The Truth About Invasive Plants

The terms "non-native", "exotic," "alien," "pest plant," "problem species," and "noxious weed" have been used for plants from other continents or distant parts of a large country which disrupt native plant communities and other desirable vegetation. Most non-native plants do not become problems, but too often plants out of their natural range crowd out natives and create adverse economic impacts.

You can help control known invasive plants and avoid introducing new threats by understanding the problem:

What characteristics make invasive plants a problem?

High productivity. More seeds mean more seedlings. Purple loosestrife produces hundreds of thousands of seeds or more, per plant. By prolific seed production they quickly establish in disturbed areas, crowd or shade out other plants, gradually spreading into less disturbed areas.

Seed dispersal. Exotics whose seeds easily get around tend to quickly surround. Sericea lespedeza seeds are eaten by birds, which deposit undigested seeds everywhere on the fly.

Growth period or seasonal advantages. When sunlight and soil conditions are right for growth, exotics will grow, even if the season is shifted from their home and the local natives aren't growing.

Lack of natural controls. Insects and plant diseases seldom travel to new habitats with their exotic host.

How do plants move from their natural range to new, distant places? Accidentally and when well-intentioned people move them. Eurasian water milfoil seeds and plant parts traveled from Europe to the eastern U. S. coast in ship ballast, then spread to the Midwest by waterbirds and boats. Exotic modes of travel: Ship ballast/boat bilge - Boat propellers Bird ingestion - Floodwaters - Nursery sales Contaminated fill soil - With agricultural seed

Whose problem is it? Exotic plant control costs millions of dollars each year. Herbicides, labor, and research top the bill in the fight against plants which threaten to clog waterways, ruin fisheries, turn pasture to wasteland, compete with agricultural crops, shade out forest regeneration and overrun natural areas.

How to stop exotics: Get to know the common exotic threats. Inform friends and neighbors. If you see these offered for sale, explain the problem to your nursery, grower or supplier. If you find any on your property, inform the Weed Control Authority. Support national, state and local efforts of early detection and rapid response to newly found invasive plants.

Some invasive plants to watch out for

Known invasive plants well established in the county

Musk thistle, Carduus nutans L.*

Leafy spurge, Euphorbia esula *

Plumeless thistle, Carduus acanthoides*

Canada thistle, Cirsium arvense (L.) Scop.*

Known invasive plants with increasing populations in the county

Purple loosestrife, Lythrum salicaria

Sericea lespedeza, Lespedeza cuneata
Description: A warm season, perennial herb in the pea family, or Fabaceae. It has an erect growth form, ranging from about 3 to 5½ feet in height, and leaves that alternate along the stem. Each leaf is divided into three smaller leaflets, about ½ to 1 inch long, which are narrowly oblong and pointed, with awl-shaped spines. Leaflets are covered with densely flattened hairs, giving a grayish-green or silvery appearance. Mature stems are somewhat woody and fibrous with sharp, stiff, flattened bristles. Violet to purple flowers emerge either singly or in clusters of 2-4, from the axils of the upper and median leaves.
Ecological Threat: Sericea lespedeza, is primarily a threat to open areas such as meadows, prairies, open woodlands, wetland borders and fields. Once it gains a foothold, it can crowd out native plants and develop an extensive seed bank in the soil, ensuring its long residence at a site. Established dense stands of lespedeza suppress native flora and its high tannin content makes it unpalatable to native wildlife as well as livestock.

Johnsongrass, Sorghum Halepense (L.) Pers.
Description: Johnsongrass is a perennial species over most of its range. Leaves are grasslike, up to 1 inch wide, with a prominent whitish midvein. The ligule is short and membranous with a hairy fringe; auricles are lacking. Stems can grow up to 8 feet in height, but our annual specimens will be closer to 3 or 4 feet tall. Large, open panicles are up to 1 foot long and emerge in midsummer. Spikelets are reddish in color and most are tipped by bent awns. Scaly, finger-thick rhizomes are produced from the crown.
Ecological Threat: Johnsongrass is an invasive grass that forms dense spreading patches that completely smother other grasses. Like all sorghums, Johnson grass can be toxic to livestock, especially during adverse growing conditions or periods of new growth. This grass is extremely difficult to control and can become a major problem in pasture and cropping areas.

Known invasive plants with few or no plants found in the county

Spotted and Diffuse Knapweeds, Centaurea sp.*
Description: Each plant produces up to 25,000 seeds that are dispersed by wind, animals, and people. Seeds may remain viable for 8 years. Spotted Knapweed is a biennial or short-lived perennial. Mature plants may be 3 feet in height and are much branched. The weed forms a basal rosette the first year and stem leaves are pinnately divided. Flower heads are abundant, ½ inch wide, and generally solitary on branch tips. Flowers are pink to purple, or occasionally white, and appear from midsummer to fall. Each stiff flower head bract has a dark comblike fringe resembling a black spot at the tip. Seeds are dark brown to tan and are tipped by plumes that fall off at maturity.

Eurasian watermilfoil, Myriophyllum spicatum L.
Description: The exotic Eurasian watermilfoil is submersed. It tolerates a wide range of water conditions, and often forms large infestations. Eurasian watermilfoil stems are reddish-brown to whitish-pink. They are branched and commonly grow to lengths of six to nine feet. The leaves are deeply divided, soft and feather-like. Leaves are about two inches long. The leaves are arranged in whorls of three to six leaves about the stem. The flowers of Eurasian watermilfoil are reddish and very small. They are held above the water on an emersed flower spike that is several inches long.
Ecological Threat: Eurasian watermilfoil can form large, floating mats of vegetation on the surface of lakes, rivers, and other water bodies, preventing light penetration for native aquatic plants and impeding water traffic. The plant thrives in areas that have been subjected to various kinds of natural and manmade disturbance.

*Designated noxious weeds in Nebraska