Rhubarb (Rheum x cultorum) leaves are not edible, but the leaf stalks (petioles) make a good pie. Wait until the plant is at least a year old, then harvest the stalks in spring and early summer. The stalks at the outer edge of the plant can be pulled off at the soil line when the leaves are fully open and developed. Just take hold of the stalk close to the soil line, and give a slight twist as you pull. Never take more than about a third of the stalks at one time. Stop harvesting rhubarb before midsummer, and let the plant continue to grow. Trim it to the ground when it dies back naturally or is damaged by freezing weather. Rhubarb will return year after year in gardens where temperatures are not too hot.