Daylilies are popular garden plants, but their green leaves can turn yellow. This yellowing is a symptom of various underlying conditions.
Watering and Drainage Issues
Improper watering often causes yellowing. Overwatering saturates soil, depriving roots of oxygen and causing root rot, which makes roots slimy and brown, preventing nutrient absorption and leading to wilting.
Insufficient watering or drought stress also causes leaves to yellow and dry. Daylilies thrive with about one inch of water per week, including rainfall.
Poor drainage exacerbates watering and nutrient issues. Heavy, compacted soils or inadequate drainage lead to waterlogging, hindering root air circulation. Daylilies perform best in moist, well-drained soil; improving soil structure helps.
Nutrient Deficiencies
Nitrogen deficiency typically causes general yellowing of older leaves, as the plant reallocates it to new growth. This yellowing may start at leaf tips and progress in a V-shaped pattern along the mid-rib.
Iron deficiency often manifests as interveinal yellowing, with tissue between veins yellowing while veins remain green. This symptom appears first on newer leaves. Less frequent deficiencies in micronutrients like magnesium can also cause leaf discoloration.
Soil pH plays an important role in nutrient availability. Daylilies prefer slightly acidic soil, ideally 6.0-6.5 pH. Outside this range, nutrients become unavailable. Imbalanced pH can also cause root burn, impairing water and mineral absorption.
Pests and Diseases
Sap-sucking insects are common culprits. Aphids, small, soft-bodied pests, feed by extracting sap, leading to deformed, yellowed leaves. They are often prevalent in cooler spring weather and excrete honeydew, promoting sooty mold.
Spider mites, tiny arachnids, cause yellow speckles or stippling on foliage, especially in hot, dry conditions. Heavy infestations can bleach leaves, causing fine webbing. Thrips, another small insect, create silvery streaks on leaves, and their feeding can stunt or distort developing flowers.
Daylily rust is characterized by small, raised orange pustules on the undersides of leaves, releasing dusty, yellowish-orange spores. This causes yellowing and browning. Daylily leaf streak presents as elongated yellow streaks along the leaf mid-vein, often progressing from the tip, causing browning.
Crown rot is a severe condition where the entire plant yellows and collapses due to rot at the crown. Various fungi and bacteria can cause this rot, often triggered by environmental stress.
Environmental Stress
Sun scorch occurs when daylilies are exposed to excessive direct sunlight, especially after transplanting from shade. This can bleach or yellow leaves. Darker-colored daylily cultivars are sometimes more prone to fading or scorching in strong afternoon sun.
Temperature extremes, such as sudden cold snaps or prolonged heat waves, can stress daylilies, leading to yellowing. While daylilies are hardy, severe thermal stress impacts physiological processes. In response to cold, daylilies may naturally enter dormancy, causing leaves to yellow and eventually brown.
Accidental exposure to chemicals is another cause of yellowing. Herbicides, certain pesticides, or excessive fertilizer applications can cause chemical burn and discoloration. Over-fertilization can result in a buildup of salts in the soil, burning roots and impairing nutrient uptake, mimicking nutrient deficiency.
Natural Causes and Other Factors
Natural aging commonly yellows older, lower leaves. As the plant matures and new foliage emerges, older leaves are naturally shed, especially as cooler temperatures arrive in fall. This process is normal energy reallocation and dormancy preparation, as photosynthesis ceases.
Transplant shock can also cause temporary yellowing. When moved, daylilies experience stress as roots adjust to the new environment. This temporary disruption manifests as yellowed leaves until established.
Overcrowding leads to competition for light, water, and nutrients. When plants are too densely packed, inner or lower leaves may yellow due to insufficient light and soil resources. Daylilies grow rapidly and form dense clumps, necessitating occasional division for vigor and health.