Henry Homeyer: how to deal with invasive plants

By Henry Homeyer
© 2023 Telegraph Publishing

Most older houses are plagued with plants brought from Asia or Europe and sold to well-meaning people who didn’t know that some of those handsome plants might become invasive. By definition, invasives come from abroad, spread rapidly, do not have any natural predators to help keep them under control, and are hard to get rid of. Most of us have a few – or will soon.

Plants including Japanese knotweed, Asian bittersweet, goutweed, purple loosestrife, yellow pond iris and multiflora roses have thrived in New England – and all are nearly impossible to get rid of, once established. Unfortunately, I now have four of the six mentioned above. But thankfully, I have no Japanese knotweed or bittersweet, which are two of the worst.

Multiflora roses showed up on my property just last year, probably “planted” by birds that dropped seeds. It is easy to see roses growing in your woods or fields, or even in a garden bed and pass them off as “just roses.” But the roses we love don’t just appear – so beware if they seem to.

The multiflora rose was introduced from Asia in the 1860’s as a vigorous ornamental rose and as a source of rootstock for grafted roses. In the 1930’s it was widely introduced as erosion control and alongside highways – a mature planting is so dense it can prevent cars from going over median strips. But the birds liked the rose hips – their seed pods – and it escaped and spread.

So what am I doing to eliminate it on my property? I am digging it out. Digging is most effective for one or two year old plants, I am using a curved, single-tine hand tool called the CobraHead to carefully excavate the roots until I can lift each plant out.

First, I dress appropriately: jeans, long-sleeved shirt, a hat with a brim, and heavy winter leather work gloves. This culprit wants to hurt anyone trying to uproot it. I cut off the branches, just leaving a foot of the stem to grab onto when pulling it out. Then I loosen the soil and pull weeds around it with my CobraHead. The roots radiate outward from the stem like spokes on a bike. I loosen each root and tug gently when they are small enough to remove.

I do not burn brush anymore because of global warming, but I don’t want any seeds to escape, so I cut up the branches and brought them to our town recycling center. I put them into the trash going to the landfill, along with the roots. It took me about an hour to remove and cut up one plant – and I have several. This plant can grow 10 feet or more in a year, and strangle trees or shrubs. I’ve read that multiflora rose seeds can stay viable up to 20 years – a good reason to dig out plants when young.

Invasive plants are always difficult to remove – usually a scrap of root can generate a new plant – or several. Cutting back the stems to ground level will stimulate the roots of multiflora rose to send up new shoots everywhere, causing a bigger problem. There is no easy answer. I fear that the rose I removed will reappear from scraps of root next year, but I’ll be watching.

Buckthorn is another invasive that is common along streams and at the edge of fields. Like multiflora rose, cutting it down stimulates the roots to send up new shoots. The best way to eliminate it is to starve the roots: take a pruning saw and cut through the bark and the green layer of cambium beneath that (but not the hardwood below it). Cut all the way around the trunk, then repeat six inches above the first cut, and repeat. This will prevent food made by the leaves from getting to the roots. It not kill the tree until the third year, but this slow death will not stimulate the roots to grow. It’s best done in winter or fall after leaf drop.

Since buckthorn are often multi-stemmed, it can be difficult to use that method. Do it up high enough so that you can get your saw in between the stems. I’ve done it, and it works.

Purple loosestrife is common in swamps and wet places – it has gorgeous purple flowers, but outcompetes many of our native wetland plants that feed pollinators and other animals. Like many invasives, it produces huge numbers of seeds and these seeds don’t all germinate the next spring – many stay dormant for years.

My approach to purple loosestrife is to dig out new, young plants when I see them show up in my garden beds. I recognize them by their square stem, the leaf shape and the color of the stem which is often reddish. But big established plants have massive root systems impossible to dig out, so I just use a curved, serrated knife to slice off the foliage once (or more than once) each summer. This prevents seed production, and reduces plant energy.

I only use organic techniques in the garden. This means no chemicals including herbicides. From what I have read, most herbicides will not kill the invasives mentioned in this article. They will set them back considerably, depending on the age of the plant and the dose of the chemical. But learning to recognize all the invasives is best. And if one appears on your landscape, get rid of it immediately! And remember, persistence is important.