Q: I wrote earlier in the week about those pesky bugs that attack our Texas mountain laurel every year, and they have already started attacking this year. I am including a photo of the critter so that you might recommend something specific to get rid of them. There are too many to hand pick and they move really fast when trying to get away.

A: The critter is Uresiphita reversalis, aka the genista caterpillar. They typically feed on new growth at the ends of branches on Texas mountain laurel. I recommend a spray of Bt (Bacillus thuringiensis) where they are feeding. Bt is a naturally occurring soil bacterium that causes disease in certain insects. Bt is considered ideal for pest management because of its specificity and because of its lack of toxicity to humans or the natural enemies of many plant pests. There are different strains of Bt, each with specific toxicity to particular types of insects so be sure to choose the strain that is labeled for caterpillar pests.

Q. When I bought my home in 2004 (built in 1986), there was a huge gardenia bush so tall that it covered the picture glass window of the living room. The house faces north. We trimmed the bush down a little so sunlight would come through the window. A few months later, I was astounded to see the bush covered with white blossoms that I recognized from proms. Beautiful fragrant gardenias! I think the bush might be as old as the house. After that, once or twice a year, we would be treated to more gardenias blooming. However, it has not bloomed again for over two years. I would water it deeply periodically, but not too often. The foliage thinned out. I added some rose food that I found in the garage and watered it again. Now, it looks even worse, becoming thin and scraggly-looking. I fear it might be dying. What can I do to bring this gardenia bush back to health?

A: The photo is out of focus but what I can see is likely in need of selective pruning and a fertilizer specifically for that plant. Unfortunately, gardenias are not well suited for our alkaline soils and require occasional help obtaining the proper nutrients for their growth, specifically iron. The optimum pH for gardenias is between 5.0 and 5.5. Around these parts the pH is typically closer to 8.0. The higher pH prevents iron in the soil from being available to the plant. Now is a good time to fertilize with something to address that need. Choose a complete fertilizer labeled for acid-loving plants. Something with a nitrogen-phosphorus-potassium (NPK) ratio around 15-5-15 would be good. You should also spread a half-inch layer of compost around the root zone because they do better with more organic matter. It’s good that you are watering deeply and infrequently because gardenias also don’t tolerate salty soil and water can wash the salt through the root zone. The ideal watering schedule would be once every two weeks to a depth of 24 to 36 inches. Leaf drop is often caused by lack of water in the desert so it’s good you noticed this symptom. Dropping leaves is a plant’s defense against water loss.

Q: What can we do to save the life of our beloved cactus! It started in December with some of the brown spots and now it is spreading. It has always been protected for frost with Styrofoam cups on tips and a burlap wrap. We have noticed other cacti (Mexican Fence Post) here in Saddlebrooke have the same markings. Any help would be greatly appreciated.

A: Brown spots on cacti may be caused by a few different things including too much sun on newly planted cacti, damage from animals, frost, hail, some pesticides, and diseases. The spots on your cacti appear to be fungal lesions that sometimes occur when we have cool, wet weather as we did this winter. The damage is usually cosmetic and won’t cause the death of the plant but it is also irreversible so it will remain as a scar. Now that summer-like weather has begun you should see less of this disease occurring in your neighborhood.

Q: We had a mountain laurel tree planted in March of 2016. Thought we followed the instructions for watering before leaving for Minnesota in May. While we were gone it was watered by the drip system. On our return in October the leaves were light green so took a sample to Civano Nursery and they said it showed that there was too much salt from not being watered deep enough. They recommended watering on a slow drip for ⅔ hours once a week, which we’ve been doing. The leaves are still pale green, not showing any improvement. Now we’re wondering if there’s any way to save the tree. Do you have any advice for us?

A: Given our salty soil and the salt contained in fertilizers, it is certainly possible your tree has an excess of salt in the root zone. A soil test would confirm this diagnosis but watering it deeply might confirm as well. The watering schedule they suggest is good for newly planted trees. Since your tree has been in the ground for a year, you can back it off a bit and water deeper. Established Texas mountain laurel trees should be watered deeply every two or three weeks to a depth of 24 to 36 inches to effectively wash the salt through the root zone. Because not all drip systems are created equal, it would be good to measure the water depth with a soil probe to make sure the running time is correct. A soil probe can be as simple as a piece of rebar that you push into the ground after irrigating to see how deep the water went. Dry soil will provide more resistance than the wet stuff and that’s how you will know when to pull out and measure the depth.

Peter L. Warren is the urban horticulture agent for the Pima County Cooperative Extension and the University of Arizona. Questions may be emailed to tucsongardensage@gmail.com