Walnut Creek's Ruth Bancroft is a national authority on drought-resistant gardening. Twice a month, she and her staff share their knowledge with readers.
Q I bought a little cactus at a garage sale and put it out on my deck when I got home. Now it is looking kind of bleached and unhappy, and my husband thinks it is sunburned. This does not make sense to me; if a cactus endures the scorching sun of the desert, how could it get sunburned on my deck?
A First of all, the cactus family is a large one, and the habitats where different kinds grow can vary tremendously. Jungle cactuses grow perched on branches in filtered light. Cactuses also are found along the foggy coast of Baja California. The native habitat of some varieties is high in the Andes of South America. Others grow happily in the blazing heat of the Arizona desert.
While the light level is important, it is not the only consideration. A plant growing in full sun at a high altitude might not have the heat tolerance needed for a place like Phoenix. Learning about where plants are found in nature can offer important clues as to what conditions they prefer.
Another important aspect is that one must take care when moving plants from one place to another. If a cactus has been growing with one side toward the south, where the sun beats on it, and the other side to the shadier north, turning it 180 degrees could be a big mistake. When the side that has been shaded suddenly is hit with direct sun, it can burn in a matter of hours.
The same concern is important when a plant has been growing under shade-cloth or in a greenhouse, where some of the sun's rays are filtered out. If the plant is suddenly moved to full sun, it easily can be scorched, even if its native habitat is a hot desert. It is the abruptness of the change that causes the problem. Moving a plant little by little into successively sunnier places gives it time to adjust.
Alternatively, you can drape a couple layers of screen or shadecloth over the plant when moving it into the sun. After two weeks, remove the top layer; after two more weeks, the remaining layer can come off. It's best to remove each layer on a cloudy day, if possible.
After a cactus or other succulent is sunburned, the affected area becomes bleached out and remains permanently scarred. This is not serious for a plant such as an Echeveria, which soon grows new leaves and sheds the old ones. However, a cactus stem stays disfigured.
Sometimes a cactus will respond to hot weather by changing from green to yellow-green. This change does not leave a scar, and the plant normally greens up again when temperatures drop.
Email questions on drought-resistant plants to email@example.com.