Rhubarb is a perennial plant, botanically a vegetable but used culinarily as a fruit due to its tart, edible stalks. As a cool-season crop, its growth is governed by the cycling of cold and warm temperatures. Understanding this temperature dependence is key to successfully cultivating and harvesting its flavorful petioles.
The Annual Cycle: Emerging from Dormancy
Rhubarb requires a period of cold temperatures, known as winter chill, to reset its growth cycle. Typically, this demands the crown—the plant’s root structure—to experience roughly 500 hours below 40°F (4°C). This necessary chill breaks dormancy, allowing the plant to prepare for vigorous growth. Without sufficient cold exposure, the plant will not produce the thick, numerous stalks desired for harvest.
Once the winter cold requirement is met, the plant responds to the gradual warming of the soil in early spring. The first sign of life is the emergence of small, reddish-green shoots, often called spears, which push through the soil as temperatures climb. This initial growth begins well before the last frost, as rhubarb is a hardy plant that thrives in cool conditions. The plant rapidly produces its large, fan-like leaves, which are the solar energy collectors that power the development of the harvestable stalks.
The Prime Harvesting Window
The period for harvesting rhubarb typically begins in late spring and extends through early summer, lasting approximately eight to ten weeks for an established plant. Stalks are ready when they reach 10 to 18 inches in length and are at least three-quarters of an inch in diameter. The color of the stalk—which can range from deep red to green—is variety-dependent and not an indicator of readiness.
To harvest, firmly grasp the base of the stalk and remove it with a gentle twist and pull motion. Cutting the stalk is discouraged because it leaves a stub that can rot and introduce disease into the crown. After harvesting, the large, triangular leaves must be immediately removed and discarded, as they contain high levels of oxalic acid and are toxic. Harvesting should cease entirely by mid-summer, generally by late June or early July, or when the average daily temperature consistently rises above 75°F (24°C). Continuing to pick past this point depletes the plant’s carbohydrate reserves, leading to thinner stalks the following year.
Preparing for Dormancy and Winter
Stopping the harvest in mid-summer allows the rhubarb plant to shift its focus to storing energy in its crown for the next season. Throughout the late summer and fall, the plant’s large leaves continue to photosynthesize, sending vital sugars down to the root system. As autumn approaches and temperatures drop, the foliage will naturally begin to yellow, wilt, and eventually die back after the first heavy frost.
This natural dieback signals the plant’s entry into its required dormant phase. Once the leaves are completely dead, they should be cut away and removed to prevent fungal diseases from overwintering near the crown. A layer of organic mulch, such as straw or compost, can then be applied around the plant, but not directly over the crown, to insulate the roots and prevent moisture loss. This preparation ensures the crown remains healthy and ready to receive the necessary winter chill hours to begin the cycle again.