An invasive plant can be defined as any plant that grows where you don't want to and does it in a way that makes it hard to control. It doesn't have to be a weed, and invasive plants are by no means always ugly specimens. A lot of it depends on the setting. For example, bittersweet vines in some settings can be quite beautiful and desirable, but if they take over your woodland garden, they are a nuisance indeed. And some plants begin as perfectly desirable landscape species that you plant deliberately, such as obedient plant (Physostegia), only to prove their invasive nature in a year or two when you discover their rampant growth characteristics.
Some invasive plants listed are quite attractive. Consider burning bush (Euonymus alatus) for example—an exotic (or "alien") shrub from Asia. Few shrubs put on a better fall foliage display. Another fall star is the vine, sweet autumn clematis (Clematis terniflora). A summer standout is Scotch broom (Cytisus scoparius). But attractive invasive plants are like some of the good-looking people one meets (you know the type): Once you get past the exterior and learn more about them, you no longer want them to hang out. Looks, after all, can be deceiving.
Many invasive plants can be thought of as exotic specimens "gone wild"—as in "out of control." The following list of 16 invasive plants comprise an introductory collection, but it is by no means an exhaustive list. Remember, too, that invasives tend to be defined regionally. Some species that pose no problems at all in some areas behave in true invasive style in other regions. Some plants are seriously invasive in the warmer climates where they are perennial but are no problem at all in colder regions where they are killed off each year.
There are three "bittersweets," and it is important to distinguish between them: oriental bittersweet (Celastrus orbiculatus), American bittersweet (Celastrus scandens), and bittersweet nightshade (Solanum dulcamara). It is the Oriental bittersweet vine that is sure to make most lists of worst invasive plants in North America. The other forms can also be invasive but are less so than oriental bittersweet. American bittersweet is the form with attractive red/orange berries that are often used in decorative displays. All forms of the plant are toxic, however, and should be avoided in the landscape.
As with bittersweet, so with wisteria: The North American grower must distinguish between American wisteria vines (Wisteria frutescens) and their Chinese counterparts (Wisteria sinensis var.). While both types are robust growers, it is the Chinese wisteria that poses a truly invasive danger south of USDA hardiness zone 4.
3. English Ivy
If you wished to crowd out weeds in an area of your landscape, you would expect to be delighted to hear about English ivy (Hedera helix), a vigorous, attractive ground cover that tolerates shade. That description fits English ivy perfectly. But that's the problem: English ivy is too vigorous, earning it a spot on the list of worst invasive plants. It easily escapes landscape cultivation and is regarded as a seriously problematic invasive, especially in the Pacific Northwest.
4. Sweet Autumn Clematis
Like the preceding three vines, sweet autumn clematis (Clematis terniflora) is another of those "good-looking" specimens that can overwhelm a landscape. It is especially problematic in the East and lower Midwest. While this plant does have a very sweet odor, that's the only thing pleasant about it. Clematis paniculata is sometimes sold as sweet autumn clematis, but this is a less invasive vine that comes from New Zealand. It is C. terniflora that you need to be careful with.
The mat-forming ajuga (Ajuga reptans), also known as bugleweed, is another popular ground cover that can turn thuggish (lily of the valley is another). Because ajuga has pleasing purple blossoms and the ability to suppress weeds, it is often planted in shady areas as a ground cover. Many homeowners, though, come to despise it when it begins to take over a garden or lawn.
Ajuga is especially problematic in warmer climates where there is no winter frost to kill the plants back annually.
The barberry shrubs (Berberis spp.) have assaulted North America from two sides. One type, Berberis thunbergii, is from the Far East; the other, Berberis vulgaris, from Europe. These invaders have come armed to the teeth, bristling with the thorns that have made them so useful in many a hedge. B. thunbergii, usually known as Japanese barberry, is so invasive that much of the Midwest has placed it on a list of dangerously invasive plants, strongly suggesting that it should never be planted at all.
7. Burning Bush
In autumn, burning bush (Euonymus alatus) puts on a show for the ages, bearing red or pinkish-red leaves. Colorful reddish-orange berries accompany the striking foliage. So why is burning bush one of the most hated exotic plants among gardeners "in the know"? This shrub is considered dangerously invasive across much of the northern United States from Maine to Minnesota, as well as in the Southeast.
