Rhubarb is a hardy perennial grown for its thick, fleshy leaf stalks, which range from deep crimson through pink to green and carry an intense, mouth-puckering sourness that has to be tamed with sugar. Botanically a vegetable but used almost everywhere as a fruit, it is the spring filling of pies, crumbles, fools and compotes — and the leaf blades atop those stalks are poisonous and never eaten.