Gardening Etcetera: Can a tree be considered a weed? | Local & More Live News

While living in Southern California, my husband and I planted a cottonwood tree next to our driveway. It grew like a weed and provided shade for parked cars while adding beauty to our landscape. We watered it frequently but failed to make certain the water soaked deeply into the soil. In just a couple of years, our beautiful shade tree was acquiring thick, gnarled surface roots from which sprouted oodles of tiny trees.

One day we discovered a wayward cottonwood root wedging its way under the driveway adjacent to the garage. Continuing to expand, it lifted the driveway to the point of becoming an obstacle to opening the garage door. The only way to salvage the situation was to cut down the tree, grind down the stump, and purchase a new garage door.

If we had done our homework before planting, we would have found that cottonwood trees have invasive roots and are not recommended near driveways, waterlines, streets, etc. They also prefer moist soils. Because we had planted this handsome tree in the wrong place, it had become a weed.

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When we purchased our home here in Flagstaff, a cottonwood was and still is growing in the front yard. We water it deeply, so we aren’t troubled with surface roots, but it has its own set of problems. Growing at 6,800 feet in elevation, the leaves often freeze before showing off any fall colors, and the tree sheds numerous twigs.

Any tree growing in the wrong place or simply not worth the work involved in keeping it handsome is a weed. Some trees tend to be weedier than others; here are a few more that grow in Northern Arizona.

Siberian Elm (Ulmus pumila)

Brought to the United States in the 1860s because of its fast growth rate and ability to withstand both drought and cold, this tree has made its home throughout northern Arizona. At first glance, you may mistake a Siberian elm for an American elm; both sport serrated, shiny green leaves. However, the leaf of the former is elliptical, while that of the latter tends to be more ovoid.

The Siberian elm can be terribly invasive. Its papery winged seeds sail along with the wind and then sprout in places that would be inhospitable to most tree species. In northern Arizona, those places are anywhere that harbors a smidgen of moisture: between cracks in sidewalks, in ditches and hedges, and even in minute crevices where asphalt meets a structure. They may also sprout by the hundreds in dry, disturbed soils. And like my cottonwood tree, the Siberian elm has a propensity to send out surface roots capable of heaving up roads and sidewalks.

Tree of Heaven (Ailanthus altissima)

This invasive tree resembles a walnut tree, but its pinnately compound leaves are large in comparison. As with the Siberian elm, the Tree of Heaven produces scads of fast-germinating winged seeds. In eastern states, seedlings often sprout from mortar in brick walls and on rooftops. In northern Arizona, this weedy tree thrives at elevations of 5,000 feet and lower. In addition to hogging water from surrounding vegetation, it produces chemicals toxic to plants, humans, and wildlife.

Lombardy Poplar (Populus nigra ‘Italica’)

The columnar, rapidly growing Lombardy poplar appears to reach for the heavens with its upwardly outstretching branches. Its green leaves morph into a lovely gold in fall, and several of them planted in a row are not only gorgeous, but also may serve as a windbreak.

But the Lombardy Poplar may be rife with problems. For one, many species of insects find it appetizing, and for another, it is susceptible to an array of diseases. Adding to that is its demand for high maintenance. Like the cottonwood, its roots may be invasive, and the tree requires deep watering. Additionally, Lombardy poplar wood is brittle, leading to small branches littering the ground.

Like me, you may loathe getting rid of any tree. But speaking from hard-earned experience, I recommend that you dispose of any tree seedling growing where it could become a nuisance. Another word of caution: if you chop weedy trees down after they’re mature, some species may sprout dozens of new trees from the stump or roots. Be prepared to rid yourself of those too.

Cindy Murray is a biologist, co-editor of Gardening Etcetera. and a Coconino Master Gardener with Arizona Cooperative Extension.

If you have a gardening question, email [email protected] or call the Master Gardener Hotline at 928-773-6115 and leave a message. A Master Gardener will get back to you.

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