Rhubarb, a perennial vegetable, sometimes develops yellowing leaves, which can concern gardeners. While green leaves signify a healthy plant, discoloration is a common issue with various causes. Understanding these reasons helps diagnose and address the problem.
When Yellow Rhubarb Leaves Are Normal
Not all yellowing leaves indicate a problem. As the growing season progresses, especially towards its end, older, outer leaves naturally yellow and die back. This allows the plant to reallocate energy and nutrients to new growth or its root system for dormancy. This natural yellowing is usually confined to the oldest foliage and is not a sign of distress.
Identifying and Addressing Common Causes of Yellowing
When yellowing extends beyond the oldest leaves or occurs early in the season, it often points to an underlying issue. Several factors can contribute to problematic yellowing, each with specific symptoms and solutions. Identifying the cause is crucial for effective treatment.
Watering Issues
Both insufficient and excessive watering can cause yellowing. Underwatering leads to dehydration, causing leaves to wilt, shrivel, and yellow, often starting from the edges. Rhubarb requires consistent moisture, especially during hot, dry periods.
Conversely, overwatering saturates the soil, depriving roots of oxygen and leading to root rot. Symptoms include mushy, swollen yellow leaves and a general lack of vigor, sometimes with a foul odor. To correct watering, check soil moisture several inches deep; water thoroughly when the top two inches feel dry, allowing excess water to drain. Well-draining soil prevents waterlogging.
Nutrient Deficiencies
A lack of essential nutrients can cause yellowing leaves. Nitrogen deficiency typically causes older leaves to turn uniformly pale yellow, and growth may become stunted. Magnesium deficiency often results in interveinal chlorosis, where tissue between leaf veins turns yellow while veins remain green, usually affecting older leaves first.
Iron deficiency primarily impacts new leaves, causing them to yellow while their veins stay green. Addressing these deficiencies often involves soil testing to determine nutrient levels and pH. Based on results, applying a balanced fertilizer or specific supplements like chelated iron or Epsom salts can correct the imbalance.
Pests and Diseases
Certain pests and diseases can cause rhubarb leaves to yellow or show other discoloration. Aphids cluster on the undersides of leaves and stalks, sucking sap and causing leaves to curl and yellow. Spider mites also feed on plant sap, resulting in yellow splotches or stippling, often with fine webbing.
Fungal diseases like leaf spot appear as small, discolored spots that can expand, causing surrounding tissue to yellow. Rust, another fungal disease, presents as orange or red pustules on leaves, which can cause severe yellowing and premature leaf drop. Managing pests might involve insecticidal soap or encouraging beneficial insects. For fungal diseases, removing infected foliage, ensuring good air circulation, and sometimes applying fungicides can help.
Environmental Stressors
Environmental factors play a role in rhubarb health and can cause leaves to yellow. Excessive direct sunlight can lead to sunscald, where leaves develop yellow or bleached spots, sometimes turning brown and crispy. Extreme temperatures, both heat and cold, can stress the plant; heat stress may cause wilting and yellowing, while frost damage can result in blackened, water-soaked leaves that later yellow.
Incorrect soil pH can hinder nutrient uptake, even if nutrients are present. Rhubarb prefers a slightly acidic to neutral soil pH, ideally between 6.0 and 6.8. Adjusting soil pH with amendments like lime or sulfur can improve nutrient availability. Providing partial shade during intense heat or protecting plants from late frosts can mitigate temperature-related stress.
Promoting Overall Rhubarb Health
Maintaining a healthy rhubarb patch through consistent practices can prevent many issues that lead to yellowing leaves. Proper planting location is foundational; rhubarb thrives in full sun but tolerates partial shade, requiring at least 6-8 hours of sunlight daily. Ample spacing (3 to 4 feet between plants) ensures good air circulation and reduces disease spread.
Rhubarb benefits from fertile, well-draining soil rich in organic matter. Incorporating compost or well-rotted manure before planting and as an annual top-dressing provides continuous nutrients and improves soil structure. Regular monitoring for signs of distress allows for early intervention, preventing minor issues from escalating.