Undesirable woody plants can creep into the landscape or grow from seed as volunteers. In some scenarios, these trees and shrubs can become a significant part of the plant population, especially when a house has changed hands several times in a short period of time, has been a rental house or has been neglected.

Weak tree species vary from region to region. Also, people disagree on exactly which species are worthy and which are not. A tree is considered to be “invasive” when it meets any of the following definitions:

• An invader plant is any indigenous or exotic plant species having a detrimental effect on the growth of commercial tree species, giving rise to particular management problems or growing where it is not wanted (Le Roux 1981).

• Invasive plants are naturalized plants that produce reproductive offspring, often in large numbers, at considerable distances from parent plants and thus, have the potential to spread over considerable areas (Richardson et al. 2000).

• Invasive species are species that are able to survive, reproduce and spread, unaided, and sometimes at alarming rates, across the landscape (van Wilgen et al. 2001).

Photos by John Fech.
Black willow can be invasive under certain conditions.

Where do you see them?

Undesirable, invasive species are often found in three areas in the landscape: along fence lines and property edges; near a home or garage; and in vegetable/fruit garden sections. If a property is adjacent to a “wild” area—a farm, acreage, industrial site or vacant lot—there is a ready supply of invasive plant material to supply your customer with a problem plant.

Many of these plants start growing under or near a fence or deck, where it is difficult to trim and remove weedy plants. Adding to the problem are homeowners who have a “plant parenting” mentality, where they can’t bring themselves to remove something that is green and growing. The mantra goes like this, “Oh, isn’t that cute? A little mulberry tree growing by the lilac hedge. Let’s just leave it for a year and see what happens.” We all know what happens—they grow at a rapid rate and often take over the area, shading out desirable perennials or grasses beneath, possibly creating a more favorable environment for foliar disease growth on desirable trees and shrubs due to reduced air flow through the landscape.

In some landscapes, an all-too-frequent occurrence is that the only shade tree on the property is one considered to be invasive, especially if the current homeowner didn’t plant it or has only a mild appreciation for the value of trees in general.

Vacant lots are prime sources for invasive species.

Know your species

Some tree species are considered to be undesirable and invasive, and others are simply misplaced. Try to distinguish between an invasive species, which tends to grow in bad locations, and a desirable species planted in a bad location, such as near a driveway, house foundation, etc.

Each region of the United States has several species that are considered to be invasive. In the Midwest and mid-Atlantic areas, mulberry, tree of heaven, willow, poplar, Siberian elm, silver maple and eastern red cedar are considered by most to be invasive. In the West, salt cedar, certain cycads and eucalyptus can be invasive, especially in riparian areas. In the South, black willow, chinaberry, tallow tree, Mimosa and royal paulownia commonly invade desirable properties, and are strong competitors for sun, water and nutrients.

Ramifications for arborists

Why are we concerned about invasive species?

Typically, these trees drop lots of seeds that sprout all over the landscape, in flower pots, vegetable gardens, flowerbeds, fence lines, near decks and even in cultivated lawns.

The establishment of an invasive species into the property of a client introduces undesirable species into the plant population, where it may not have been part of the mix previously. This tends to be problematic as it may have susceptibility to diseases and insects that hardy, well-adapted trees don’t have, but could become infested with. In addition, they usually crowd out desirable species by producing a large canopy, and thus too much shade for them. Other problems include that most break easily in storms and produce undesirable fruit.

Pesky invasives pop up all over the landscape.

What to do

Clearly, invasive species create problem specimens in the landscape, customers are reluctant to remove them, and your ISA training tells you that you should. How should you handle these situations? Use this simple three-step approach:

1. Audit/site analysis—Educate the client regarding the pitfalls of having an invasive tree in their landscape by performing a site analysis. If a total renovation of the landscape is required, it is best done as a two-step process, where you start by walking the property with a photocopy of the survey/plat, and write down observations about the state of plant health of various species. Write phrases such as “several hangers present” or “crowding exists” directly on the survey or plat. Next, make a second pass through the landscape and make a value judgment about the importance of the initial observations. Convert words “crowding exists” into “needs to be thinned” or “possible drop-crotch here.” If only one or two trees are being looked at, you can combine the steps. Get help from a landscape designer if necessary.

2. Encourage elimination or at least corrective action—Use the information that you’ve gathered in the site analysis to document the need for improvements. Take another walk with the client present, pointing out the faults of each tree, and making special note of the weak structure, propensity to produce undesirable fruit, likelihood to create bothersome seeds and other problematic features.

3. Replant with desirable species based on the site analysis and needs of the client—Once you’ve convinced the client to remove the invasive tree, you may be looking at a big empty space in the landscape. Work with a landscape designer/ architect to create several options for replacement with hardy, well-adapted trees.

Providing good customer service involves more than simply doing a good job for a fair price. Customer education and follow-up care will go a long way towards improving the dynamics and tree diversity of your customer’s properties, as well as set your company apart from those who are just in business to cut trees down and make a quick buck.

The author is a horticulturist and certified arborist located in Omaha, Neb.