Roadsides as runways: trend spotting invasive plants

I can’t help but notice the roadside plants while I am driving. This spring, after an otherwise violent brush cutting along Route 7, I spotted a large and glorious patch of trillium; the other day I noticed that the super-spreader Japanese stiltgrass has taken over the sides of a local thoroughfare that shall not be name-shamed. Roadside plants are both a seasonal timestamp and a leading indicator of what invasives we will soon be grappling with inside the boundaries of our own properties. A kind of drive-by trend spotting.

A new-ish umbellifer dotting the roadsides this summer, Wild parsnip, Pastinaca sativa, is a tall, fennel-shaped invasive that has, thankfully, now withered to a brown skeleton. For cyclists this summer it was a surprise, then a worry. Skin contact with the plant can cause burns and blistering welts. If it gets into the eye it can even cause blindness.

The plant that is currently engaging my peripheral vision from the road is purple loosestrife, Lythrum salicaria. By the time you read this it will be winding down its blossom and going to seed. One is forgiven for not taking this one seriously; with its skinny stems and distractingly vibrant flowers it is arm candy to the native goldenrod it often accompanies. I saw it in a bouquet at a dinner last week and thought that we should all be making purple loose strife bouquets in an effort to stop its spread. True to its name, this loosestrife is a troublesome invasive that bullies out native neighbors with a combination of its dense perennial rootstock and prolific seeder. According to the Connecticut Invasive Plant Working Group, a mature plant can produce more than 2.5 million seeds annually.

Purple loosestrife is most often seen in wetland areas; its seeds can germinate even after a couple of years spent under water. Cut it down to prevent the seeds from spreading; it is hard to pull out by the root. If you only have a few, consider using a shovel to dig out the roots, which you will need to do consistently over a few years. It does get better. Chemical control, especially in wetlands, is best left to a professional.

There have been attempts at biological control against purple loosestrife; four types of insects were approved for release in Connecticut years ago to slow down the plant’s spread including a leaf eating beetle and a root eating weevil. My sense was, over the last few years, populations of the invasive plant were declining. This year, perhaps due to the mild winter or the heavy rainfall, it seemed to be everywhere.

There are both native and invasive loosestrifes co-existing in our area so a quick lesson to help you distinguish between them. There is a smaller, paler purple-flowered native called Winged loosestrife, Lythrum alatum, that I have never seen here and would like to know if any readers have encountered the species nearby. If you have, please send me an email at dee@theungardener.com

The other three are all yellow-flowered. Yellow loosestrife, Lysimachia vulgaris, is the invasive type. Like its purple relative, it grows in full sun and prefers wetlands. The yellow flower has more of a golden hue than that of its native relatives, Fringed loosestrife, Lysimachia ciliata, and Whorled loosestrife Lysimachia quadrifolia. Both of these native plants have distinctly paler yellow flowers that are less abundant on the stem than they are with the invasive type. Fringed loosestrife is abundant on the banks of the Housatonic where we live but so is the invasive version which I admit to not identifying it in time to keep it from establishing a robust stand in an area where I cannot shovel out the roots without causing erosion to the area. I will keep cutting it and see how it responds.

Back to the earlier-mentioned Japanese stiltgrass. If you intend to eradicate this prolific plant now is your last opportunity before it spreads its seeds, which are emerging from its slender stems. Consider first the area where it resides. In meadows and lawns you will do well to pull it out rather than weed whack so that there is no chance of seeding. Be sure to get your fingers down near the ground before you tug as the stem is as thin as a thread and will easily break with the root still in the ground. Not a disaster but it does risk re-growth this fall. For large and dense swaths, a weed torch will work wonders so long as you use with caution and safety measures. Remember to use this method only after a rain or early in the morning when the dew is thick and fallen leaves are not easily flammable. Always have a fire blanket or other fire eradication system by you. Perhaps most importantly, to avoid stiltgrass infestation, ask your lawn care company to hose down their mowers before starting to mow your lawn. This time of year stiltgrass seed will travel on mowers from other people’s lawns to new spots.

The shrub and small tree called Burning bush, Euonymous alatus, is the next invasive plant to put on a show for us on the roadside runway with its brilliant bluish-red and then pink leaves trying to distract driving eyes. Don’t succumb – eyes on the road!

Dee Salomon ‘ungardens’ in Litchfield County.