Wet, humid spring weather caused influx of aphids

Q: We have lots of aphids in our good-sized oak trees that are dropping sap on our cars and the sidewalk. Short of cutting the trees down, do you have any suggestions on how to get rid of the aphids?

A: Blame their high population numbers this year on our wet and humid spring weather. The fastest and probably cheapest way to get rid of them is to drench the soil beneath the tree with a systemic insecticide diluted in a bucket of water. This dilution will help you spread it more evenly everywhere under the tree.

The pesticide I’m telling you to use has the active ingredient imidacloprid in it. This insecticide is systemic, so when you pour this around the base of the tree, diluted in water, the roots will take it up and transport it to the leaves. Because the aphids are sucking juices out of the leaves, they ingest this toxin and die.

There are several trade names of this pesticide that contain imidacloprid. The most popular with homeowners is the Bayer product with the trade name Tree and Shrub Insect Control. Any product that you use must contain imidacloprid in the ingredients. The easiest and probably safest way to apply it is a soil drench, but it can be sprayed on the leaves.

The label will tell you how much of this pesticide is applied to one tree. Follow the directions on the label for a soil drench application.

When you are finished with the application, rinse the bucket three times with fresh water and apply these “rinsates” under the tree. Wear unlined plastic gloves and eye protection when mixing and applying this product and rinse and dispose of them afterward.

Consider spraying these trees during the wintertime from top to bottom with a dormant oil. Apply an extra dose to the base of the tree where aphids might be hiding. Dormant oil sprays applied during the winter are effective in reducing the populations of aphids, scale insects and spider mites for the next growing season.

Ants love aphids. They milk ants of this sugary sap for their own use. If you look at the tree trunks carefully, you will see a steady stream of ants going up and down the trunk. They are also part of the aphid problem and should be controlled.

Follow the stream of ants back to the opening of the underground nest. Sprinkle 10 or 15 crystals of ant bait directly at the opening of the nest. I use a product called Amdro. The nest will be empty the next day.

Q: I planted two mandarin oranges from different nurseries this past spring. One did very well in full sun from the get-go. The other had leaves that were turning yellow. I applied iron and nitrogen fertilizers, but it didn’t do much after a week. So, I constructed some shade over it, and it started looking green again.

A: It sounds like you’ve got your answer; intense desert sunlight caused leaf yellowing. But it begs the question: Why only one of the trees if they were both mandarin orange? You said the trees came from different nurseries. That might be part of the answer.

But first, eliminate other potential problems before I get to the nursery explanation. Make sure your trees share the same microclimate, the soil in both locations was similar with similar drainage, the soil mix used to plant both trees was the same, that the roots were kept moist and planted no more than a half inch below the soil surface, and that they were staked and watered thoroughly after planting.

You might be observing differences in how and where your trees were grown versus our harsh, desert climate. A local nursery used to bring in citrus trees for sale in containers and put them in an area that had partial shade. They could put them in full sun and sell them from there, but they didn’t. Too risky.

Plants grown in a greenhouse under partial shade or in a cool, coastal climate produce leaves that are different from leaves grown during our harsh desert sunlight and low humidity. When tender plants are plunged directly into our strong sunlight and low humidity, the leaves might scorch, yellow or drop from the plant.

The plant is not dead but quickly produces a new set of leaves very different from its old ones. The newer leaves are smaller, thicker, tougher and better capable of handling desert sunlight and humidity. The plant has become acclimated.

Observe where nurseries are selling plants. Plants sold from shady areas might struggle when planted in full sun. Plants grown in a greenhouse need two or three weeks of acclimation before they are plunged into an intense desert landscape. That is true of vegetable transplants too.

If you suspect you have a plant that might be acclimating to its new environment, sometimes it’s easier to strip off the leaves or prune the plant so its new growth is better acclimated to its new desert environment. In your case, wait until fall and remove the shade. Let the mandarin orange acclimate to its new home during the cooler fall weather.

Q: I was thinking of using soaker hoses around my trees instead of drip emitters. Any advice on that?

A: You are calling them “soaker hoses,” but I prefer to call them “drip tubing.” Drip tubing is about a half-inch in diameter with drip emitters embedded in the tubing walls during its manufacture. Its best use is in areas that need water applied evenly to the same depth, repeatedly.

Drip emitters are added to blank tubing — no emitters in the sidewalls — after it’s installed. It’s best for watering individual, smaller plants with spaces between plants that should stay dry. Drip emitters are best for watering different kinds of plants of different sizes with varying amounts of water.

Drip tubing is ideal for watering trees over 20 feet tall. A coil of drip tubing can be placed under the canopy of a tree and enlarged as the tree gets bigger.

The embedded emitters in drip tubing should be 12 to 18 inches apart under the canopy of the tree. The length of tubing needed depends on how much water is applied. When water is applied under medium trees, it should penetrate 18-24 inches deep. When water is applied under large trees, it should penetrate 24-36 inches deep.

Q: We planted a butterfly bush that was doing good but suddenly took a wrong turn. I’m very grateful for any help and guidance.

A: There are problems sometimes using the common names of a plant. Do me a favor and Google “butterfly bush.” If what you are calling butterfly bush is Buddleia, then you have a bush that struggles in hot desert climates unless you keep it out of the afternoon sun. You must improve the soil around this plant at planting time and never plant it in rock, aka desert landscapes.

It is a shrub that grows well in Chicago. It can be planted as far south as Atlanta, so it probably will flower this far south, but I have never heard of it planted in the hot, desert Southwest.

In our climate, plant it on the east side of a building where it gets shade in the afternoons. If you planted this shrub in the wrong spot, baby it through the heat of the summer and move it to a new location in October.

Plant in soil amended with compost. Use wood chips at the base of this plant in a circle at least 6 feet in diameter. Water it to a depth of 12-18 inches and use four to six drip emitters after it becomes established. Two emitters should be enough for the first two years. Watering frequency would be the same as fruit trees and other non-desert landscape plants.

This shrub is beautiful when it flowers. It flowers on new growth, so it’s best if it’s pruned to the ground during the winter of each year after it is established. Its floral display depends on the plant vigor, fertilizers applied and its overall health. Apply iron fertilizer with a fertilizer used for roses in late fall just before leaf drop or very early spring.

Bob Morris is a horticulture expert and professor emeritus of the University of Nevada, Las Vegas. Visit his blog at xtremehorticulture.blogspot.com. Send questions to Extremehort@aol.com.