The best time to harvest rhubarb is typically in the spring, from April to May. You should begin harvesting when the stalks reach about 10 to 15 inches long. The rhubarb season generally lasts for eight to ten weeks, usually ending by mid-June or early July to allow the plant to store energy for the winter. Important tips for harvesting include:

• Only take about one-third to one-half of the stalks at a time to avoid weakening the plant.
• Use the "twist and pull" method to harvest the stalks rather than cutting them with a knife, which can harm the plant.
• Remove and discard the leaves after harvesting, as they are toxic.
• Avoid harvesting in the first two years after planting to allow the plant to establish.
• Stop or significantly reduce harvesting by late June or early July for the health of the plant.
• If rhubarb flowers appear, cut them off immediately as they divert energy from stalk production.

You can harvest rhubarb until frost but should do so sparingly in late summer to prevent damaging the plant. Overall, for the best flavor and plant health, stick to harvesting mainly in spring through early summer.