Terrestial Plants Out of Place

Giant and Japanese Knotweed    Leafy Spurge    Purple Loosestrife 
    Spotted Knapweed      Wild Chervil

What is wrong with having exotic species as part of your natural surroundings? Here's the problem - Exotic (invasive) species have no habitat value for the wildlife of Polk County.  They tend to grow so profusely that they overtake and strangle out any native species growing in the vicinity.  Once the habitat for our wildlife changes, the wildlife will move elsewhere. The following invasive species have been found in Polk County:

Giant and Japanese Knotweed

Description
The arching stems of this perennial (which grows to heights of 5-10 feet in large clones up to several acres in size) are hallow and bamboo-like, a reddish-brown to tan color. Even though they die in winter, they remain upright. Each spring it regrows from its extensive root system at a temendous rate. Mature leaves are 3 - 5" wide and 4 - 9" long, lighter on the lower surface and egg shaped; young leaves are heart shaped. Lacy 2" long clusters of tiny greenish-white flowers are produced in late summer and held upright at the leaf base. Japanese knotweed reproduces occasionally by seed, but spreads primarily by extensive networks of underground rhizomes, which can reach 6 feet deep, 60 feet long and become strong enough to damage pavement and penetrate building foundations.

Impacts
Introduced in the late 1800s, Giant and Japanese Knotweed is now found throughout much of North America. It is especially widespread in the Midwest. It is often considered to be the most troublesome weed in Great Britian. It grows in a variety of habitats, in many soil types, and a range of moisture conditions. Of particular concern is its tendency to invade valuable wetland habitat and line the banks of creeks and rivers where it often forms an impenetrable wall of stems, crowding out native vegetation and leaving banks vulnerable to erosion when it dies in winter. It is also found along roads, railroads, utility pathways and strip-mining areas. In addition to spreading by rhizomes and seed, it is often spread by streams, by transportation of fill dirt, or through roadside plowing.

Control
Attempting to remove Giant and Japanes Knotweed by pulling or digging is generally ineffective due to its extensive underground rhizome network; it may even promote further spreading if pieces of the plant are not disposed of properly. Herbicide application has been effective, when the entire clone is treated repeatedly. Applications of herbicides containing glyphosate are typically used after spring leaf out and on re-sprouts emerging after cutting.

Return to top of page

Leafy Spurge

Description
Leafy spurge grows in dense clumps with one or more shoots coming from a woody root crown.  It has non-woody, hairless stems, which can reach 3 feet or more in height.  Leaves are alternate and pointed.  Flowers are yellow with large, round, greenish-yellow bracts which develop in mid-June.  Pink buds on the root crown sprout readily and produce new  shoots, adding to the weed's rapid spread. Although the plant appears to be delicate, its strength is in its extensive root system. New infestations often appear in small clumps, which quickly spread.

Effects of Invasion
Leafy spurge is an aggressive invader and, once present, can completely overtake large areas of open land. Leafy spurge possesses several characteristics that allow it to spread rapidly and displace native grasses in only a few years. The species puts on growth early in the spring, allowing it to shade natives and dominate available moisture and nutrients. It is allelophatic, which means it emits chemicals from its roots that retard the growth or seed germination of other plants. Also, like other exotic invasive plants, it lacks the insects and fungi that preyed on it in its native land. The plant can reach densities of up to 1,800 stems per square yard. Roots can reach depths up to fifteen feet, and laterial spread of up to thirty-five feet. This deep root system makes eradication of the species extremely difficult. Even if the foliage of the plant is destroyed, the roots will regenerate new shoots.

Leafy spurge can be catastrophic to grasslands for both economic and ecological reasons. The plant is not palatable to cattle and is considered a serious pest in the grasslands of the Great Plains. It is estimated that the plant reduces the productivity of grazing land by 50 to 75 percent. It currently inhabits about three million acres of rangeland in the U.S., and economic losses in just four states alone exceed $100 million annually. In natural areas, leafy spurge reduces species diversity and habitat for wildlife, creating a monoculture with little to no forage or shelter value.

Control
Several systemic herbicides have been found to be effective if applied in June, when the flowers and seeds are developing, or in early to mid-September, when the plants are moving nutrients downward into the roots. Preliminary research suggests that chemical treatment in the fall followed by a spring burn to reduce seed germination may be an effective strategy for reducing leafy spurge infestations. Multiple treatments are necessary every year for several years, making leafy spurge control an extremely expensive undertaking. If left uncontrolled for a single year, leafy spurge can re-infest rapidly.

Mechanical Control has not been found to work effectively as control depends on the destruction of the root system. Fire, cultivation, obscuring sunlight and mowing have not been successful. Hand-pulling, digging and tilling are only successful if the entire root system can be excavated and may increase the number of plants if any remnants remain in the soil.

