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 that 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 that a wayward cottonwood root had wedged its way under the driveway adjacent to the garage. Here it had continued to expand, lifting 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 and not recommended near driveways, waterlines, streets, etc. It also prefers moist soils. Because we had planted this handsome tree in the wrong place, it had become a weed.
Any tree that you don’t want or is growing in the wrong place is a weed, and some tend to be weedier than others. The San Francisco Weed Management Area here in Northern Arizona has listed the following tree species as weeds.
The Siberian Elm
Brought to the United States in the 1860’s because of its fast growth rate and ability to withstand both drought and cold, this tree has made its home throughout Flagstaff. 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.
But the Siberian elm can be terribly invasive. Its papery winged seeds sail along with the wind and then rapidly sprout in places that would be inhospitable to most tree species. In this town those places are anywhere that harbors any 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 roads and sidewalks.
Tree of Heaven
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 roof-tops. In Arizona Northern this weed-tree thrives at elevations of 5,000 ft. and lower. In addition to hogging water from surrounding vegetation, it produces chemicals toxic to plants, humans, and wildlife.
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. If you chop it down after it’s mature, it may sprout dozens of new trees from the stump or roots.