GV?Gardeners: Oleander — loved for beauty, maligned for invasiveness

Oleander (Nerium oleander), a native of the North African deserts, is one of the most heat- and drought-tolerant shrubs in our area.

There are hundreds of varieties, which vary greatly in size and bloom color. On the negative side, it has been much overused in our landscapes. When it gets too much water, it grows into a towering giant that smothers small properties.

Its invasive roots may be running amok beneath our yards. Oleander can sink its python-size roots into the ground, where it can destroy underground pipes and crack or raise walls. A backhoe may be necessary to remove mature plants and their roots.

These vigorous growers can produce a dense hedge up to six feet high in three growing seasons of favorable conditions. Plants are generally half as wide as they are high, and mature oleander can easily reach 25 feet tall and taller. An area example is located on the south side of Continental Road from the post office east to Abrego Drive.

Stems are cane-like and are densely covered to the ground with dark green, narrow leaves. Because of their overwhelming size, oleanders are best used at the edges of larger properties. Some plants have been trimmed up as trees for use in smaller intimate spaces such as patios. Because of their dependability, they are also used in median strips or as street trees.

Flowers may be single or double, with heaviest bloom in mid- to late spring. Some will bloom throughout summer until nights become cool.

There are varieties with red, pink, salmon, soft yellow, or white flowers. Many have a sweet fragrance. Pods may follow, releasing airborne seeds which can cause hay fever. They are also known to often reseed where they are not wanted.

“Sister Agnes” is the largest and most vigorous grower. It is a white single-flowering specimen. The more refined “Petite” series is a fairly recent introduction, available in pink, salmon, red, and light yellow. These are dwarf plants growing 3 to 5 feet high. They are compact enough to be used in containers, or for low borders or background plantings.

Oleander revel in heat. However, they can be damaged by cold below 20 degrees and severely damaged at 10 degrees. Dwarf varieties are even more cold sensitive. All recover rapidly in spring.

There are more negatives. All parts of the oleander are poisonous. Cattle have been known to eat from illegally dumped oleander yard waste in the desert, resulting in illness or even death. The sap may easily irritate skin or eyes. Sometimes new leaves will yellow if the plants are overwatered, and along with spent flowers produce a lot of litter. Plants trained as trees prolifically sprout suckers from the base. Instead of being cut off, these need to be pulled out.

Yellow oleander aphids, scale, and warty bacterial growths called galls often occur on these plants. For galls, the plants must be cut back beyond the growth. To avoid spreading of the disease, between each cut disinfect with 50/50 bleach solution, both the pruning cut and the trimmers used.

Oleander blight, or leaf scorch, is a devastating disease that kills mature plants quickly. Oleanders 40 to 50 years old are dying in other areas. Spread by the smoke tree sharpshooter insect, it has appeared in California and recently as near as north-central Phoenix. Without a defense to this bacterial disease, many oleander will die.

University of Arizona plant pathologists feel that for the time being, this disease is isolated. The sharpshooter insect seems to prefer conditions where they are now found. However, it is suggested to watch for signs of leaf scorch disease: initial yellowing of leaves; browning and decaying of leaf tips and edges; and continuing decline of the plant over time.

In place of oleander, Arizona Rosewood (Vauzuelinia californica) a native look-alike is recommended. It too has dark-green, lance-shaped leaves, can grow as high as 20 feet, and produces clusters of cream-colored flowers. A slower grower than oleander, it takes about five years to reach its mature height. There is a rosewood just south of the entry at the Arid Garden, off Camino Encanto.

Among other large shrubs that can be planted in place of oleander are Hopbush (Dodonaea), pittosporum (P. tobira), Texas Mountain Laurel (Sophora secundiflora), sugar bush (Rhus ovata), and Texas olive (Cordia boissieri).

To quote Phoenix garden author Mary Irish, “To some, the oleander is a spectacular summer-flowering shrub; to others it is a monstrous pariah, indicative of all that is wrong in desert gardens.” Take your choice, but keep in mind that there are alternatives to this beautiful summer bloomer.

Mary Kidnocker is a Master Gardener who writes frequently about the subject from a local perspective. Her columns appear Sundays. She is president of The Green Valley Gardeners.