Borer damage to trees, shrubs is obvious this time of year

This is the time of year when borer damage in trees and shrubs is most obvious. Limbs are dying. Their damage can be seen from a distance now, but they’ve been working hard feeding on the inside of trees and shrubs for months.

The telltale sign of borer damage is a single limb or branch with leaves that turn brown and the branch dies. That’s during dry weather which happens a lot here.

Borer damage is oftentimes associated with damage to plants from intense sunlight, sometimes called sunburn or sunscald. Damage is oftentimes seen on the hot exposures of a trunk or the upper sides of limbs.

A good strategy is cautiously to prune plants that get borers. These include many fruit trees, pyracantha, Arizona cypress, loquat, several types of landscape trees and shrubs.

One borer in a small branch can kill it. But it takes two or three borers feeding in the same area of a limb to cause a larger branch to die the same season it is attacked. It’s probable that borers were present in trees, chewing away on the soft succulent inside just under the bark, for years before obvious summer dieback is seen.

Resistance to borers depends on the health and age of the tree. Healthy trees withstand several attacks by borers before damage is seen. Smaller-sized trees are easily killed while larger ones are more resistant. Sometimes trees are healthy enough that damage is never seen and they outgrow it.

The best time to inspect a tree for borer activity, even if you don’t suspect anything, is immediately after a rain. The rain softens the surface of limbs or the trunk. Tree sap associated with the feeding of borers oozes from the trees, at the damaged area, resembling varnish remover oozing from the trunk or limbs.

If there is a lot of “varnish remover” coming from a limb or trunk, then there is heavy damage in those areas. Take a sharp, sanitized knife and remove the bark from the trunk or limb. The borers, or flat-headed worms, lie just beneath the surface in those damaged areas. Remove them and clean up the wound and let it heal. If the damage is too severe, remove it.

Q: I think I have an ash tree. I’ve been reading about ash tree decline disease with some apprehension. I noticed my tree has bark with damp spots, and it’s starting to separate from the trunk. Is this a sign of ash tree decline or something else?

A: It probably is not ash decline, which starts as dieback of the limbs from the outside, higher up on the tree. A common problem on many ash trees is sunscald or severe sunburn of the trunk and limbs in full sunlight.

In our climate, sunlight is very intense and will burn exposed surfaces of some trees. These exposed surfaces can be the trunk, limbs and even the unprotected fruit of some fruit trees. Most of this sort of damage is seen on the west or south sides of tree trunks or on the upper surfaces of large limbs.

We see this most often on trees that have thin bark covering the trunk and limbs such as some ash trees, locust and honeylocust, and many fruit trees. We don’t see it as often on trees with thick bark covering the trunk and limbs such as pine trees.

It can be important to leave small stems growing from the trunk and limbs so they provide shade to help prevent damage from intense sunlight. Remove these small limbs when they get bigger than pencil-sized in diameter.

Scientists don’t agree whether these sunburned areas attract boring insects or not. It’s a “chicken and egg” kind of thing: Did sunscald attract the borers or borers contribute to the sunscald? Regardless, oftentimes borers are found damaging the tree near these areas.

It will not hurt the tree to remove the dead surface of the trunk or limb for a closer inspection. Either pull the loose bark from this area or cut it away with a clean knife. Inspect these areas for borer damage.

Removing this dead surface area with a sharp knife helps the trunk or limbs heal more quickly. Make sure the tree is getting enough water at each watering and correct an irrigation problem if one is found.

If you see some borer activity in fresh wood in the sunburned area, the tree will probably recover unless the damage is severe. Get the tree healthy and let it heal on its own.

Q: I have a lemon tree with some possible issues. It has been in the ground 10 years and is about 8 feet tall, but recently the leaves have turned yellow to brown and the branches are losing leaves in some spots. There is about 3-4 inches of rock under the tree and thick weedblock under the rock.

A: If you read my column regularly, you may know what I’m about to say. In our desert soils, placing rock on the soil surface beneath fruit trees is a no-no. You might get away with it in other deserts where there has been some form of agriculture but not here.

Frankly, I’m surprised it has taken 10 years for the leaves to begin yellowing and browning. That may be a record.

Soils are made up of two major components: the mineral component that is sand, silt and clay and the organic component. Our desert soils are extremely low in the organics, the good stuff that makes soils come alive. We refer to this component as the organic component or “organics” or “organic matter” of the soil.

Organics in the soil should rot or break down over a few years and disappear if not replaced regularly, every year in vegetable and herb beds or at least every two to three years around trees, fruit trees and shrubs.

Trees and shrubs that originate in deserts, i.e., desert adapted, can tolerate soils with little organics in them. But that’s not true of plants that don’t come from deserts. This includes citrus.

Your lemon tree is behind the curve regarding organics in the soil. The soil is probably extremely depleted. Adding compost or other sources of organics to the soil surface may not make much difference for a couple of years.

I would remove the rock, punch holes in the soil, pour compost in these holes so that compost can impact the roots quickly. Or lightly mix compost into the upper surface of the soil.

Compost tea may help. Add a cup of compost to a 5-gallon bucket of water, stir it, let it seep for a couple of hours and pour it over the top of the rocks. But ultimately you must get organics into the soil near the tree roots.

Check the irrigation and make sure the tree is getting enough water. An 8-foot citrus tree should get about 20 to 30 gallons of water each time it’s irrigated. The water should be applied to at least half the area under the tree. Next February, add iron chelate to the soil to correct any possible leaf yellowing because of iron chlorosis.

Q: I want to grow grapes to provide a screen between my neighbor’s yard and mine. We have a 5-foot block wall between us. I want to grow grapevines to 7 feet and 2 feet away yet parallel to the wall. This wall faces south and receives full sun all day. I want a variety that tolerates the sun and heat in this spot.

A: Most hot climate grapes handle the heat extremely well. You probably want a table or desert grape. Most of the grapes available in grocery stores can be grown in the hot desert. The most popular table grape is Thompson seedless and handles the heat and southern exposure very well.

When growing grapes, remember the problems with varmints that many landscape plants don’t have. Insect problems include the skeletonizer, leafhoppers, whiteflies and a few other lesser problem insects.

Birds are a big problem with grapes as well as ground squirrels if ground squirrels are in the area. As the berries begin to ripen and get sweet, it will be a battle for the fruit between you and them.

Remember these plants are deciduous so they drop their leaves in the winter. They won’t provide much of a visual barrier during the winter. Prune to keep them productive year after year.

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

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