Rhubarb (Rheum rhabarbarum) is a robust perennial vegetable, often one of the first plants to emerge in the spring garden. Despite its hardy nature, a sudden decline in health indicates that its cultural requirements are not being met. The cause is typically related to its environment, the soil, or a biological threat. Understanding the specific symptoms associated with each problem helps diagnose and reverse the plant’s distress.
Stress from Sun, Water, and Temperature
Rhubarb is a cool-weather crop, and its health is compromised by prolonged exposure to high temperatures. It performs best when summer temperatures remain below 75°F; periods above 90°F trigger a rapid decline in production. When subjected to excessive heat, the plant often wilts dramatically and stops producing edible stalks.
A severe reaction to temperature stress is “bolting,” where the plant prematurely sends up a tall flower stalk from the center of the crown. Bolting diverts the plant’s energy away from producing consumable petioles and towards reproduction, weakening the plant for the current and following season. Removing the flower stalk immediately by cutting it near the base slows the energy drain. To prevent recurrence, the underlying heat stress must be addressed, often by planting rhubarb in a location that receives afternoon shade in warmer climates.
Water management requires a delicate balance, as rhubarb needs consistent moisture but is highly susceptible to “wet feet.” Wilting leaves signal drought stress, corrected with deep watering. However, wilting combined with stunted growth often indicates root or crown rot. The crown should never sit in saturated soil, as this encourages soil-borne fungal pathogens. To prevent rot, ensure the crown is planted slightly above the soil line or on a small mound to encourage drainage.
Soil Health and the Need for Division
Rhubarb is a heavy feeder, requiring a significant supply of nutrients, particularly nitrogen and potassium. When the soil is depleted, growth becomes stunted and stalks are thin and weak. Nitrogen deficiency appears as uniform yellowing (chlorosis) across the older, lower leaves as the plant moves this mobile nutrient to support newer growth.
Potassium deficiency is characterized by yellowing or browning (necrosis) starting at the tips and margins of older leaves. This condition, known as marginal necrosis, indicates the plant struggles to regulate water and transport nutrients. Annually amending the soil with well-rotted compost or manure in early spring supplies these macro-nutrients and helps maintain the slightly acidic soil pH (6.0 to 6.8) that rhubarb prefers.
Soil compaction, especially in clay-heavy soils, restricts the extensive root system. Compacted soil hinders the roots’ ability to access water and nutrients, starving the plant and exacerbating drainage issues. Furthermore, an established crown that has not been divided in five to eight years will become overcrowded and decline. Dividing the dormant crown every few years rejuvenates the plant, resulting in renewed vigorous growth.
Identifying Pests and Fungal Diseases
Rhubarb is susceptible to specific biological threats, most notably the soil-borne fungal disease known as Crown Rot. This destructive disease causes the crown tissue to become mushy, brown, or black, often accompanied by a foul odor. Above-ground symptoms include wilting foliage and a lack of vigor. Once the rot is extensive, the plant cannot be saved and must be removed and destroyed to prevent the fungus from spreading.
Fungal Leaf Spot, caused by organisms like Ascochyta and Ramularia, is a cosmetic issue, manifesting as small, reddish-purple spots on the leaves. Although the infection can reduce photosynthesis, it rarely threatens the plant’s life. Management involves removing affected leaves and ensuring good air circulation. Proper garden sanitation, such as removing dead foliage from the base, is a preventative measure against fungal threats.
The most common serious insect pest is the Rhubarb Curculio (Lixus concavus), a large snout beetle that feeds on the stalks and crowns. Its presence is identified by circular feeding and egg-laying sites on the stalks, which often ooze a sticky sap (gummosis). While the Curculio rarely kills the plant directly, the damage creates entry points for fungal and bacterial diseases, increasing vulnerability to Crown Rot. Management focuses on hand-picking the adult beetles and removing nearby weeds, such as dock and thistle, which serve as alternate hosts.