That depends on the age and size of the plants. I usually harvest just once in the spring taking the largest leaves and again in early summer before the 4th of July. (Rhubarb pie is great with blueberries and white whipped cream!) Then let the plants rest for the rest of the summer. New or young plants should be harvested just in the spring leaving half the foliage behind.   Rhubarb plants love a mulch of rotted manure in the fall or early spring. Keep growing, Marianne Binetti