Oleanders (Nerium oleander) are evergreen shrubs with showy blooms and deep green foliage. While most varieties of the fragrant perennials grow 8 to 12 feet tall and equally wide, others grow up to 20 feet in height, while dwarf varieties grow between 3 and 5 feet tall. Their long blooming seasons make oleanders a wonderful addition in gardens. Yellowing leaves on an oleander indicates a variety of problems that require immediate attention.
Caused by Xylella fastidosa bacterium, oleander leaf scorch is a serious disease that kills the plant in extreme cases. The bacteria spread through insects feeding on the plant's xylem. Symptoms are similar to drought but cause yellowing and drooping leaves on one branch at a time. Left untreated, leaf margins turn deep yellow or brown and die. Although leaf scorch occurs at any time of the year, it is more common during warm temperatures of late spring and summer. Because oleander leaf scorch has no cure, heavily infected plants eventually die. Pruning infected parts prevents the disease from spreading if it is caught in time.
Improper irrigation or poor drainage causes the leaves of an oleander to appear yellow. Oleanders cannot tolerate over-watering, especially when planted in soils that do not drain well. Rain or irrigation water that stands around the base of the plant prevents roots from absorbing oxygen and carrying it to the foliage, thus causing stressed leaves to turn yellow. The plant develops root rot, a condition that causes the roots to turn black, slimy and emit an unpleasant odor. Ideally, water your oleander when the soil begins to feel dry to the touch. Instead of overhead irrigation, use a trickle irrigation system that provides water to the roots, without wetting the leaves.
Lack of rainfall or irrigation dehydrates the plant, especially during periods of prolonged sun exposure. The leaves of drought-stressed oleanders turn yellow along the central veins. Severe drought adversely affects plant growth and flower production. Unless the damage is severe, drought-stressed plants bounce back to health with proper cultural practices. However, severely stressed oleanders drop yellow leaves that fail to regain their natural color and vitality.
Proper cultural practices strengthen the oleander bush and prevent different problems, including yellowing leaves. Grow the oleander in well-draining soil with full sunlight exposure. The plant tolerates a variety of soils, from moist clay to dry and sandy. Although established oleanders are moderately drought tolerant, provide the plant 1 to 2 inches of water every week, especially during the summer, to promote flowering. Similar to watering, established plants survive without nutrition, but providing 2 lbs. of a well-balanced fertilizer two times a year, once in early spring and again in fall, encourages healthy growth and abundant flowering. Avoid over-fertilizing the plant, because excess nitrogen causes yellowing leaves.