By midsummer, after a few weeks of regular mowing, the grass in your lawn could probably do with a feed to keep it healthy, green and vigorous through the rest of the season. The fastest way to green it up and give it a boost is to use a nitrogen-rich liquid feed, specially formulated for lawns. Apply the feed on a dull day or as the sun goes down to reduce evaporation from the surface. Grub out any large weeds first so that you are not feeding them, too. You should notice an improvement within days.