Photo by Brian Lilley

Q: I am considering planting an African sumac, but my neighbor says this is not a good tree. Any advice?
A: The Arizona Native Plant Society (AZNPS) lists African sumac (Rhus lancea) as invasive. It produces an abundance of seeds and spreads by underground suckers into natural areas, competing with native trees for water, notes University of Arizona Maricopa County Cooperative Extension Master Gardener Christine Bahto. Wherever you see one mature sumac tree, there are most likely dozens of saplings growing nearby. “I removed sumac in my backyard three years ago, and seedlings still pop up,” she says, and reports that her neighbor has one growing out of the trunk of a canary date palm. As alternatives, AZNPS suggests planting sugar sumac (Rhus ovata) or velvet mesquite (Prosopis velutina). For more information about invasive plants, download the society’s brochure at aznps.org.

Q: I have gorgeous tomato plants growing in a raised bed filled with mulch. They’re enormous, leafy and dark green. However, there are very few blossoms and even fewer tomatoes. How do I improve fruit production?
A: Your tomato plants may be receiving too much nitrogen, a nutrient that produces lush, green foliage at the expense of flowers and fruit, says Maricopa County Cooperative Extension Master Gardener Dick Gross. If you are fertilizing regularly with a nitrogen product, reduce applications. (Nitrogen is the first number on the fertilizer package. For example, 15-30-15 contains 15-percent nitrogen by weight, 30-percent phosphorus and 15-percent potassium.) Phosphorus promotes flowering and fruiting; however, it does not move readily through soil, as does nitrogen. Phosphorus is most effective if placed in the bottom of the hole at planting time, although you could scratch some into the soil around the plant, taking care not to damage roots. Also, flowers that don’t set fruit may not have been pollinated by bees or other insects. Try hand-pollinating by transferring pollen from one flower to another with a small artist’s brush or cotton swab. Gently tapping the plant early in the day, when pollen is more viable, is sometimes sufficient to transfer pollen.

Q: What are the tiny oval bright-yellow bugs I am finding on my oleanders?
A: These vividly colored insects are called oleander aphids (Aphis nerii), explains University of Arizona Pima County Cooperative Extension Master Gardener Linda Drew. They regularly appear on desert milkweed (Asclepias subulata), as well. They usually do not cause any harm and can be washed off with a forceful spray of water. However, some gardeners leave them alone to attract beneficial insects to the garden. Lady beetle and green lacewing larvae consume aphids voraciously. The parasitic wasp—another beneficial insect—also targets many oleander aphids; infested aphids swell, turn brown and die. There is no need to fear this wasp species; they are only interested in the aphids.