Why Are the Leaves on My Oleander Turning Yellow?

Determining what is turning the leaves of an oleander (Nerium oleander) yellow may enable you to save the shrub or small tree. Hardy in U.S. Department of Agriculture plant hardiness zones 9 through 11, oleander has abundant dark green leaves that form the perfect backdrop for its showy, colorful flowers. When diseases, pests or improper care get the better of this poisonous evergreen, however, yellow leaves and ultimately the death of the plant may be among the unpleasant, sometimes preventable, consequences.

Oleander leaf scorch is a disease caused by Xylella fastidiosa bacteria. Mainly found in California, the disease spreads from infected plants to healthy ones via 1/2-inch-long insects known as glassy-winged sharpshooters. When an oleander is infected, the transportation of nutrients and water from its roots to its leaves is blocked. That situation causes yellowing of the leaves from the tips inward, then drooping of the leaves, eventually followed by the death of the plant. Although pruning affected plant parts may preserve the aesthetics of the oleander temporarily, no cure for this disease exists. If you have other susceptible plants in your landscape, however, rather than pruning, consider removing and disposing of infected oleanders altogether to help stem the spread of the disease. Before pruning any plant, sanitize the blades of your pruning tool by wiping them with a paper towel or clean rag that's been dipped in isopropyl alcohol, which also helps prevent the spread of disease.

Pests such as the oleander aphid and false oleander scale suck sap from the plant, robbing it of nutrients, and may be to blame for yellow oleander leaves. Oleander aphids are small, soft-bodied, yellowish insects that tend to roam on the undersides of the leaves, reproducing prolifically, excreting honeydew and attracting ants. To help manage a large infestation, knock the insects off with a forceful spray of water. Apply water as often as needed early in the day so the moisture has a chance to evaporate by nightfall. To detect an infestation of false oleander scales, start looking for small, white flecks on new leaves in early spring and manually scrape them off if there are just a few. If the infestation is large, spray both sides of the leaves thoroughly with ready-to-use horticultural oil three times at five- to six-week intervals. Prepare the plant for spraying by watering it thoroughly the day before; limit spraying to late afternoon or early evening when temperatures are between 45 and 90 degrees Fahrenheit.

Although quite drought-tolerant most of the year, oleander benefits from watering during hot, dry summer weather. Insufficient watering during that time can cause drought stress, which triggers the root tips to die and the leaves to yellow uniformly. Encourage the oleander to recover by providing at least 1 inch of water each week, lowering the amount after rainfall. A 2-inch-thick layer of mulch, such as pine needles or grass clippings, can promote soil-moisture retention. Also, grow oleander in well-drained soil because wet, soggy soil can trigger root rot, which can impede nutrient uptake.

Although an established oleander often doesn't need fertilizer, yellow or light-green, small leaves and sparse flowering may be indications of a lack of nutrients. A fertilizer boost three times annually, using a complete fertilizer at a rate of 1/2 pound of nitrogen per 250 square feet, can be beneficial. Sprinkle the fertilizer over the oleander's root zone, or 1 1/2 to 2 times the canopy diameter, and thoroughly water it into the soil.