Pick individual outer leaves from the arugula plant when they reach a length between 2 and 3 inches. Grasp the leaves by the stem near the base and pull them from the main plant.
Harvest clusters of outer leaves at once if they have reached mature size. Cut them off at the base with a sharp pair of small shears. Leave the interior of the plant to continue growing.
Pluck leaves every three to five days once the plant is producing. Frequent harvesting encourages the plant to continue producing and helps prevent the arugula from setting seed.
Cut the entire plant off at soil level and harvest all the leaves when the arugula sends up a flower stalk, but before it flowers. The leaves become bitter and inedible after blooming.