Plants: Out of Control

27 July 2016

Not long ago, we told you about plants that are tough to grow in Arizona. But there are also plants that seem to love Arizona, grow easily and maybe grow too well. Then, when you decide to get rid of them, it’s hard to make them go away.

Be wary of Arizona plants that can take over your yard.

That sounds just like a tree called the Dalbergia sissoo, commonly known as Sissoo or Indian Rosewood. This is a tree that many homeowners plant because it’s very lush and fast-growing. Sissoos grow to a moderate height of 35 to 40 feet but their vigorous root systems can threaten underground irrigation lines, sidewalks, block walls and even lawns. The invasive roots can take over a yard after only a few years. Some homeowners complain about the many pods and leaves that drop off the tree, but its roots pose the biggest problem. Then when a decision is made to remove the tree, the real headaches begin.rosie on the house sissoo dalbergia

“Although Dalbergia sissoo is not listed as an invasive species,” says John Eisenhower of Integrity Tree Service in Phoenix, “it is considered by many horticulturists to be a nuisance tree because of its aggressive rooting and sprouting.”

Homeowners often call our radio program to ask how they can stop the Sissoo roots and suckers from spreading in their yards. According to Eisenhower, most of the problems begin when homeowners decide they want to cut down a Sissoo. “Usually when you cut down a tree, it will be gone, but not Sissoos. After the tree is cut down, they don’t just send up volumes of sprouts from the cut stump like some other trees do. They send up shoots from every root that remains in the soil, creating an unwanted forest of trees sometimes 100 feet from the stump,” he says.

If this happens to you, there are some steps you can take to control the roots and sprouts, according to Eisenhower. You start by cutting down the tree and leaving a stump that’s 18 to 24 inches tall. Then you drill holes into the outer sapwood ring on top of the stump and fill the holes with a contact herbicide. This will kill the stump and roots to a certain radius from the tree. Additional stump treatments plus spot spraying of new shoots will then be needed over the next few months to achieve full control of sprouting. Due to the massive amount of root material that Sissoos generate and the corresponding high volume of chemicals required to kill them, some people choose to first dig out as many roots as possible before applying chemicals. Soil removal and replacement to the full depth of the deepest roots is another option.

Eisenhower added that the chemicals needed for stump, root and sprout treatments are not available over the counter and need to be applied by a licensed applicator with the Arizona Office of Pest Management.

Despite the risk that aggressive rooting trees like Sissoos present, they remain very popular. If they have adequate room to grow where they will not encroach on surrounding landscape elements, they can be a beautiful and serviceable evergreen shade tree. It’s not that these are always bad trees; they’re just the wrong trees when planted in the wrong place.

You can read further on John's article Dalbergia Sissoo Root Problems.

Another plant that can refuse to die is Cat’s Claw. Homeowners often plant this drought-tolerant vine to cover up walls and fences. It’s hardy, loves the sun and gets yellow trumpet-shaped flowers in the spring. It can reach 25-feet-high and 25-feet-wide. But as I know from personal experience, once you plant it, you might have it forever. In fact, if you want to get rid of it to make it easier to paint your walls or a fence, you should use the same method of killing it as is recommended with the Sissoo tree. It’s not impossible to get rid of Cat’s Claw. It just takes a long time.

Speaking of flowering plants, the Oleander is another one that has done really well in Arizona and that many homeowners have used over the years. Oleanders are often planted as hedges because they can reach 20 feet in height and be very wide as well -- making for a great privacy screen. Sometimes they’re even turned into trees.

But Eisenhower recommends being cautious about planting Oleanders right now because many of them in the Phoenix area have been infected with Bacterial Leaf Scorch (Xylella fastidiosa) spread by flying insects that feed on its leaves. The Oleander’s leaves get yellow, dry up and drop off, and the entire plant usually dies within a few years.

“In central Phoenix, many neighborhoods have been decimated,” he says. “And there’s no known treatment available at this time to stop the disease.”

If you’re looking for a different plant to use as a hedge, try Sour Orange or Arizona Rosewood. Other options include Hopseed Bush and Yellow Bells. These last two are more susceptible to frost so they are not appropriate if you live in an especially cold area. If your Oleanders are healthy, consider yourself fortunate. Nothing provides the fast growth and screening, not to mention the beautiful flowering of a large, dense Oleander hedge.


Photo Credits:

  • Header image: Rosie on the House
  • Sissoo: Chris A. Martin, Arizona State University




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