Alien Invaders: Noxious Weeds and Invasive Ornamentals

Waterprimrose in the Delta Ponds south of the gravel path
Waterprimrose in the Delta Ponds south of the gravel path

Noxious weeds may be pretty, but after we get to know them we discover their lifestyles push us beyond tolerance. These bullies aggressively out-compete other plants for available water, nutrients, light, and space. Reproductively dominant, they form single species stands that render an ecosystem inhospitable for birds, amphibians, fish, and invertebrates.

A weed may be defined as a plant that is not valued where it is growing. Unquestionably this definition is anthropocentric.

  • NOXIOUS WEED A plant, usually non-native, classified as “injurious to public health, agriculture, recreation, wildlife, or any public or private property” by the Oregon Department of Agriculture (ODA, 2012).

  • INVASIVE ORNAMENTAL An exotic plant introduced deliberately for landscaping or gardening use that escapes and become problematic in wildlands (Emerald Chapter, NPSO, 2008).

How did noxious weeds get here?

Some arrived long ago from foreign lands in the ballast of ships or were introduced by pioneer families as garden ornamentals. Others were intentional plantings by farmers for erosion control. Some hitch hiked in on vehicular tires or heavy equipment blades. There were natural introductions, as well, from guts of birds and animals or by wind and water.

Some ODA listed weeds flourished in the fertile soils of Goodpasture Island when it was in farmland. Others grew unchecked following gravel extraction in the area we now know as the Delta Ponds (Simpson, 2011). Ornamentals were introduced by developers and homeowners in landscape plantings without concern of future environmental impact. Invasive aquatic plants in home fish aquariums were dumped into Goodpasture Island sloughs and ponds.

Economic Cost

The economic impact is staggering. Noxious weeds degrade ecosystems, contaminate food crops, damage roads and drainage systems, and constitute safety and fire hazards.

Researchers at Oregon State University (Cusack, Harte & Chan, 2008) estimate the annual cost of 21 noxious weed species in Oregon to be $125 million (in 2008 dollars). This estimate includes production losses, fire damage and control costs. In addition there are future costs, not easily quantifiable, for inaction.

Strategic Planning

Land stewards design weed programs around management strategies:

  1. Maintain vigilance. Stop unwanted introductions before they arrive.

  2. Eradicate small populations while they are still manageable. No action or late action leads to “out of control” populations.

  3. Draw a line in the sand. Contain “out of control” populations.

  4. Assimilate them into the landscape. Learn to live with them.

growing on land

  • Himalayan blackberry (Rubus bifrons)
    One of the most tenacious plant invaders is Himalayan blackberry. Main plants have large, deep, woody root balls that sprout at the nodes. Grubbing out non-native blackberries that have grown to impenetrable thickets is a tough, backbreaking job. The control of re-sprouts and new seedlings is an ongoing process that can take years. City of Eugene Natural Areas Restoration Supervisor Trevor Taylor (2012) says that in recent years herbicides have been added to the toolbox at Delta Ponds as manual efforts alone were not adequate.

  • Scots or Scotch broom (Cytisus scoparius)
    Scots broom was introduced to the US by immigrants as a garden ornamental. It found the fertile soils of the Pacific Northwest much to its liking where it grew unchecked having left its enemies behind. The weed stands out thanks to its bright yellow flowers. See it along the Willamette River corridor.
    Scots broom plants have an extremely efficient seed dispersal system. The seed pods split at maturity and eject the seeds some distance from the plant. Eric Wold, Natural Resources Manager for the City of Eugene, is quoted in a KVAL interview (Nelson, 2011): “The seeds can live up to 50 years in the soil so even if we remove all the big plants, we still have to go back and look for seedlings that are germinating.”

  • English ivy (Hedera helix; H. hibernica)
    Walk along the Ruth Bascom Willamette River trail to see English ivy clambering up trees or over embankments. Pioneers brought it to the new world where it became popular with gardeners because it grows quickly, suppresses other plants and doesn’t need much care (Orlando, 2011). However, these very qualities are major reasons for ivy’s devastating impact.
    In 2002 the City dedicated $30,000 to ivy removal. “But at the cost of $4,500 an acre, it would take years and millions of dollars to make Eugene an ivy-free city,” writes Diane Deitz in the Register Guard (2002). She adds that Hendricks and Skinner Butte parks are blanketed with the stuff.

  • Canada thistle (Cirsium arvense)
    Canada thistle is probably the most widespread serious weed in Oregon. It occurs in every county in Oregon (French and Burrill, 1989). It became established on Goodpasture Island on farmland grazed by sheep (Simpson, 1984). Canada thistle spreads by seed and an extensive, branching root system which may extend as deep as 2-1/2 feet in cultivated soil. Once established, it is difficult to control.

