Rhubarb is a perennial vegetable prized for its tart, edible stalks. The plant’s growth is strongly seasonal, and its productivity relies on a period of winter dormancy. Harvesting in September, a late-season month, requires understanding this growth cycle and the plant’s preparation for the cold season. Generally, the answer is no, due to concerns about the plant’s future health and the concentration of a naturally occurring compound.
The Ideal Rhubarb Harvest Season
The most productive and safest window for harvesting rhubarb begins in early spring and typically extends into early or mid-summer. Gardeners usually start picking stalks once they reach a length of 12 to 18 inches, often beginning in May and concluding around the end of June or early July, depending on the local climate. This early-season timing ensures the stalks are tender and flavorful.
Harvesting involves grasping the stalk near the base and removing it with a gentle pull and twist, rather than cutting it with a knife. Pulling allows the stalk to separate cleanly from the crown, which encourages regrowth and prevents a decaying stub that could invite disease. To maintain the plant’s vigor, a gardener should never remove more than one-third to one-half of the stalks from a single plant at any one time.
Why September Harvesting is Risky
Harvesting rhubarb in September is generally not recommended because it interferes with the plant’s preparation for winter. The primary reason to stop picking is to allow the foliage to remain on the plant, where it can photosynthesize and store energy. This energy, stored as carbohydrates, is transferred to the underground crown and root system to fuel the next year’s growth.
Removing stalks late in the season significantly depletes these carbohydrate reserves. A weakened plant will produce a much smaller, less vigorous yield the following spring, or it may even fail to survive a harsh winter. The gardening tradition of ceasing harvest by mid-summer protects the plant from this exhaustion, ensuring its long-term health.
A secondary concern involves oxalic acid, a compound found in all parts of the rhubarb plant, which is highly concentrated in the leaves. While the stalks remain safe to eat throughout the summer, some gardeners notice the stalks become tougher and thinner as the season progresses. The main risk is to the plant’s energy reserves, not direct toxicity in the stalks themselves. The safest practice is to stop harvesting when the stalks become thin or by the end of July at the latest.
Preparing Rhubarb for Winter Dormancy
Once the growing season is over, the next step is to prepare the crown for the cold months ahead. Rhubarb is a cold-hardy perennial, but it requires a period of temperatures below 40 degrees Fahrenheit to enter dormancy and establish new buds for the spring. The plant’s own leaves should be allowed to die back naturally, which typically happens after the first hard frost.
Once the foliage has completely browned and collapsed, it should be trimmed away and removed to prevent disease from overwintering near the crown. A protective layer of organic mulch, such as well-rotted compost or straw, should be applied around the plant. This layer insulates the crown from extreme temperature fluctuations and enriches the soil for the next season’s growth. It is important to avoid placing the mulch directly on the crown itself, as this can trap too much moisture and encourage rot.