University of Maryland Extension

Introduction to Invasive Plants

This information focuses on specific examples of invasive plants and their impact on people and wildlife.

Canada thistle


Canada thistle crowds out the pussytoes that were planted in this garden bed. Garden volunteers planted the pussytoes because they are the host plant for the painted lady butterfly.  

For three years, these volunteers set aside the joyful work of teaching others about native plants and pollinators to do the joyless work of Canada thistle control. After attempting mechanical means such as pulling and smothering, they eventually resorted to the use of chemicals. At the end, the pussytoes were relocated.

Photo Details: Canada thistle rosettes (at left) crowding out pussytoes, the silvery, native, groundcover plant (at right).

I am a gardener why should I care about invasive species?

Food Gardeners & Food Eaters

Many Marylanders grow fresh fruits and vegetables for their families. Unlike commercial operations, home food gardens rely almost exclusively on native pollinators. As our wild native plant populations succumb to invasive plants, the pollinators that rely upon them disappear.

Carrots, onions and turnips ready for sale at a Maryland farmer's market.

Photo Details: If it were not for pollinators, there would be no vegetables in this photograph.

japanese honeysuckle vine strangles a rare wildflower

Birders & Hunters

Light cannot reach the foliage of the native wildflower (lavender blossoms at center) because it is blocked by the foliage of Japanese honeysuckle. Eventually, this will prove fatal for the wildflower. Marylanders find their outdoor adventures duller as botanical treasures like this one disappear, and to worsen matters, with the wildflowers and other native plants go the insects, and with the insects go the birds. Recreational bird watching and game bird hunting are very popular sports in Maryland. As we know, songbird populations are in precipitous decline and some of the most popular game birds (woodcock, bobwhite) have all but disappeared.

Photo Details: Japanese honeysuckle smothers the foliage of hairy beardtongue, a rare wildflower, depriving it of light and eventually killing it. This eliminates the plant as a source of the nectar, pollen, and leaf tissue on which insects depend.

Pedestrians and motorists visit local park in early spring.

Motorists & Pedestrians

Motorists and pedestrians pass through a county park during an April shower in 2014. Without these alien shrubs and trees, which leaf out much sooner than native ones do, these park visitors would have clear views of the massive bedrock outcrops to the left, and a gurgling stream to the right. They would also see drifts of spring ephemeral wildflowers such as bloodroot and trout lily. These wildflowers are disappearing because they cannot get the light they need below the dense cover of invasive shrubs and vines. As invasives spread, scenery, and the people who enjoy it suffer.

Photo Details: Every green plant in the photograph is alien. At left, English ivy blankets the slope and climbs up tree trunks. At left and right, Japanese honeysuckle shrubs and privet shrubs leaf out nearly a month earlier than native shrubs do.

alien vines drag down native wildflowers, shrubs, and young trees

Park Staff & Park Visitors

Oriental bittersweet vines smother native plants in a meadow at this county park. In addition to their other duties, park staff are charged with preserving the biological diversity, ecological function, and natural beauty of the land in their jurisdiction. The proliferation of invasive plants makes it necessary to divert staff time, volunteer hours, and budget dollars to the unpleasant task of invasive plant control. The alternative is a park devoid of the natural beauty that would make people want to work or visit there in the first place.

Photo Details: The black cherry sapling (back right), Joe Pye weed (center), common milkweed (center left, invisible at this scale), and black raspberries (foreground) would support a plethera of butterflies and songbirds if it were not smothered by oriental bittersweet.

Animal Lovers

In a Prince George's County bog, Asian vines smother all the native plants, obscuring the stems where a dragonfly would normally perch to bask in the sun. This dragonfly landed too close to a mile-a-minute leaf and became fatally ensnared in its prickles. The proliferation of alien plants doesn't just impact pollinators and songbirds, the majority of our state's animal species will be affected, as will citizens who appreciate the beauty and diversity of our wildlife heritage. 

dragonfly caught in the thorns of an alien vine




Photo Details: Mile-a-minute vines are soft and flexible with prickles that run under the leaf, along the petiole, and down the stem. This dragonfly became entangled in the vine's prickles.


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