From a Correspondent:
Unlike one of your recent correspondents who wished to kill his oleander hedges, my partner and I, who are new to Arizona, would very much like to nurture ours. Unfortunately, we've received conflicting advice about how much to water them. One neighbor recommends a brief daily drip; another suggests a single monthly soak. What is the best method for watering oleanders in the desert?
Ben, Tucson, Ariz.
Good morning Ben,
For all the disparaging remarks to which oleanders (Nerium oleander) are subject (and I freely admit to maligning them on a regular basis), there is no doubt that they have certain qualities that recommend their place in the Desert Garden. They're green, tough, heat-and sun-tolerant, fast growers, they spend much of the year covered in big bright flowers, and they make very good screening plants. Out of respect for your desire to nurture yours, I won't mention that they are large, poisonous, overbearing weeds capable of taking over the world.
Neighbor number two called this one correctly: Oleanders do best with deep but infrequent watering, as do all plants in the Desert Garden. The idea is to develop a deep and strong root system to support the plant and to help it tolerate drought. Plant roots will only develop in the presence of moisture, so if the moisture is confined to the top layer of soil, as a result of shallow watering, then the roots will tend to stay in the top layer as well, making them more subject to damage from sudden or prolonged drought. Water no more than once a month, and water so that the soil is moist a good two feet down; during rainy times of the year, skip the watering altogether.
Most oleanders that I've encountered receive far more water than they need to remain healthy and attractive; I think we tend to believe that any plant with large colorful flowers needs lots of water. Too much water on an oleander leads to vigorous and soft growth, which then calls for severe pruning to maintain the plant's size and shape. Heavy pruning on an oleander also leads to vigorous and soft growth, and in short order you're in a never-ending cycle of pruning, watering, and pruning again.
Also pay attention to what water your oleanders may be getting from other parts of your or your neighbors' landscape. An oleander planted alongside turf (even on the other side of a neighbor's wall) may be getting all the water it needs from the grass nearby.
Watering deeply and infrequently is the best way to nurture your oleander and be certain that it develops the deep, strong, and healthy roots that future residents of your house will someday curse to Heaven.
I hope this helps,