Vegetative Invaders, Part II

A while back, I wrote a piece about some of the common invasive shrubs and trees in my part of the southeastern United States.  The trouble with attempting to write concisely about nonnative plants in any geographic region is that there are so many species to cover.  For example, in North America, many of the plants that we take for granted in our flower beds, gardens, and lawns were not present prior to colonization by Europeans; believe it or not, an unbroken expanse of uniformly green grass is not really natural, and common flowers such as Queen Anne’s lace were imported long ago.  The difference between a more typical naturalized plant species and an invasive is often a matter of degree, but it’s also fair to say that most people understand pretty intuitively which plants have gotten out of hand in their new home ranges.  In many cases, it's impossible to miss these invasives!

One of these out-of-control plants is so abundant in the southeastern U.S. that it has even had festivals created around it.  Kudzu (Pueraria montana) is a leafy, purple-flowered vine in the legume family.  Although it was originally imported to the U.S. (from Japan in 1876) as an ornamental, and was used for erosion control in the early 20th century, it has since become a noxious weed, infamous for covering entire landscapes and choking out native vegetation.  In ideal circumstances, it can grow up to a foot a day.  Controlling kudzu is difficult partly because of the stolons (or runners) that it produces, as well as rhizomes, or underground stems that produce roots and shoots; uprooting the plant so that it's incapable of forming new growth is next to impossible.  Herbicides can work if there is enough product to treat the many acres of ground that kudzu frequently smothers.

Kudzu creates odd, alien-looking landscapes.

Interestingly, one of the natural predators, the kudzu bug, was accidentally introduced into the U.S., with the first individuals recorded in Georgia in 2009.  However, because the kudzu bug tends to cause more damage to soybean crops than it does to kudzu, it can't be relied on as a biological control method.

Another invasive plant that can dramatically transform its environment is the multiflora rose (Rosa multiflora), a sometimes vinelike shrub that was introduced from Asia to the U.S. in the 1860s.  Like kudzu, it was widely used for erosion control, and it also shares the ability to form impenetrable thickets.  Its thorny stems can attain heights of 10 to 15 feet, and it has attractive flowers that have five white petals with blunt or slightly indented tips.  Herbicides are effective at controlling multiflora rose, but follow-up treatments must be consistently applied to prevent germination of seeds, which can lie dormant, ready to sprout at any time, for up to 20 years in the soil.  

Multiflora rose flowers

Multiflora rose certainly isn't the only attractive invasive plant that often appears innocuous.  Two of the most widespread introduced legume species in the U.S. are Chinese and Japanese wisteria (Wisteria sinensis and W. floribunda, respectively).  Because these species can and do hybridize in the wild (rather like the red and black imported fire ant species), identification is often difficult.  Both species have long, compound leaves and clusters of purple or white flowers.  Given the toxicity of wisteria's fruits, native wildlife species are almost certainly not helping these plants to spread.  Instead, wisteria's quick growth and tendency to strangle and shade out native vegetation provide the explanation for its range expansion after its introduction to the U.S. in the early 19th century (1816 for Chinese wisteria, and about 1830 for the Japanese wisteria).  According to the Mississippi State University Extension Service, wisteria control is extremely difficult.

Wisteria flowers

Wisteria growing over another invasive: privet.

I put kudzu at the top of this list for a reason, but I also find Japanese honeysuckle (Lonicera japonica) to be a fairly horrifying invasive climbing plant.  Like all of the plants discussed thus far, Japanese honeysuckle was imported (in 1806) as an ornamental.  It has attractive and fragrant blossoms, and many people have fond childhood memories of sucking the nectar from these blooms.  Like wisteria, however, Japanese honeysuckle's thick, woody stems quickly strangle native vegetation—and, as with kudzu, biological control doesn't work.  In fact, there is no available biological control for Japanese honeysuckle.

Japanese honeysuckle flowers

Because of the limited ecological benefits and the difficulty of controlling Japanese honeysuckle, the Mississippi Forestry Commission recommends planting several native vines, instead—including crossvine, coral honeysuckle, and Virginia creeper.  That site and others, such as the Mississippi State University Extension Service, are well worth perusing for detailed information about native trees, shrubs, vines, and herbaceous plants.  Although complete eradication of kudzu, multiflora rose, wisteria, Japanese honeysuckle, and other long-established invasives is almost certainly a pipe dream at this point, there are still plenty of options for keeping them at bay, and native gardening is one of the many methods.  Natural ecosystems will thank you.