Lantana (Lantana camara) is a broadleaf evergreen shrub native to tropical areas, and it is a notable invasive in Florida and Georgia and across the South all the way to California. But it poses no danger in colder climates north of zone 9, where it is often used in hanging baskets. In warm zones, however, it can easily escape gardens and naturalize in dangerous profusion.
9. Butterfly Bush
Butterfly bush (Buddleja spp.) is among the worst invasives in the Pacific Northwest, where growing conditions resemble its native habitat. It also an invasive problem in areas of the Southeast. In areas colder than zone 6, it is less problematic, since the plant dies back to the ground each winter. An alternative plant to grow for attracting butterflies is butterfly weed (Asclepias tuberosa). Butterfly bush is so-named because it attracts butterflies (as well as other pollinators), but to humans, the plant is somewhat unpleasant in odor.
10. Common Privet
As with barberry, a privet hedge is a familiar sight. That very familiarity of privet (Ligustrum vulgare) may make it difficult to see such exotic plants as invasives, but this species is on official lists of problematic plants in much of the Midwest and Northeast from Pennsylvania north to Maine. Privet's popularity is based on the fact that it responds well to pruning and tolerates the pollution that typically plagues plants in urban settings. But privet shrubs grow so fast that they can easily escape the boundaries of cultivation and become naturalized in the wild.
11. Norway Maple Trees
Full-sized trees can be invasives, too, as in the case of Norway maple (Acer platanoides), which is considered invasive in much of the Northeast, and dangerously so in Maine, Vermont, New Hampshire, and Massachusetts. Originally planted as a landscape species, its seeds easily disperse on the wind to naturalize in other environments.
12. Japanese Knotweed
Japanese knotweed (Fallopia japonica) is a clumping perennial plant that has virtually no redeeming landscape value. The best that can be said for its appearance is that it produces a fluffy-looking flower in early autumn (thus one of its alternate common names, "fleece flower"). The opinions of 19th-century plant collectors notwithstanding, most 21st-century Westerners agree on this one: Japanese knotweed is an ugly nuisance and an easy pick as one of the worst invasive plants. It is considered invasive in every state, though is more so in its natural hardiness zones, zone 5 to 9.
Kudzu vine is in the pea family. So far, so good, right? After all, you're familiar with peas from your experiences at the dinner table. Along those lines, kudzu has even been employed as livestock feed. But this perennial vine from Asia is one of the very worst invasives of all time, and is sometimes ruefully called "the vine that ate the South." It is an enormous problem in all Southern states. Initially planted to shade porches on southern plantations, the plant quickly spread to surrounding territory, where it now gobbles up nearly all that it touches. It grows well in both sun and shade and is dangerously invasive throughout the South and Southeast and well up the Atlantic seaboard.
A recent control effort involves bringing goats into areas infested with kudzu and turning them loose to eat their fill.
Tansy, unlike the exotic plants considered above on this invasive species list, is an herb—albeit a poisonous one. Tansy's toxicity belies a rich tradition of medicinal and culinary usage. But there's more to worry about than just its toxicity: Tansy plants are invasives that spread via both seeds and rhizomes.
15. Purple Loosestrife
Purple loosestrife is an invader of wetlands. Many people who have no clue about the name of the plant have nonetheless seen it innumerable times and remarked upon its beauty. In fact, it is a lovely plant when massed together—which is the norm, since this is a plant that spreads incredibly vigorously.
Purple loosestrife is thought to arrive in North America as seeds in soil used as ballast in sailing ships in the early 19th century. Now, 200 years later, it is found in every state in the United States, except Hawaii and Alaska. The plants take over wetlands by forming dense root mats that choke out native plants, degrading wildlife habitat. Since 1992, the governments of Canada and the United States have used to European beetles, Galerucella calmariensis and G. pusilla, to feed on the plant—an experiment that appears to be successful.
16. Japanese Honeysuckle
Japanese honeysuckle (Lonicera japonica) is another attractive, sweet-smelling specimen that turns out to be a sinister foe. This vigorous, fast-growing twining vine has fragrant yellow flowers that appear from June to October, and it grows to 30 feet. When planted deliberately, it is used as a ground cover, but it is considered an exotic invasive across the entire Midwest.
If planted in the garden, great care should be taken to keep the plant in check, including aggressively cutting it back regularly. When it escapes, this plant can break tree limbs with its heavy weight and kill shrubs and trees by girdling them with strong vines.