Rhubarb is a hardy perennial vegetable prized for its edible stalks. When rhubarb leaves turn yellow, a condition known as chlorosis, it signals that the plant is unable to produce sufficient chlorophyll, the green pigment necessary for photosynthesis. This yellowing is a non-specific symptom, requiring a careful diagnostic approach to uncover the underlying cause, which can range from nutrient imbalances to environmental stressors or biological threats. Understanding the specific patterns of yellowing is the first step toward restoring the plant’s vigor.
Nutrient Deficiencies and Soil pH
Rhubarb is considered a heavy feeder, requiring a rich supply of nutrients to support its large, rapid-growing foliage. The availability of these nutrients is heavily regulated by the soil’s acidity or alkalinity, measured as pH. Rhubarb prefers a slightly acidic to neutral soil environment, thriving in a pH range of 5.5 to 6.8; outside this range, essential minerals can become chemically locked away, even if present in the soil.
A general yellowing, especially in older, lower leaves, often indicates a nitrogen deficiency, as nitrogen is a mobile nutrient the plant moves to new growth. Magnesium deficiency typically presents as interveinal chlorosis, where the tissue between the leaf veins turns yellow while the veins themselves remain green. Conversely, a lack of iron or manganese causes yellowing in the newest leaves first, because these are immobile nutrients that cannot be relocated from older tissue.
If the soil pH is too high (alkaline), iron may become unavailable, causing the yellowing of new growth, even if iron levels are adequate. A simple soil test is the most reliable way to assess both nutrient levels and pH, allowing for targeted correction through balanced fertilization or soil amendments like sulfur to lower pH. Annual additions of organic matter, such as compost, help buffer pH changes and provide a steady supply of micronutrients.
Improper Watering and Environmental Stress
Physical conditions related to moisture and temperature frequently cause rhubarb leaf yellowing, often mimicking nutrient problems. Rhubarb requires consistently moist but not saturated soil, performing poorly in dry or waterlogged conditions. Overwatering is a common mistake that leads to root suffocation, preventing roots from absorbing oxygen and nutrients, which results in yellowing and wilting foliage.
Drought stress can cause leaves to wilt dramatically, often followed by the edges turning yellow or brown and becoming crisp. The large leaves transpire a significant amount of water, making the plant susceptible to drying out during hot, dry spells. Proper soil drainage is necessary to maintain the ideal balance of moisture and air pockets around the roots.
Rhubarb is a cold-climate perennial that struggles with prolonged heat stress. When summer temperatures consistently exceed 75°F, or especially 90°F, the plant’s growth slows, and leaves may yellow and die back prematurely. Providing afternoon shade in warmer regions and ensuring deep, consistent watering during hot periods can mitigate this stress.
Identifying Pests and Pathogens
Biological threats, including insects and diseases, can damage the plant’s vascular system, leading to chlorosis. The Rhubarb Curculio, a snout beetle up to three-quarters of an inch long, punctures the stalks for feeding and egg-laying. These punctures cause sap leakage (glistening, gummy drops) and can lead to dark streaks and decay, resulting in wilting and yellowing of the associated leaves.
Sucking insects like aphids can also cause yellowing by feeding on plant sap, often concentrating on the undersides of leaves and along the stalks. Large infestations extract moisture and nutrients, causing leaves to curl and turn yellow, while also potentially spreading viral diseases that cause mottling or ring spotting.
Pathogens, particularly fungi and bacteria, present serious threats. Crown Rot, caused by various fungi like Phytophthora, is the most devastating disease, typically occurring in poorly drained or waterlogged soil. Initial symptoms include wilting and yellowing leaves, but inspection of the crown reveals mushy, brown, or black decay at the base, which eventually kills the entire plant. Fungal Leaf Spot diseases, such as those caused by Ascochyta, begin as small spots that may have white centers and yellow margins, eventually causing the entire leaf to yellow and wither.
Assessing Damage and Intervention Strategies
Once the specific cause of yellowing is determined, immediate and long-term actions can protect the rhubarb plant. If a nutrient deficiency is the culprit, applying the correct fertilizer, such as a nitrogen-rich or balanced formula, will quickly resolve the issue once the soil pH is corrected. For pest issues like the Rhubarb Curculio, the most effective intervention is often hand-picking the large, visible adults from the stalks and leaves, as chemical controls are often ineffective.
In cases of fungal diseases like Crown Rot, intervention is limited. The entire plant should be dug up and destroyed to prevent the pathogens from spreading to other plants in the bed. For localized problems like leaf spots, removing and destroying the infected leaves and stalks immediately can limit the spread of fungal spores. When pruning damaged leaves, remove only the stalk and leaf blade, ensuring the crown—the central growing point—remains undisturbed.
Long-term prevention focuses on improving the growing environment, including ensuring excellent drainage, avoiding fertilization directly on the crown, and maintaining good garden sanitation. Yellowed or dead rhubarb leaves contain high levels of oxalic acid and must never be consumed. Mulching around the plants helps regulate soil moisture and temperature, but keep the mulch away from the crown to prevent moisture build-up that encourages rot.