Rhubarb, a perennial vegetable often used as a fruit, is a favored addition to many gardens. Its large, vibrant green leaves are crucial for its growth. However, yellowing leaves signal underlying issues, prompting gardeners to investigate. Understanding why rhubarb leaves yellow is the first step toward restoring plant health.
Common Reasons Rhubarb Leaves Turn Yellow
Yellowing rhubarb leaves can stem from several distinct problems.
Nutrient Deficiencies
Rhubarb plants are heavy feeders, requiring ample nutrients. A lack of essential nutrients like nitrogen, magnesium, or iron often manifests as yellowing foliage. Nitrogen deficiency typically causes an overall yellowing of older leaves, while insufficient iron or magnesium can lead to yellowing between the veins, with the veins remaining green.
Watering Imbalances
Both too much and too little water can cause rhubarb leaves to yellow. Overwatering leads to waterlogged soil, depriving roots of oxygen, potentially causing root rot and hindering nutrient uptake. Conversely, underwatering stresses the plant, leading to wilting and shriveling before leaves turn yellow. Rhubarb thrives in moist, but not overly wet, soil.
Environmental Stressors
Rhubarb prefers cooler climates and can struggle in excessive heat. Intense sunlight can scorch leaves, causing them to turn yellow or brown (sunscald). Incorrect soil pH, ideally between 6.0 and 6.8, can also affect nutrient availability.
Pests and Diseases
Various pests and diseases can contribute to leaf yellowing. Pests like aphids and spider mites feed on plant sap, leading to discoloration and stunted growth. Fungal diseases such as crown rot can cause leaves to yellow, wilt, and eventually die. Leaf spot diseases can also cause yellow spots that may turn brown.
Natural Senescence
Older, outer rhubarb leaves naturally yellow and die back as part of the plant’s life cycle. This process, known as senescence, allows the plant to reallocate resources to new growth or storage in the crown. It is a normal occurrence, typically affecting only the lower, older foliage.
Diagnosing the Cause of Yellowing
Determining the reason for yellowing rhubarb leaves involves careful observation. Examine the yellowing pattern on leaves (entire leaf, edges, or between veins) for clues about nutrient deficiencies. Check soil moisture levels a few inches deep and inspect the underside of leaves and the plant’s crown for pests or disease signs like spots, holes, or mushy areas. Consider recent weather patterns, such as extreme temperatures or prolonged dry spells, which might indicate environmental stress. If multiple factors seem possible, a soil test can provide detailed information about nutrient levels and pH.
Effective Solutions and Prevention
Addressing yellowing rhubarb leaves requires targeted solutions and preventative measures. If nutrient deficiencies are the cause, apply a balanced fertilizer or organic matter like well-rotted manure or compost in early spring. For nitrogen deficiency, a nitrogen-rich fertilizer can be beneficial, while iron chelate can address iron deficiencies. Ensure the soil pH is within the optimal range of 6.0 to 6.8, adjusting if necessary.
For watering imbalances, establish a consistent schedule with deep, infrequent watering, allowing the top 2-4 cm of soil to dry out between waterings. Improve soil drainage by incorporating organic matter into heavy clay soils or by ensuring the plant is not in a waterlogged area. To mitigate environmental stress, provide afternoon shade during hot periods to prevent sunscald. For pest issues, consider organic pest control methods like hand-picking or insecticidal soap. If fungal diseases like crown rot are present, remove affected plant parts and ensure good air circulation.
Harvesting Rhubarb with Yellow Leaves
When rhubarb leaves turn yellow, gardeners often question the edibility of the stalks, though minor yellowing of older or outer leaves due to natural aging or slight environmental stress generally does not impact their safety or quality. The edible stalks remain safe for consumption even if some leaves have yellowed. However, if the plant is severely affected by disease, extensive pest infestations, or if the stalks themselves show signs of rot or significant discoloration, discard them. Remember that only the rhubarb stalks are edible; the leaves contain high concentrations of oxalic acid and are toxic to humans and animals, regardless of their color. Always remove and discard the leaves before consuming the stalks.