Alien Invaders

At the time, creating two small ponds and an associated wetland habitat in an urban wildlife park seemed a worthwhile objective.

Thus it was that Phil, a fellow board member for Modie Park Conservancy, and I went out on a fine summer morning to direct the actions of a backhoe operator. By the end of the day, we felt modestly satisfied with our efforts, and could easily envision the ponds providing habitat for frogs, dragonflies, and red-wing blackbirds.

Indeed, within a year, kids from the neighborhood were chasing tadpoles, and there was a pair of nesting mallard ducks.

Tragedy soon struck, and by the third year, the area was so smothered and choked by Himalayan blackberry that nothing else had a chance.

Such is the power of invasive, alien plants. Control and eradication are difficult, some would say impossible. It is similar to Hercules attacking Hydra’s head--you cut off one vine and three quickly appear.

An invasive plant has the ability to thrive and spread aggressively outside its natural range. Normally, in its native habitat, natural controls of climate, insects, various diseases, and foraging animals assist in keeping its growth in check. The problem occurs when that plant is introduced to a new habitat.

For years, Kudzu vine (Pueraria montana) was the poster child of invasive species; however, with the ease of transporting plant species around the globe, numerous other plants have joined that list. Nationwide, these include the infamous tree of heaven (Ailanthus altissima), the aptly named mile-a-minute vine (Persicaria perfoliata, previously Polygonum perfoliatum) and purple loosestrife (Lythrum salicaria).

Although purple loosestrife is alluring and attractive with its pink-fuchsia flowers, this wetland perennial is able to suppress native plants, and even impede water flow.

The magnitude of the invasive problem is such that over $100 million a year is spent in the U.S. combating invasive plants in wetlands alone. A more startling figure is the estimated $35 billion per year nationally in crop and livestock losses, property damage, eradication efforts, and reduced agricultural exports.

The Ramifications

Spending project money on controlling invasive species translates to reduced funds available for the desirable parts of projects.

In the Atlanta region, Kerry Blind, FASLA, Vice President of Landscape Architecture for Pond l Ecos, says, “We do have many problems with invasives on projects. Primary invasive pest plants are Kudzu, privet, Hedera helix (although many would dispute this since every nursery sells it!), and wisteria. We often inform clients early on that a portion of their landscape budget will be allocated to IPP removal/eradication.”

Landscape architects and designers understand that invasive plants constitute a real threat to biodiversity and habitat loss. Rich, diverse plant communities can become barren, inhospitable expanses of invasive plants with little value to wildlife.

We all love the diversity that non-native ornamental plants can bring to planting plans. Often, there will be no negative consequences if the species in question remains in cultivation, and does not have the propensity to escape and naturalize a region of the country.

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).

In Washington state, Scotch broom (Cytisus scoparius) clearly belongs on the invasive-species list. I have seen it in its native Scotland, and it is beautiful with its butter-gold blossoms. However, in the mild, moist climate of western Washington, with no natural controls to hold it in check, it blankets the countryside. The official lists of noxious weeds also include butterfly bush (Buddleia davidii), which plant nurseries promote for butterfly gardens, and salt cedar (Tamarix ramosissima).

What can landscape architects and designers do? Gaining knowledge and understanding of the issue is a logical first step--to learn to identify the invasive plants of the region in which we practice. The lists provided for each state on the website noted above can help, as can a conversation with the local county-extension agent. You can find a list of the federal noxious weeds at .

In our designs, we do not need to feel restricted to native or indigenous plants, but ethically we should only specify non-invasive plants. Furthermore, if a project property borders a natural area, strong consideration should be given towards using only native plants.

If the site already harbors invasive species, advise a client on the need to remove invasive plants or prevent their spread. The Center for Invasive Plant Management ( ) offers assistance in the ecologically sound management of invasive plants.

Plants naturally need to reproduce, and when those plants are alien invaders, the result has an undesirable ecological impact. Hopefully, landscape architects will employ best practices and be part of the solution and not the problem.

Don Brigham, Jr. is a landscape architect with over 29 years of experience designing spaces for the enjoyment of people. His firm is headquartered along the Washington-Idaho border from which he serves clients throughout the northwest. Since 1984, he has served as Adjunct Faculty in the Landscape Architecture Department at the University of Idaho.