to the directions found on the label. The spray can be purchased already mixed, or you can buy a concentrate and dilute it. Place the spray in a hand-held or backpack sprayer.
Apply the glyphosate spray to the weeds. Glyphosate is the active ingredient in weed killers like Roundup, and it binds to soil or cement after it dries so that if it comes in contact with water, it will not cause run-off that could kill nearby plants, flowers or lawns. However, glyphosate spray will kill greenery on contact, so only use enough spray to target the unwanted growth.
Wait a few days for the weed or grass to die before removing it. This will ensure that you are not leaving a living root behind that could eventually grow into another weed. If any portion of the weed remains green one week after spraying, reapply the glyphosate weed killer.