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Invasion of the Purple Loosestrife

I’m sure you know purple loosestrife, even if you don’t know its name. It’s a big, tall clumping plant bearing bright pinkish-purple flowers and it’s appearing now in your neighborhood — especially in wet areas. What you may not know is that it’s an aggressive foreign invader that can out-compete many of our native species. It has been around for a couple of centuries — it came over, allegedly, in soil used as ballast in sailing vessels in the late 1700s.

But if it grows well and it’s pretty, what’s the problem, you may wonder? A number of things. It’s a big plant that can dominate the landscape and shade out smaller, less-vigorous native plants. A mature loosestrife can produce up to a million seeds, seeds that can be carried by streams to new locations. Wildlife can be negatively impacted. Many insects and birds depend on native plants for food and nesting materials — native plants that evolved with them. Invasives like purple loosestrife often do not meet their needs.

According to Barbara McIlroy of Etna, who has been active in the movement to control invasive species, the number of bird and butterfly species present on properties that grow native species is much, much higher. “I’m converted,” she told me. “Grow natives. Period. The positive effect on the wildlife is dramatic.”

Unfortunately, it’s almost impossible to rid your property of purple loosestrife. These plants have amazingly tenacious roots. Before I knew better, I tried digging out some large plants from a swampy area on my property. Yes, I got the plants out — but some of the roots broke off and re-sprouted. And by clearing out a space, I also opened up a new site for seeds to grow. If you have had purple loosestrife in the past, there are seeds waiting for a chance to germinate — like little time bombs.

But there is good news: two species of beetle (Galerucella calmariensis and G. pusilla) have been approved for and found effective in controlling purple loosestrife. These beetles cannot eliminate the loosestrife, but can reduce the numbers dramatically. Loosestrife is not a problem in Europe the way it is here, in part because it has natural enemies — the beetles. By introducing the beetles, an effort has been made to help Mother Nature keep loosestrife under control. Once established, they eat enough of the leaves and stems that the plants do not bloom. And theses beetles have been well tested: they will not eat your garden plants or valuable crops.

In 2001, Saint-Gaudens National Historic Site in Cornish released two species of beetles to see if they could control the purple loosestrife that was taking over the wetlands at Blow-Me-Down Pond. I met recently with Steve Walasewcz, who monitored the effort from 2001 to 2008. He explained that the beetles did not have an immediate effect. In fact, from 5.75 stems per square meter in 2001, the numbers increased to 10.5 by 2003. But then they declined each year until reaching only 1.05 in 2008.

I went to the area where the beetles were released in 2001 and although there were a few blooming stems of purple loosestrife, there were very few. According to a report done for Saint-Gaudens, it is believed that even low numbers of beetles prevent flowering; at higher numbers, “the beetles can strip the plants of its photosynthetic layer, resulting in high mortality in the winter due to a depleted starch reserve.”

Unfortunately, the beetles are not readily available for purchase. But for several years I have been experimenting with my own way of controlling the problem near my brook. Each year in early July I take pruners and cut back the plants to the ground. I identify them by their square stems that have a reddish color near the ground, and of course their leaves. Then a month later, I do it again — in essence, acting like a beetle. By cutting back the plants, few are able to bloom, greatly reducing the number of seeds spilling into my environment.

As a second step, I planted some big, tall native plants that I hoped would compete with the purple loosestrife: Joe Pye weed (Eupatorium purpureum) and Canadian burnet (Sanguisorba canadensis). I also added marsh marigold (Caltha palustris), which though not tall, is tough. These natives have given purple loosestrife a run for its money. Instead of filling three wheelbarrows with cut loosestrife in early July, I was able to get it all in one.

Purple loosestrife is just one of many invasive plants. If you see a new, vigorous plant show up on your property, try to learn its name and find out if it’s a problem elsewhere. Meanwhile, there is good news: scientists are working on biological controls for both Japanese knotweed (commonly called bamboo) and garlic mustard. In England, certain beetles are now well established to reduce the vigor of Japanese knotweed.

Henry Homeyer is the author of four gardening books and a children’s fantasy-adventure about a boy and a cougar. His web site is www.gardening-guy.com.