Rhubarb is a cool season, perennial plant that is very winter hardy and resistant to drought. Its crop is produced from crowns consisting of fleshy rhizomes and buds. Following a season of growth the rhubarb crown becomes dormant and temperatures below 40 °F / 5 °C are required to stimulate bud break and subsequent growth. The first shoots to appear in the spring are edible petioles and leaves. These emerge sequentially as long as temperatures remain cool (below 90 °F / 32 °C). As temperatures increase, top growth is suppressed, even appearing dormant in periods of extreme heat. With declining temperatures in later summer, foliage growth resumes.