As desert plant, acacia trees are opportunists with water

Q: My young acacia tree has abundant growth but only on the top half of the branches. Each of these branches is losing many leaves halfway up the branch. There is a lot of growth at the top of the tree but not much below. Am I watering too much or too little? I water every five days during the summer.

A: Acacia trees are desert plants. Most desert plants are opportunists when it comes to using water. In other words, when water is present, they grow like crazy. When water is absent, their growth slows, and they then try to use as little water as possible. Desert plants may even stop their growth and drop their leaves when water is not available

All plants are tremendous competitors for water, nutrients and light. They want to be “top dog” in their plant community by taking as much water, nutrients and light as possible when it’s available. By doing this, they take away these building blocks of growth from other plants.

When water is present, trees try to get as tall as possible as rapidly as they can before they start to fill out. They grow upward first and then put energy into horizontal growth once they’ve established some height.

This growth in height takes away light and shades competitors. This early growth in height, when there is a plenty of water and nutrients, oftentimes is at the expense of putting on lower growth.

Watering schedules take two different forms: how much water is applied and how often water is applied. It’s difficult to say with certainty without seeing the tree, but it sounds like it is receiving water too often.

Watering every five days means nothing to me. I can take a sip of water hourly, and someone might think I am drinking plenty of water. But another person might ask, how big are your sips? A teaspoon or a pint?

How much water should you apply? When watering trees, give them enough. Apply enough water to wet the soil at least 24 inches deep. Apply this water to at least half the area under the canopy of the tree. Once it enters the soil, the water spreads horizontally farther than this.

Use ⅜-inch diameter rebar that is 3 feet long. After irrigating, push this rebar in the soil in three or four locations to check the watering depth. Wet soil allows the rebar to slip in easily to the same depth as the wet soil. Dry soil makes it hard to push further.

It should slip into the soil at least 24 inches deep. Once you know how many minutes this takes, the amount of time you water won’t change. Each irrigation will be 24 inches deep.

If using drip irrigation, space emitters about 2 feet apart. If using a basin or moat under the tree, the basin should be as wide as half the area under the canopy. Trees grow. This means the basin must be expanded every three years. If using drip emitters, add more emitters every three years.

How often should you apply water? Look at the tree canopy. It will tell you. When the canopy of the tree starts to thin out, it’s time to irrigate. Desert trees tell you when to water when their canopies begin to thin out.

Q: I was reading your blog about worm castings, the chitinase enzyme produced by worms and its ability to control insects. Chitinase has been proven to degrade the chitin that holds insect skeletons together. Chitin is necessary for strong insect exoskeletons. So using worm castings in garden soils will control insects.

A: Scientists think this may be true about worm castings being vermicompost, but the research hasn’t linked everything together yet. Chitinase occurs in the soil because of earthworms, but does this chitinase control insects? Is the soil transferring this chitinase to the plants?

Some preliminary research claims it can. The research is going on right now to find out how much value chitinase has controlling problem insects in the garden.

Gardens are filled with insects. There are good insects, and there are bad insects. Can the chitinase produced by earthworms only kill the bad bugs, or will it also kill the good bugs? This is why research is needed.

There is a similar problem with some of the organic insect control chemicals. Soap sprays and oils don’t differentiate between good bugs and bad bugs. They kill them both. We have to rely on our knowledge about good bugs and bad bugs and how it might be applied to control only the bad guys.

Q: Where I can I buy good roses to plant in my garden in Las Vegas? All my roses are about 30 or so years old. What I see in the nurseries do not look good enough for all the work I put into planting them.

A: Sometimes you get what you pay for. Better roses cost more money.

Roses are sold in three different grades: Grade 1, Grade 1½, and Grade 2. Grade 1 is the highest grade while Grade 2 is the lowest. These grades were established by the American Association of Nurserymen to differentiate better quality roses from lower quality plants.

The grade refers to the number of canes or stems the plant has and how vigorous the plant will be after it’s planted. Roses sold in nurseries or garden centers should have the grade listed on the plant label.

Buy Grade 1 whenever possible to get your plants off to the strongest start. But if you see a variety that you must have and it’s only available in Grade 1½, then go ahead and buy it. The least expensive roses will be a Grade 2 and will not perform to your liking after planting.

Personally, I have always liked roses produced by Weeks Roses, a wholesale grower, but they may only be available here by mail order. Its website has lots of good information on general rose care. It also has a list of roses that perform best in different climates including the hot, dry desert.

Roses perform well for eight months of the year here when planted properly in the Mojave Desert. When planting, mix the soil with about 50 percent compost and cover the soil around the roses with woodchips. Make sure your roses are well watered during and immediately after planting. Roses do beautifully when watered with drip irrigation.

Q: I have an ornamental purple leaf plum that’s producing fruit. My dog has been eating the fruit. Is it safe to eat?

A: The ornamental purple leaf plums were selected for their leaf color and low fruit production, but they originally came out of the orchard industry. They became popular in landscaping for their ornamental beauty: beautiful spring flowers and purple leaf color. Some ornamental purple leaf plums leaf out green and then turn purple later. Others leaf out purple and stay that way.

Occasionally they do produce a fair amount of edible tart fruit with a very high sugar content. The fruit has very good flavor. It is used for making jams, jellies, pie filling, pastries, flavoring, gelato and juices.

But a warning: The fruit can develop a very high sugar content in the desert but at the same time be very tart. The tartness of the juice masks its high sugar content. A chef friend of mine jokingly complained that the sugar content was so high that it had to be diluted with water so it could be used to make gelato.

Two common varieties of ornamental purple leaf plums, Krauter Vesuvius and Thundercloud, have purple leaves, while other variants have green leaves that turn purple as the leaves get older. They are all called ornamental purple leaf plum and can produce fruit.

A similar plum that produces abundant fruit, cherry plum or myrobalan plum, is grown for its tart fruit. Two common varieties of this plum, Sprite and Delight, are personal favorites of mine because of their extremely high sugar content but tart flavor. Still another variant of this fruit tree is used as a rootstock for other plums. A very versatile plum.

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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