Grubbing out blackberry at Delta Ponds
Grubbing out blackberry at Delta Ponds (C. Simpson)
IIvy clambering up trees along Ruth Bascom trail
Ivy clambering up trees along Ruth Bascom trail

rising above the water

  • Yellow flag iris (Iris pseudacorus)
    Yellow flag iris is a garden escape from Europe. It was first documented as having spread into natural areas in 1911. Once established, yellow flag iris colonizes in large numbers, forming single species stands that out compete native wetland plants (Hill, 2006). City of Eugene Parks and Open Space staff contracted a specialist to navigate to known populations of yellow flag iris in the Delta Ponds area. The contractor cut live emergent vegetation down to the water line. Seeds were bagged and hauled off site (Integrated Resource Management website). In addition to seed reproduction the plant propagates by rhizome. Rhizome fragments left behind become new plants. Yellow-flag iris has been found in landscaping and at lake margins of developed properties on the Island.

  • Purple loosestrife (Lythrum salicaria)
    Purple loosestrife is a large, showy plant that is invading wetlands in the United States and Canada at an alarming rate. It was first introduced as a horticultural plant. Klaus Richter (1994) wetland ecologist working for King County, Washington, writes that infestations can become so dense that they smother breeding and cover habitats for ducks, geese, herons and many other water birds. Purple loosestrife forms impenetrable stands with little food value. The strategy used by Parks and Open Space staff in DeltaPondsPark is to “take out” individual plants before they can establish a foothold. This requires labor intensive pulling and bagging to contain seed escape.

  • Water primrose (Ludwigia hexapetala)
    Water primrose is an aggressive South American weed that grows in ditches and ponds. It is a fast grower and reproduces vegetatively and by seed (Hill, 2006). The plants have a massive root system making hand removal extremely difficult. Root fragments left behind resprout. Parks and Open Space staff have been mapping water primrose at the Delta Ponds since 2008 with the strategy of keeping it contained to the pond south of the gravel trail and just north of the new pedestrian/bike bridge as much as possible (Holts, 2012).

Yellow flag iris
Yellow flag iris
Yellow flag iris
Yellow flag iris at margin of Island Lakes main lake

living in the water

Goodpasture Island’s lakes, ponds and sloughs contain invasive aquatic plants that reproduce quickly. They were likely introduced into our area by the dumping of home fish aquariums. In other Pacific Northwest locations they are introduced on the rudders and propellers of recreational boats.

Eurasian watermilfoil (Myriophyllum spicatum)

Eurasian watermilfoil is a submersed aquatic plant from Europe, Asia and North Africa that forms dense mats in nutrient rich ponds and lakes. It shades out other native vegetation and provides poor habitat for waterfowl, fish and other wildlife. It was likely introduced into the U.S. in the 1940’s through release from fish aquariums or escaped from private ponds (Wiedemer and Chan, 2008). It is found at IslandLakes and in the Delta Ponds.

Parrot’s feather (Myriophyllum aquaticum)

Parrot’s feather is indigenous to South America. It was introduced to the US in the late 1800s or early 1900s as an aquarium and pond ornamental. It has escaped into shallow water of ponds, lakes and ditches (DiTomaso and Healy, 2003), and has become a serious weed shading out native vegetation. See it in the Delta Ponds.

Water Primrose
Water Primrose
Water primrose and parrot’s feather
Water primrose and parrot’s feather
  • Cusack, Chris, Michael Harte, and Samuel Chan. 2009. The Economics of Invasive Species. Oregon Sea Grant, Oregon State University. Corvallis, OR.
  • Deitz, Diane. April 7, 2002. “An Alien Invader: English ivy threatens native plants, trees and wildlife” in the Register Guard. Eugene, OR.
  • DiTomaso, Joseph M. and Evelyn A. Healy. 2003. Aquatic and Riparian Weeds of the West. Agriculture and Natural Resources, University of California. Oakland, CA.
  • French, Ken. and Larry C. Burrill. March, 1989. Problem Thistles of Oregon. Oregon State University Extension Service and Oregon Department of Agriculture. Corvallis, OR.
  • Hill, Deborah. 2006. Exotic Invasive Plants in Natural Areas and Ornamental Ponds. Informational brochure of The Oregon Garden. Silverton, OR.
  • Holts, Lauri J. September, 2012. Personal communication.
  • 2012 Oregon Noxious Weed Policy website
  • Muir, Maya. August 1995. “Spring’s Silent Invaders: How Alien Weeds are Winning the West” in Cascadia Times. Vol. 1 No. 5 page 10-12. Cascadia Times Publishing Co., Portland, OR.
  • Native Plant Society of Oregon, Emerald Chapter. April, 2008. Exotic Gardening and Landscaping Plants Invasive in Native Habitats of the Southern Willamette Valley. Eugene, OR.
  • Nelson, Kristina. June 22, 2011. Scotch broom in full bloom: ‘It just starts taking over.’ KVAL News. Eugene, OR.
  • Oregon Department of Agriculture. 2012. Noxious Weed Policy and Classification System. Salem, OR.
  • Orlando, Cynthia. August 25, 2011. “Invasive invaders” in the Register Guard, Home and Garden Supplement. Eugene, OR.
  • Palmer, Susan. July 18, 2010. “Alien Creatures” in the Register Guard. Eugene, OR.
  • Richter, Klaus O. October, 1994. “Exotic Weeds: Biological Pollutants” in Northwest Parks and Wildlife 4:77-82.
  • Simpson, Charlene. 1984. Personal recollection.
  • Simpson, Charlene. 2011. Goodpasture Island’s Changing Landscape: Before 1846 to the Present.