Spray pre-mixed Roundup Weed and Grass Killer on the leaves of unwanted plants. Prevent over-spray on nearby garden beds or lawns with plastic, as any product reaching plants' leaves will kill them. Apply Roundup's weed prevention spray directly on areas prone to weed growth, such as concrete crevices and gravel.
Roundup recommends applying its products on mild-weather days with low winds and temperatures above 50 degrees Fahrenheit. Some products work in as few as three hours, but wait to replant treated areas at least 24 hours for flowers and shrubbery, three days for grasses and edible plants, and seven days for full lawns.