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Spraying dormant oil now controls bugs for coming year

Maybe gardeners aren’t busy in Minnesota or Iowa during the winter, but for gardeners living in the Mojave Desert, it is probably our busiest time of year. I’m asked about spraying dormant oils on trees, but most people don’t know anything about them. It’s a shame because it’s probably the most important method to control bugs that may become problems during the coming year. It’s good insurance.

There isn’t much time remaining to get an application of dormant oil on landscape plants and fruit trees. This one application, applied early in the season, is probably the single most important method to control bad bugs hiding in your trees and shrubs and ready to spread their numbers in 2019. And it’s very safe.

This spray, however, does nothing to control borers or that ugly leaf-footed plant bug which flies from yard to yard and appears in early summer. But it does help control what we call “soft-bodied insects” such as aphids, mites, scale insects and others that live out the winter on trees and shrubs, patiently waiting for spring growth to appear.

Don’t confuse these oils with things like Neem oil or Cinnamon oil or any of the many oils out there that are currently popular. Dormant oils are made from mineral oil or paraffin.

They have been around for decades and are not very expensive to buy and apply if you do the job yourself. If you hire a landscape professional to apply it, they must understand how this oil works and how to apply it correctly. Application methods are explained on the label.

It works best when applied on warm days without wind. Never apply dormant oil to plants that are flowering. These sprays will harm honeybees visiting the flowers and may damage the flowers as well. If there are flowers present and open, delay the spray until the flowers are gone.

Apply the oil to the entire tree or shrub, top to bottom. Give an extra shot of dormant oil at the base of the plant where these types of insects often wait out the winter. It works by suffocation. These oils are not poisonous to pets or yourself, but they are deadly to all insects that are sprayed.

I know it doesn’t feel like spring, but it’s right around the corner. Now is the time to get fertilizers applied to trees, shrubs, fruit trees and anything else that will start to grow in a week. You want fertilizers available to plants when they are ready to grow. For most plants, this is around the first week of February.

Q: With the leaves off, I got a good look at our apricot tree and found two limbs with bark severely damaged. It looks like the bark is completely gone down to the wood underneath. What, if anything, can be done to salvage these limbs?

A: Bark easily lifted from tree limbs or the trunk no longer protect anything that’s alive. Plant parts below this bark is dead, whether killed by borers or intense sunlight. Dead is dead. Unlike how animals heal, the living wood surrounding this dead area must roll back on top this dead area and cover it.

If the damaged area is more than half of the way around the limb, then consider removing it. If the damaged area is half or less, remove the loose bark and encourage it to heal properly through irrigation and fertilizers.

Removing loose bark covering this dead wood helps the plant heal faster and new growth to roll over it. Healing is a waiting game. Healing is faster if the tree is getting adequate amounts of fertilizer and water. You won’t get it to heal faster by giving it excessive amounts of water and fertilizer.

Take a sharp, sanitized knife and remove all loose bark down to fresh wood. Remove all the loose bark and slightly cut into the living wood surrounding this dead area. You will not harm the tree by cutting into the living wood if the knife is sanitized; the tree will heal faster.

Sanitizing the knife, just like the surgeon’s scalpel, will keep serious disease problems from entering the tree that might slow down the healing or worsen the health of the tree. The tree will heal quickly if it is healthy. But this knife must be sanitized with alcohol or a good household sanitizer before cutting away at it.

It is possible that apricot, unlike many peach and nectarine, will sucker or show new growth below this damaged area. If the limb must be removed, select a sucker growing in a good direction to fill the vacancy left by removing the limb. If the limb remains, remove the suckers and let the wound heal.

Q: We found borers in a large sumac as well as a Chinese pistache ornamental tree. My ash and oak tree have aphids. I want to spray dormant oil but do not have enough information about it. Do I spray only trees without leaves or can I spray evergreen trees such as holly oak and African sumac?

A: Read the label to make sure your trees are included but there should be no problem spraying trees that have leaves. It just requires applying more liquid spray because there is more surface area to spray. It’s important to spray the undersides of the leaves because that’s where most of the bugs will be hiding.

Q: My tree has a trunk that has split badly on the side facing the sun. This split is very deep and 6 feet long. Can I wrap the trunk in burlap to keep the intense sunlight off of it while it’s healing?

A: The split is in dead wood all the way through to the center of the tree. All of this wood is dead probably because of sunburn. Sunburn is a big problem with trees that have thin bark protecting them such as ash, locust, ornamental plum and even some of the desert trees.

The tree will heal this area over time if the tree is healthy and growing vigorously. The question is more about your patience and if you can wait or not. This healing might take up to four or five years depending on how the extent of damage. Trees that are vigorous growers heal most rapidly.

The damage is most likely finished, and it will probably not spread any further than this. The outside edges, just beyond the damage, will roll over and enclose this damaged area over time. How much time? It depends on how vigorous the species is and the tree’s health.

Water this tree regularly but not daily. Always skip at least one or more days when watering. Soak the soil around the tree under the canopy deeply and then wait until it needs it again.

Fertilize this tree in early spring with a fertilizer container of nitrogen. This means a big first number on the fertilizer bag. Many lawn-type fertilizers will work. You could fertilize the tree a second time in April with half of the amount of a single application, but I don’t think it’s necessary.

When new tree growth rolls over this damaged area, this new growth will be more tolerant of intense sunlight. This is because it grew there under desert conditions. No need for paint. No need for burlap trunk wrap. In fact, I worry that a covering of brown burlap might trap more heat than just leaving it exposed to the open air.

If it’s possible to cast some shade on that damaged area while it’s healing, the tree will appreciate it. But unless there are some unusual circumstances, such as a location with lots of reflected sunlight from windows or similar, it shouldn’t be necessary.

I also worry a little bit about borers infesting the area near this damage. In a few months or couple of years, these bugs may create more problems for the tree. Consider applying borer control pesticides as a soil drench around the tree after it flowers. This might help while it is recovering and keep it free of borers.

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.

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