Alien Invaders

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

Scotch broom and Japanese knotwood are among the "alien invaders" in the state of Washington.

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.

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