The best time to pick rhubarb stalks in the United States and Canada is from April to September. Growers should only harvest stalks from plants that are two or more years old.
Rhubarb stalks are ready to harvest when they are firm and about 10 to 15 inches long. Growers can pick them every four to five weeks during the summer. The color or thickness of the stalks does not indicate if they are ready for picking but rather reflect weather conditions and variety. If frost settles on rhubarb, it's good for picking only if the stalks are still firm and not mushy.