Rhubarb is a perennial vegetable, often utilized as a fruit due to its tart flavor profile. Gardeners and cooks are advised to cease harvesting the stalks around the end of June or early July. This guideline is rooted in two reasons concerning human health and plant vitality. One involves the changing concentration of a naturally occurring chemical compound. The other focuses on the plant’s need to prepare for the subsequent growing season.
The Role of Oxalic Acid
The primary health concern associated with late-season rhubarb harvesting is the potential increase in oxalic acid concentration within the stalks. Oxalic acid is a naturally occurring organic compound, and rhubarb is known for having a high amount of it. This compound is present in all parts of the plant, but the leaves contain consistently toxic levels, which is why they are never consumed.
As the plant matures, or is stressed by heat, drought, or frost, the concentration of oxalic acid in the edible leaf stalks can increase. Consuming high levels is dangerous because it acts as an antinutrient, binding with minerals like calcium to form insoluble salts. These salts, called oxalates, can then travel through the bloodstream and accumulate in the kidneys.
This accumulation can lead to the formation of kidney stones, and in extreme cases, it may cause acute kidney damage or failure. Nonetheless, the common advice to stop harvesting by mid-summer is a protective measure to avoid the seasonal escalation of this compound.
Late Summer Botanical Changes
The second reason to cease harvesting after July is related to the plant’s long-term health and the quality of the stalks themselves. Rhubarb is a hardy perennial, meaning it must store energy to survive the winter and fuel the next year’s spring growth. As the days shorten and temperatures rise in mid-summer, the plant naturally shifts its focus from producing new, tender stalks to storing carbohydrates in its underground root crown.
The late-season stalks produced during this energy storage phase are significantly different in texture and flavor. They become tougher and stringier as the plant directs resources toward structural rigidity. The taste also tends to become more intensely sour or bitter, diminishing the overall culinary quality.
A plant that is continually harvested late into the season is stressed, diverting energy away from necessary root replenishment. This excessive stress can weaken the plant, leading to lower yields and less vigorous growth the following spring. Late in the summer, rhubarb is also more likely to “bolt,” sending up a tall, thick flower stalk, which signals the plant is diverting energy toward reproduction instead of producing edible stalks.
Guidelines for Safe Harvesting
The optimal window for harvesting rhubarb is typically from early spring, as soon as the stalks are large enough, until the end of June or the first week of July. To ensure the plant’s health, only about one-third to one-half of the stalks should be removed over the course of the season, allowing the remaining leaves to photosynthesize and replenish the root crown. If the plant experiences significant stress, such as a prolonged drought or excessive heat, the harvest should be stopped even earlier.
When removing the stalks, the best technique is to grasp the stalk firmly near the base and twist and pull it away from the crown, rather than cutting it with a knife. Pulling the stalk cleanly minimizes the risk of leaving a stub that could rot and introduce disease into the plant’s crown. The leaf blades must be immediately trimmed from the stalk and discarded, as they contain the highest concentration of toxic oxalic acid.