Biological control (using one organism to control another) offers a highly promising management tactic for leafy spurge. The U.S. Department of Agriculture has shown success using six natural enemies of leafy spurge imported from Europe. These include a stem and root-boring beetle (Oberea erythrocephala), four root-mining flea beetles (Aphthona spp.) and a shoot-tip gall midge (Spurgia esulae). Large scale field-rearing and release programs are carried out cooperatively by federal and State officials in many northern states. The results are not as immediate as when herbicides are used but, if pesticide use is kept to a minimum, large numbers of these agents build up within a few years and have shown impressive results. 

Return to top of page

Purple Loosestrife

Description
Purple loosestrife is an attractive wetland perennial plant from Europe and Asia that was introduced to North America without the specialized insects and diseases that keep it in check in its native lands. Freed from its natural controls, purple loosestrife grows taller and faster than our native wetland plants. These advantages and prolific seed production have allowed it to invade many Wisconsin wetlands to the near total exclusion of most other vegetation. Once established, it literaly shades everything else out. Loosestrife has spread rapidly in Wisconsin over the last 20 to 30 years. This perennial can range in height up to 9 feet with a dense bush growth of 1-50 stems. Showy flowers vary from purple to magenta and bloom from July to September. Leaves are opposite, nearly linear, and attached to four- to many-sided stems without stalks. 

Purple loosestrife was first introduced as a garden perennial from Europe during the 1800's. The plant was first detected in Wisconsin in the early 1930's, but remained uncommon until the 1970's. It is now widely dispersed in the state, and has been recorded in 70 of Wisconsin's 72 counties. Low densities in most areas of the state suggest that the plant is still in the pioneering state of establishment.

This plant's optimal habitat includes marshes, stream margins, flood plains, sedge meadows and wet prairies. It is tolerant of moist soil and shallow water sites such as pastures and meadows, although established plants can tolerate drier conditions. Purple loosestrife has also been planted in lawns and gardens, which is often how its seed has gotten into many of our wetlands, lakes and rivers. Purpleloosestrife spreads mainly by seed, but it can also spread vegetatively from root or stem segments.

The plant's ability to adjust to a wide range of environmental conditions gives it a competitive advantage. Purple loosestrife displaces native wetland vegetation and degrades wildlife habitat. As native vegetation is displayed, rare plants are often the first species to disappear. Eventually, purple loosestrife can overrun wetlands thousands of acres in size, and almost entirely eliminate the open water habitat. The plant can also be detrimental to recreation by choking waterways.

How Does Purple Loosestrife Spread
A mature plant can annually produce over 2 million tiny seeds, its primary dispersal agent. It can also grow from bits of root or broken stem that readily root in moist soil. Water, animals (especially birds), boats, construction equipment, and people can transport its seeds long distances and they may remain viable in soil for many years. Some uniformed gardeners still buy or trade and plant purple loosestrife.

Preventing the Spread of Purple Loosestrife

• Inspect for and remove aquatic plants, mud and seeds from boats, trailers, gear, clothing and footwear used in infested areas before moving to un-infested areas.
• Learn to identify purple loosestrife, pull young, small plants wherever found, and report all infestations to the DNR or Polk County LWRD Office.
• Encourage your local highway department to find alternatives to mowing the plant. Mowing usually helps to spread roots and seeds if timed poorly.
• Help curb local use of the plant - discourage local cultivation and distribution.
• Actively work to control purple loosestrife in a wetland near you.
• Education others about purple loosestrife.

Control
Effective control might consist of the use of physical and/or chemical measures. Physical control of purple loosestrife includes a range of options from hand pulling and shearing, to mowing, and cultivating.

Because of the seed bank that a purple loosestrife stand can have, a control program would have to be an ongoing process.  One-time control measures would have only a temporary effect due to new plants constantly springing up from the extensive seed bank. Chemical controls that have been used in the United States include Rodeo and 2,4-D. 

Biological agents (using one organism to control another) for purple loosestrife have also been affective in places.  Three beetles were approved for release in the United States on June 26, 1992-Hylobius transversovittatus is a root-infesting weevil, and Galerucella calmariensis and G. pusilla are leaf-feeding beetles.  Feeding by these insects at high densities of attack resulted in defoliated mature plants, killed seedlings, and destruction of or prevention of formation of flower spikes.  In addition to being effective, these insects are highly host-specific as has been confirmed during trials between 1988 and 1990.

Current Staff Reports - In the past, the Polk County Land and Water Resources Department (LWRD) has reared and released beetles as a means to manage purple loosestrife.   Beetles are obtained through the DNR in the early spring and allowed to multiply on caged purple loosestrife plants.  Usually by August, the beetles have had a chance to multiply and are ready to release.  Most recently, beetles were released in the Luck wetland in 2010 and the White Ash Lake Channel in 2011.  If you are interested in raising and releasing beetles in Polk County please contact Polk County LWRD for more information or assistance. 

