It’s tempting to gather colorful wildflowers growing robustly along New Hampshire roadsides, especially pretty purple loosestrife, with its long-lasting flowers. This Eurasian plant escaped from perennial and herb gardens and now thrives in roadside ditches and wetlands.

Later in the fall, the bright orange bittersweet berries accented by contrasting yellow husks, burst open in pretty clusters along vigorous vines. These tough vines wrap around tree trunks, eventually strangling them. Common along roadsides, the berries lure those looking for attractive fall decorations.

But don’t pick these invasive plants! It’s illegal to transport viable plant parts of these and other decorative but invasive plants in New Hampshire, because picking spreads their seeds. Horror stories abound.

One bittersweet door wreath became a jungle of bittersweet vines that took hold under a deck as the berries fell from the wreath and rolled under the deck. They sprouted and created a nasty eradication problem because it was difficult to crawl to the plant.

Loosestrife is pretty because of the millions of flowers on each stalk, on its many branched, sturdy stems, but they develop as many as two-and-a-half million seeds per plant each year.

The non-native plants on the state’s prohibited invasive species list have all been carefully evaluated for their nuisance potential; they possess certain traits that give them an advantage over most native species. In their new habitat, these invasives have escaped the natural predators and biological controls that keep them in check in their native lands. Their abundant seed production, aggressive roots, lack of insect predators and other traits give them a competitive advantage over our native plants.

Burning bush, for example, comes into its glory in the fall because of the flaming red color of its leaves. But as this robust Asian plant matures, each branch drips with hundreds of orange berries.

Rose hips, the round seed pods from multiflora rose, have caught the eye of crafters who make lovely wreaths from the thorny-stemmed hips. This Japanese native was first introduced in 1886 as a rootstock for cultivated roses. Later, the U.S. Soil Conservation Service promoted it for soil erosion control, and nurseries even promoted it as a “living fence” for livestock. Now it is a plague on old farms.

Birds eat and spread all these berries: bittersweet, rose hips and burning bush. At one time they were promoted as sources for food for birds in winter! These seeds germinate easily, and they sprout up at the edges of fields where birds rest in the hedgerows, near birdbaths and under shrubs where they nest.

Japanese knotweed, colloquially known as “bamboo,” flourishes along roadsides and flowers in the late summer to early fall. The six-foot-tall, hollow, arching branches produce clusters of white flowers along the stem. With the first frost, the entire plant dies back to the ground. This non-native plant, imported as an ornamental shrub, spreads by a dense mat of underground roots, totally overtaking other plants in its way. Any node that finds soil will root and form a new plant.

The milkweed-like pods of black swallow-wort are a curiosity in the fall when they burst open and release white fluff attached to a seed. Breezes and wind blow this fluff all over, spreading this nasty perennial vine far and wide. It has tenacious white roots that look like spaghetti when dug up. These root clumps get tangled up with shrub and flower roots and can quickly ruin a garden. They are especially annoying when entangled with lilacs because the swallow-wort leaves resemble lilac leaves.

As the trees begin to turn their vibrant fall colors, the ugliness of the Crimson King Norway maple becomes conspicuous. While all the native trees put on an exciting fall color show, this tree’s foliage turns black­a good time to make the case for cutting the Crimson Kings down. Many of these trees are now mature enough to produce seed, worsening their invasive impact.

The N.H. Department of Agriculture, Markets & Food took decisive action to prohibit certain invasive plants because these aggressive and prolific aliens crowd out native plants, reduce natural biodiversity, create overgrown jungles in our forests, old pastures and orchards, and block scenic views. The purple loosestrife that ruins wetlands is regulated as an aquatic invasive by the N.H. Department of Environmental Services.

For more information

Complete list of 26 New Hampshire legally prohibited invasive plants, photos of each, and tips for controlling them: http://bit.ly/invasives
 
Disposing of invasive plant materials: http://bit.ly/invasivesdisposal

Alternatives to Invasive Landscape Plants http://bit.ly/invasivealternatives
 
Doug Cygan, Invasive Species Coordinator for the Division of Plant Industry, works with community groups can advise and assist with specific problem situations: 271-3488, dcygan@agr.state.nh.us

Anne Krantz is also a member of the UNH Cooperative Extension Invasive Plant Outreach Group

Photo caption and credit: Purple loosestrife blooms along a highway. Photo by Anne Krantz.