Four Emerging Invasive Plant Species in Princeton, NJ

Some invasive species have been in Princeton for a century or more, while others are just now arriving. Kramer's 1960s ecological study of Herrontown Woods mentions only Japanese honeysuckle as being overly aggressive in the landscape. Since then, as deer numbers rose and the native flora became intensely browsed, many more nonnative plant species have spread into local preserves to fill the void. No ecological study of Princeton in the 21st century would be complete without mention of a multitude of additional invasive species that gradually, quietly became established and now present a tremendous challenge for any land manager or backyard gardener. Call them too much of a good thing, with some good features but rejected by native herbivores and overwhelming in their dominance. Privet, bush honeysuckle, winged euonymus, Photinia, multiflora rose, linden viburnum, porcelainberry--their deeply embedded presence represents a deferred maintenance of daunting proportions.

Though we often think of them as a generalized menace, each invasive species has its own story of where and how it was introduced to the continent, and the extent of its spread. Having lived in Michigan, North Carolina, and New Jersey, I've seen how an invasive plant deeply entrenched in one region has yet to spread to another. The stiltgrass and lesser celandine that smother large areas of Princeton are just now starting to show up in Michigan and Wisconsin. The garlic mustard long common in the northern U.S. is relatively new to North Carolina. Even within Princeton, invasions can be very localized, affecting one yard but not yet another.

Here, for instance, is one solitary invasive plant out standing in a field near Carnegie Lake. By its scraggly tilted shape, you can tell it is Chinese bushclover, Lespedeza cuneata, also called Sericea lespedeza. I first encountered it in North Carolina, where the DOT (Department of Transportation) planted it along new highways to hold the soil on steep slopes. While serving one environmental goal--reduced soil erosion--this non-native bushclover's tenacious grasp of the soil abetted its spread into nearby piedmont prairies. Promoters of the plant's use for erosion control claimed its seeds would feed wildlife, but the small seeds moved through the birds' guts undigested. I've seen how it can take over, displacing diverse native meadows to form monospecific stands. The tragedy of Chinese bushclover's spread now extends to what few original tallgrass prairies remain in the great plains.

For decades, Chinese bushclover has been spreading along the gas pipeline right of way in Princeton. It also began invading Princeton's Greenway Meadows preserve years ago, became entrenched before anyone took much interest, and now the municipality is having a very tough time combatting it. 

What a pleasure, then, to remove just one plant down near Carnegie Lake, and thereby protect a whole field in five minutes. That's the magic of "early detection, rapid response"--intervening before a new invasive species can become established. Many weed warriors savor a big battle, but for me, being able to declare victory after the tiniest of skirmishes brings the greatest reward.

By these whitish flowers you can see how many seeds that one Chinese bushclover could have produced.

There are native bushclovers. One with pink flowers is called slender lespedeza (L. virginica). Another is roundhead bushclover (L. capitata). Typical of native species, they "play well with others," integrating into a plant community rather than taking over. I fought to save a stand of these from encroachment and takeover by Chinese bushclover in a preserve I helped create in Durham, NC, but wasn't able to devote enough time to succeed. Thus the preference for catching invasions very early, before the work becomes overwhelming.

Another invasive species knocking ever more loudly at Princeton's door is a prickly vine that grows so quickly it's called Mile-a-Minute. This patch is growing on a bluff overlooking the canal, just off Alexander Road. Having seen how the vine can smother vegetation in rural areas outside Princeton, I've snuffed out small patches I have found growing in town along roadsides and field edges, lest the seed be eaten and spread by birds. Keep an eye out for its distinctive triangular leaf. It's an annual, so wear gloves and pull its prickly vines out before it they can create seed.

This past summer, you may have noticed some yellow in your lawn that wasn't dandelion. Chances are it was a legume called birdsfoot trefoil. The way it's spreading across lawns at the high school, and here along the gas pipeline right of way in Princeton Community Village, it could easily come to dominate untreated lawns in coming years. 

Some will be aghast at the thought of calling Queen Anne's lace an invasive species. The flower is one of the most beautiful creations, and it's the wild version of the carrots we eat, Daucus carota. Highly likeable! But see how it is beginning to take over the upper detention basin at Princeton High School. 

I'm not surprised by this behavior because years ago I saw how aggressive Queen Anne's lace could be in prairies and along roadsides in the midwest. Seeing it begin to spread in New Jersey, I had hoped that it would behave more tamely here. Apparently not.

As managers of the detention basin vegetation at Princeton High School, our Friends of Herrontown Woods crew let them bloom, then pulled every last one before the seeds could mature. Here is a pile of hundreds of plants, the largest of which I estimated could produce 5000 seeds each. 

Wild carrot is a biennial, producing leaves the first year, flowers the next before dying. Though we removed hundreds of thousands of seeds, there are plenty of first year plants remaining that will produce flowers next year for people to admire and, in an act of romance without finance, maybe cut to share with a loved one. 

There are many other emerging invasive species in Princeton, such as Japanese angelica tree and black jetbead. Each year, the municipality hires the NJ Invasive Species Strike Team to remove some patches of these. With followup by local nonprofits, the goal is to avoid the mistakes of earlier decades, when invasive species were allowed to spread undeterred.