Purple loosestrife has been present in Polk County for many years.  An elaborate inventory was conducted in 2000 to understand its presence and try to reduce its spread.  Sites were also prioritized for control of the infestations.  With support from a WDRN grant, several sites were re-evaluated in 2005 to see if control had been effective and to re-attempt control measures. 
See Purple Loosestrife Report.

Return to top of page

Spotted Knapweed

Description
Spotted knapweed is a short-lived perennial who got its name from the spots formed by black margins on the flower bract tips. Spotted knapweed typically forms a basal rosette of leaves in its first year and flowers in subsequent years. Flowers are purple to pink with 25 to 35 flowers per head. Plants bloom from June to October, and flower heads usually remain on the plant. Flower heads are oblong or oval shaped and are single or in clusters of two or three.

Control
The most cost effective management strategy for spotted knapweed is to prevent its spread to non-infested areas. Spread by seed can be minimized by avoiding travel through infested areas; by cleaning footwear, clothing, backpacks, and other items after hiking through infested areas; by not grazing livestock when ripe seeds are present in the flower heads; and by using weed free hay.

Small infestations of spotted knapweed can be controlled by presistent hand-pulling done prior to seed set. Gloves should be worn because of the possibility of skin irritation. Because spotted knapweed can regrow from the base, care must be taken to remove the entire crown and taproot.

Control of spotted knapweed infestations using three chemical herbicides (2,4-D, clopyralid, and picloram) has been reported but this is problematic. Existing plants can be killed with 2,4-D but it needs to be reapplied yearly to control new plants germinating from seed stored in the soil. Picloram is a more persistent herbicide and has controlled knapweed for three to five years when applied at a 0.25 lb/acre at any stage of plant growth. In the absence of desirable native grasses, longevity of control may be increased by revegetating with competitive grasses and forbs. Picloram may pose a risk of groundwater contamination where soils are permeable, particularly where the water table is shallow.

If desirable grass competition is evident in spotted knapweed stands, judicious herbicide application that does not injure grasses may allow them to compete effectively with the weeds. Irrigation (where possible) may help stimulate grass competition in these cases. However, rangeland or pastures often are degraded, allowing knapweed encroachment, and herbicides alone will not restore the land to a productive state. Seeding suitable perennial grasses is necessary to prevent weed reinvasion.

Biological control (using one organism to control another) has been fairly affective in controlling Spotted Knapweed.  The seedhead flies Urophora affinis and U. quadrifasciata have been released in many Front Range counties. These insects cause plants to produce fewer viable seeds and abort terminal or lateral flowers. Biological control agents may be more effective when combined with other control methods such as herbicides, grazing and revegetation with desirable, competitive plants.

Return to top of page

Wild Chervil

Description Wild chervil is a biennial or short-lived perennial plant of the parsley family. Seedlings develop into a rosette during the first year. In the second year, the plants produce hollow flower stems, usually 3-4 feet tall (can reach 6 feet). The stems are branched and covered in soft hairs, particularly near the base. The leaves are alternate, nearly hairless, and divided into smaller, fernlike leaflets. The base of each leaf surrounds the stem. Tiny, white flowers with 5 notched petals bloom from late May to early July of the second year. Individual flower stems form a small cluster. The flowers of several of these small umbels together form a larger umbel resembling an umbrella. The thick taproot of wild chervil has lateral buds at the top of the root which allow for resprouting. The taproot of mature plants may be up to 6 feet deep.

Impacts and Habitat Wild chervil will grow in a variety of soil types, but prefers rich, moist soils. It may be found along roadsides, open woods, pastures, and disturbed areas. Spreading can be caused by the movement of seeds in water, by birds and by mowing equipment; as well as by lateral budding at the top of the root. This plant completes with others for light, space and nutrients and may shade out surrounding vegetation.

Control Wild chervil is difficult to control because of its extremely deep taproot and its resistance to herbicides. Pulling up the flower stalks without removing the entire taproot can lead to resprouting. Mowing may deplete root reserves if done repeatedly before the plant forms seeds. Herbicide application (clopyralid and dicamba) shortly before blooming and one month after a pre-bloom cutting has shown some success. In recent trials, metsulfuron methyl has killed chervil plants three weeks after application; 2,4-D has been ineffective.

Campers:

Landowners and Gardeners:

Additional Information

Ecology and Management of Invasive Plants Program, Cornell University
Common Terrestrial Invasive Plants in WI - pdf
Invasive Plants Association of Wisconsin
Midwest Invasive Plant Network
WI DNR Terrestrial Invasives
A Field Guide to Terrestrial Invasive Plants in Wisconsin - pdf
Wisconsin Headquarters Invasives Partnership
Wisconsin Invasive Species

For more formation, contact:   

Jeremy Williamson
Polk County LWRD
100 Polk County Plaza, Suite 120
Balsam Lake, WI 54810
(715) 485-8639
jeremyw@co.polk.wi.us

Home

Return to top of page.