Some trees will grow back after being cut near to ground

Q: If a dogwood were cut off a few inches from the ground, would a new dogwood tree grow from the stump? This is a theoretical question, but I’m talking about a wild dogwood that was transplanted into a landscape in Virginia, where these plants normally grow wild. The tree was transplanted from its wild location several years before it was maliciously cut down.

A: Flowering dogwood will sucker from its base after it has been established. How do I know this? I looked it up. So cutting this plant 8 to 10 inches above the ground results in sucker growth that can be used to create a new plant.

Sometimes I cut plants shorter than this, but it depends on the plant. After cutting it back, you must prune it during the first couple of years to create a desirable plant in future years. This means removing undesirable growth while it is young.

But this brings up an interesting question: How do you know if a plant will sucker from its base or not if you can’t look it up?

Plants have three options when they are cut nearly to the ground: grow from the base vigorously, grow from the base slowly or not sucker at all because this plant doesn’t sucker and you’ve killed it. There is a fourth option for grafted plants. Plants grafted to different roots, called a rootstock, may send up suckers from below the graft. This type of new plant may be worthless and should be removed.

For the first two options, look at the base of the plant. Are there suckers already present or not? If it is less than 5 or 6 years old, the presence of suckers tells you this plant can be cut back, leaving a stump or stumps, and it will sucker vigorously from this stump.

Some plants grow slower than others. For instance, if this kind of pruning were done to oleander it would grow back vigorously making a new shrub in a year. In the case of Texas ranger, it grows back slowly so it might take two years to re-establish itself.

Some plants, such as most fruit trees and some ornamental/shade trees, are grafted so that this graft makes a union between two trees. The top tree is called the scion and the bottom tree, used for its roots, is called the rootstock.

There is a characteristic dogleg that eventually grows where the two plants are joined together. If the pruning cut is made 10 or 12 inches above this dogleg, any growth from the scion can be kept but any growth below — or from this dogleg — should be removed.

Most commercial fruit trees are grafted to a rootstock. Many shade trees like ash and flowering plum are grafted the same way. Some ornamental trees, such as southern live oak, are not. Look for the dogleg to be sure.

Q: I have one purple leaf plum tree with very few leaves and another with lots of leaves. I was told the reason it has few leaves is that I am watering only the trunk of the tree. This tree is watered twice daily for eight minutes on a trip system. The second purple leaf plum tree also gets full sun all day and the same watering schedule, but it’s full of leaves. What am I doing wrong?

A: Watering might be the only problem if your tree was growing anywhere but in the Mojave Desert. Here there are two potential problems: irrigation and desert soils. But your watering schedule sounds skimpy and too frequent.

Desert soils can create problems for nondesert trees even though the tree is getting enough water. To make sure, the tree roots from this tree should be growing into soil that has compost mixed with it. Too late for that to happen? Read on.

A second contributing problem can be caused by the rock covering the soil after the tree was planted. Purple leaf plum is a nondesert plant and covering the soil with rocks easily contributes to many future plant problems.

Purple leaf, or ornamental plum, is not a desert tree like vitex or desert willow are. Rock is frequently used to cover the soil surface in desert landscaping. But using rock on the surface of the soil around purple leaf plum can create problems in about five years after planting.

If this is the case, try vertical mulching around this tree. If you want to keep the rock, rake the rock away from the tree at its base a distance of 3 feet from the trunk. Drill or auger vertical holes in the soil about 18 to 24 inches from the trunk and to a depth of about 2 feet. Fill these holes with compost, water it, and replace the rock. Otherwise, remove the rock from this area, fill the holes with compost but replace the rock with woodchip mulch.

The wrong amount and timing of irrigations can also create this problem. Minutes of watering doesn’t tell me anything. These trees need between 7 and 15 gallons after they are first planted from a 15-gallon container. Water application varies with the time of year but watering in midsummer should only be three to four times a week, not daily.

In about three to four years, the amount of water applied should be increased to 20 to 30 gallons. The number of times it’s applied each week doesn’t change. Increasing the amount is accomplished by adding more drip emitters under the tree canopy and further from the trunk, not increasing the number of minutes.

But eight minutes of water under the best conditions only gives the tree perhaps 2 to 4 gallons each time it’s irrigated. Watering trees twice each day may be convenient but it is a bad idea.

Q: We planted a sweet broom shrub in March of this year and it did very well. This fall I noticed the leaves were gone and only tan spikes remained. I found the culprits: 11 caterpillars doing the damage. I used a soap and water spray and the next day they were dead.

Two days later I picked off 25 more caterpillars. I then switched to a rose and flower insect killer and killed these. But 10 days later I picked off 25 more. I didn’t think desert plants got caterpillars. This is crazy! What should I do?

A: It’s a common misconception that desert plants don’t get bugs. They do. Sometimes they get more bugs than plants not native to the desert. That’s because their pests are here to begin with and they easily migrate to desert landscape plants.

This is probably the fall webworm. They lay eggs on the bottom sides of leaves and spin webbing around where they are feeding. This webbing makes a water- and bird-resistant tent where they feed protected. These caterpillars can keep coming back for several weeks during late summer and fall. Sometimes they don’t.

Soap and water sprays kill any bug, whether it’s a good bug or a bad bug. It’s a powerful insecticide. But it has one major disadvantage. It doesn’t last long. Once it is sprayed, this killing machine is finished. It doesn’t persist. To be effective, it must be sprayed over and over, every few days, when these bugs are seen.

Conventional insecticides, like the one you selected, is different from soap and water sprays because they persist. This is a good thing and a bad thing. It’s good because you don’t require spraying as often. It’s bad because it sticks around in the environment longer.

Another possible spray to use is Bt. It is not organic, but it is a natural pest control product. It persists longer than soap and water but not as long as most conventional insecticides like the one you were using.

Sweet broom is a good desert landscape plant. But it does have this problem so keep on the lookout in the spring and fall months for this pest and be prepared to deal with it when it arrives.

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.