Monitor the plant during the growing season in the early spring and summer. Look for seed heads -- large buds that have white clusters of clumpy flowers inside of them -- starting to form. Typically, seed heads form in the middle of the plant and grow on stalks that are taller, rounder and thicker than leaf stalks.
Find the stalk on which the seed head is growing, following it all the way down to the base of the plant. Cut it as close to the bottom of the plant as possible with a sharp knife.
Continue removing flower stalks for as long as necessary. Flower stalks will form throughout most of the growing season, depending on the variety.