You can pick rhubarb generally from April or May when the stalks reach about 10 inches (25 cm) or more in length. The color of the stalks (red or green) is not a reliable indicator of readiness; rather, stalk size is key. Early cultivars may be ready as early as March, while maincrop varieties are ready by late April or May. The rhubarb season typically lasts about 8 to 10 weeks, ending around June or early July. After this, it's best to stop or significantly reduce harvesting to allow the plant to store energy for next year. When harvesting, only take about one-third to two-thirds of the stalks at a time to prevent harming the plant. The best way to harvest is to gently pull or twist the stalks at the base rather than cutting them, as cutting can lead to rotting. Also, always remove the leaves, as they are toxic. In summary:

• Start harvesting when stalks are about 10 inches (25 cm) long.
• Harvest from April/May through June or early July.
• Avoid harvesting too much to keep the plant healthy.
• Twist or pull stalks off rather than cutting.
• Stop harvesting after about 8-10 weeks for plant health.

You can continue to harvest rhubarb sparingly through summer (up to August) but it is not recommended to harvest heavily after early July to ensure the plant's health and future productivity.