Answer: This depends on the age of the plant--a well established rhubarb plant will yield several pounds of rhubarb every season. As you probably know, it is best not to pick rhubarb the first season, and the second season harvest only those stalks that are at least 1" thick. After that, if the plant looks healthy and hearty, you can harvest almost continually for a 4-6 week period in the spring. Once summer hits, you should stop harvesting and let the new shoots mature. If you're just making the occasional pie or a few pints of preserves, two to three plants should be plenty for most households.
When planning a rhubarb patch remember that rhubarb is a perennial and takes up quite a bit of space (3-4 feet in diameter!), choose a permanent home for it on the edge of your garden or flower border. They make nice specimen plants, too with their colorful leaves and grand flower stalks, so you don't need to grow it in your vegetable garden if space is at a premium.
Rhubarb requires at least 5 hours of full sun to produce well, and prefers a deep, rich, slightly acid (5.5-6.8pH) soil. Spread compost around plants in spring and mulch them to keep soil moist. Water soil around plants during dry spells, and foliar feed plants during the growing season for higher yields.
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