Some invasive plants may be ugly, depending upon whom you ask. For instance, at the end of the growing season, Japanese knotweed litters the landscape with ragged dead canes that many find to be about the closest thing to "trash" that the natural world produces. But the collectors who brought these thugs to the West in the first place did so for what they considered to be their ornamental quality, of all things (there's no accounting for taste).
And that's how many of the examples in this picture gallery of invasive plants first breach enemy walls: they're invited in as ornamentals, with warmth and fanfare. Good looks make their conquest a breeze. Fact is, the invader doesn't always assault the gates wearing a horrific visage and battle scars.
Some of the invasive plants listed below are quite attractive. Take burning bush, that exotic (or "alien") shrub from Asia. Few shrubs put on a better fall foliage display. Another fall star is the vine, sweet autumn clematis. A summer standout is Scotch broom (Cytisus scoparius). But the attractive invasive plants are like some of the good-looking people one meets (you know the type): Once we get past the exterior and learn more about them, we realize we wouldn't necessarily want them hanging out in our yards. Looks, after all, can be deceiving.
Invasive plants can be thought of as exotic specimens "gone wild," as in "out of control." The following "Sweet 16" list of invasive plants comprises a collection convenient for introductory purposes. It is obviously not meant to be an exhaustive list, nor would one in any way deny that other invasive plants not listed here have perhaps rendered equal harm to native habitats. The list was inspired mainly by some of the worst offenders that one commonly encounters in New England, in the United States.
But don't stop reading, just because you, yourself don't live in New England. The aliens colonizing that region are known as marauding invaders in other regions, too. In fact, one of the examples listed below, lantana, earned its "invasive plant" label far to the south of New England, where the weather is warm enough to sustain it as a perennial. Nor are warnings about the following problem children pertinent to North America alone; for example, Japanese knotweed is widely recognized as a menace in the United Kingdom.
There are three "bittersweets," and this article distinguishes between them: Oriental bittersweet, American bittersweet and bittersweet nightshade. The Oriental bittersweet vines are sure to make most lists of worst invasive plants in North America.
As with bittersweet, so with wisteria: the North American grower must distinguish between American wisteria vines and their Chinese counterparts.
If you wished to crowd out weeds in an area of your landscape, you'd be delighted to hear about a vigorous ground cover -- an attractive vine that tolerates shade -- right? That description fits English ivy perfectly. But that's the problem: English ivy is too vigorous, earning it a spot on my list of worst invasive plants.
Like the preceding three vines, sweet autumn clematis is another of those "good-looking" specimens about whose charms I warned you. Perhaps making its apparent innocuousness even more convincing is the presence of the word, "sweet" in its name. But this is no sweet little wallflower. Do not let it sweet-talk you into growing it.
My fifth entry in this invasive plants list is yet another vine (albeit a smaller one): the mat-forming ajuga (or "bugleweed"). Like English ivy, it is a common ground cover; another popular ground cover with thuggish tendencies is the sweet-smelling lily-of-the-valley. But because of its combination of pleasing blossoms and weed-suppressing capability, ajuga's siren song may be the most powerful of the three.
One could easily go on and name more names when it comes to thuggish vines: They are perhaps the least well-behaved class of landscape plants. For example, creeping Charlie is notorious for spreading into lawn areas, where it is unwanted (although the smell it gives off when you mow it is quite delightful).
The barberries have assaulted North America from two sides. One, Berberis thunbergii, 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.
In autumn, burning bush 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 piece sheds some light on why burning bush generates so much heat.
Here in New England, our idea of "Florida landscaping" is often a hanging basket of lantana dangling over a patio. But folks who actually live in Florida know better. This exotic plant is a shrub that can easily get out of control and "take over" in warm climates. Indeed, lantana, that darling little hanging plant in New England, is one of Florida's worst invasives.
Butterfly bush is among the worst invasives in the Pacific Northwest. An alternative plant to grow for attracting butterflies is butterfly weed (Asclepias tuberosa). While butterfly bush attract humans, too, another demerit against this exotic plant, besides its invasiveness, is that its flowers have a truly miserable smell.
As with barberry, a privet hedge is a familiar sight. That very familiarity may make it difficult to see such exotic plants as invasives. Privet responds well to pruning and tolerates the pollution that typically plagues plants in urban settings. Privet shrubs are also fast growers -- too fast, in fact, as these invasives have escaped the bounds of cultivation in some regions.
The last four items have been shrubs. Lest the trees should feel left out, we are compelled to include a tree on this list. Although silk tree and tree of heaven can each make a case for being the poster child in this category, the dubious honor goes to Norway maple.
As stated earlier, Japanese knotweed is not one of the more seductive invasives. About the most 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.
Kudzu vine is in the pea family. So far, so good, right? After all, we are all familiar with peas from our experiences at the dinner table. Along those lines, kudzu has even been employed as livestock feed. Problem is, it is one of the worst invasive plants in North America. But wait: Maybe the solution lies in that whole livestock feed thing. Well, that's what some people have surmised, at least. Because, the fact is, companies have even sprung up -- going by such names as "Goat Busters" -- that bring goats to areas infested with kudzu and turn them loose on it. That's right: Goats are now devouring the vine that famously "ate the South."
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.
Purple loosestrife is an invader of wetlands. Many people who have no clue as to the name of this 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.
The beauty of purple loosestrife was just mentioned, but, the pleasing appearance of invasive plants may be more of a rule than an exception, as noted in the opening, above. Except for Japanese knotweed (and even that menace is reputedly edible), every plant on this list can boast either of being useful in some way or of being easy on the eyes. Japanese honeysuckle plays right into that theme. It has a pretty flower and a sweet-sounding name to match.
In fact, it is rather annoying that the invasive plants on this list should be so darn good-looking. It is a lot easier to pass up on a plain-looking plant than on a gorgeous one that beckons us to grow it. Many people feel the same way about candies and other decadent foods: What conspiracy lies behind the fact that they taste so good and yet are so bad for your health? Sometimes, life just isn't fair.