Alien Invaders

Purple loosestrife is pretty--but potentially hazardous to local ecosystems.

As innocent as it may seem, purple loosestrife was imported in the early nineteenth century as a medicinal herb for treatment of ulcers and dysentery.

Why do landscape architects need to have an understanding of this issue? Beyond the altruistic desire for the preservation of healthy native-plant communities, we should know that federal agencies will not fund work that contributes to the introduction or spread of invasive species unless all reasonable measures to minimize risk have been considered. Consequently, federal funds cannot be used for construction, re-vegetation, or landscaping activities that include invasive plant species.

The Natural Resources Conservation Service (part of USDA) has an Invasive Species Policy whose purpose includes “preventing the introduction of invasive species, managing existing invasive species, and minimizing economic, ecological, and human health impacts that invasive species may cause.”

Part of this agency’s responsibilities includes coordinating with other federal agencies in the establishment, application, and use of an Invasive Species Management Program approach for the control and containment of invasive species, as well as providing national leadership for invasive-species management.

Problem Children By Region

One of the best practices for landscape architects in the design of projects is preserving vegetation by strategically minimizing the disturbance to existing site vegetation. Witness the wildlife-park wetland project discussed above.

Preserving healthy, existing site vegetation is more effective at maintaining stormwater quality than removing and planting new vegetation. If the ground must be disturbed and existing vegetation removed, timely re-planting with appropriate species is paramount.

Removal of the problem children is another aspect that is crucial to controlling invasive species. Some species can be eradicated by hand-removal, employing goats, or a sensitive application of herbicides. Other species prove more formidable: Himalayan blackberry is notorious for its robust life-force, often surviving even when bombarded with herbicide chemical cocktails, severing limb from limb with the brute force of the most Machiavellian of brush-whackers, or even taking flame-throwers to the monsters.

Kerry Blind says, “One interesting aspect on the Atlanta BeltLine project is we are doing an organic maintenance and operations plan–that certainly affects how you approach removal/eradication.”

The United States encompasses a vast diversity of ecological types. While some invasives, such as tree of heaven and purple loosestrife, are a problem throughout the country, these species are not likely to show up on too many planting plans of landscape architects.

The potential problem may be specifying ornamental plants for a project in blind ignorance of certain species’ propensity for escaping cultivation. With the wealth of information available through the Internet, ignorance of the problem plants is no longer a valid excuse.

The U.S. National Arboretum website provides information on invasive plants, as well as links to the official lists of offenders for each state (

There, for instance, you can learn that Virginia’s list of highly invasive species includes the tree of heaven, porcelain-berry (Ampelopsis brevipedunculata), winged burning bush (Euonymus alata), and the infamous Kudzu vine (Pueraria Montana). Recognizing the geographic diversity of the state, the list is divided into coastal, piedmont, and mountain regions, and the plants’ preferred moisture levels (hydric, mesic, xeric).

Moving to the Deep South, in Alabama the tree of heaven has again made the “Most Un-Wanted List.” Also included are plants which designers in other parts of the country freely incorporate into planting plans: Princess Tree (Paulownia tomentosa), Bradford flowering pear (Pyrus calleryana ‘Bradford’), shrubs such as privet and honeysuckle, and numerous vines–English ivy (Hedera helix) and Chinese wisteria (Wisteria sinensis).

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