By Nancy Ostman
Plantations

F. Robert Wesley

Cornell Plantations gardener Krissy Faust (left) and summer '06 intern Ann Kelley (right), with a truckload of garlic mustard pulled from the Mundy Wildflower Garden.

Plantations’ natural areas now encompass more than 4,000 acres on 35 biologically diverse sites. These ecological communities include bogs, fens, gorges, ravines, meadows, and old growth forests. The natural areas are used primarily for research and teaching by Cornell faculty and students, but they are also important places for recreation, inspiration, and relaxation.

Unfortunately these communities have also become home to invasive plants, which continue to arrive — and thrive — in our precious natural areas. Every season we find common invasives like garlic mustard in new locations, pushing out and replacing our native plants. Our staff spends more and more of its time finding new ways to beat back invasive plants. We are not alone; natural-areas managers and gardeners continually report the arrival of aggressive new weeds. If left alone, these weeds may dominate a garden or, in our case, an entire ecosystem, often reducing its habitat value for wildlife. The scale of the problem in natural areas is overwhelming. Where are all of the invasives coming from? How did we get in this position?

Invasive plants probably arrive in all the usual ways, deposited in bird droppings, dropped from clothing or animal fur, blown on the wind, carried by water, or walked into sites attached to shoes, paws, or hooves. Gardeners may plant invasives in their gardens, either purposefully or inadvertently. But once there, these plants don’t tend to stay put. Some species like garlic mustard, Japanese knotweed, and even kudzu were once considered desirable food plants. Our tastes may have changed, but the leftover plants remained in a fairly steady state at abandoned home sites for decades. And then they began to spread. Ground covers like periwinkle, English ivy, and crown vetch are desirable for their ability to hold off other weedy plants. This same characteristic can go badly awry, however; these ground covers now march across the forest floor. Shrubs widely considered suitable for hedges, such as privet, multiflora rose, and Japanese barberry, share these same fortunate and unfortunate traits. In fact, any garden plant described as tough or easy to maintain could become invasive, given the right conditions.

Invasive plants often have lots of viable fruit, as well as lots of flowers — hence names like multiflora rose. Last year was a notably great year for flowers all through the spring, from the first spring flowering of periwinkle, to early fall, when Japanese knotweed was covered in frothy blooms. Then we had a remarkable fruiting season for both natives and non-natives alike; the fiery red winterberries in our swamp forests remained a spectacular sight through December. A more dire sight was the abundance of black berries hanging from the branches of the invasive European buckthorn into spring. In late fall, along Route17 heading from Ithaca toward New York, invasive Japanese barberries created a red haze above the forest floor.

In natural areas, invasive species often arrive first along transportation corridors. We see garlic mustard “walking” down or along trails and sweeping into natural areas along swales and small creeks. Pale swallowwort, with its fluffy, milkweed-like windborne seeds, first showed up along the windy edges of the lake cliffs. Seeds from the Asian hedge maple twirled in the wind from their horticultural homes into our meadows and forests. Birds ate and dropped the seeds from many beak-sized berries.

But why the recent glut? Perhaps global warming is favoring the non-natives. Invasives may also be responding to changes in the way we do business. For example, we use large equipment, traveling over long distances, to dig out roadside ditches and mow roadsides — thus creating a seed bed that is wide open to weed seeds. Invasive phragmites, a tall wetland grass, and Japanese knotweed both do very well in such disturbed wet ditches. We use lots of road salt, and some invasive plants can survive under such harsh roadside conditions where natives cannot. As development spreads, disturbed sites may be more connected than undisturbed sites, favoring the spread of invasives over the spread and recolonization of native plants.

F. Robert Wesley

Cornell Plantations staff, ably assisted by professor Tim Fahey and his natural resources undergraduates, pull garlic mustard in the woods along the Morgan-Smith Trail, a Cornell Plantations Natural Area.

What are we doing about invasives at Plantations? We pull them, cut them, treat them with herbicides, and smother them with black plastic. We torch them and use weed-whackers just to slow down their growth. We cut off seed heads to slow their spread. Every treatment, especially herbicide treatment, depends on understanding the growth habits of the specific plant involved, so we can hit them when they are most vulnerable. For many woody plants, that means in the fall. We cut them near ground level and immediately paint the exposed stems with herbicide. Because the plants are then translocating nutrients back to the root systems for winter storage, the herbicide is drawn to the root system, with a better chance of killing it. In spring, when nutrients are moving up from the roots, herbicides are ineffective.

When considering a treatment in a natural area, we must first consider what plants and animals we are trying to protect. This means we find most herbicides unacceptably strong or long-lasting. We rely almost exclusively on judicious applications of glyphosate (better known as Round-up or Rodeo), which quickly breaks down into nontoxic components in the environment. We may also find traditional broadcast spray treatments unacceptable. For example, to treat swallowwort last summer, we used a labor-intensive application method we dubbed “the bloody glove.” Using a thick rubber glove to protect our hands, we put a cloth glove over it, soaked it with herbicide, and then carefully wiped the stems of individual plants. To treat phragmites, we first cut off the tops of the stems at about waist height. Then, using a chemical squirt bottle, we squirted a small amount of herbicide into the hollow stem. We may treat a species either very late or very early in the season, either before or after the understory of native species has gone dormant. For example, the over-wintering rosettes of garlic mustard remain green all winter, growing and photosynthesizing opportunistically and gaining a leg up on the native plants that emerge in the spring. A warm December week may find us spraying herbicide on these rosettes while our native plants are safely dormant.

We always remind ourselves that more herbicide is not necessarily better — either for the ecosystem or for our purposes. We can use far less herbicide if we apply it to the stem of a plant than if we spray it on the leaves. Using too strong a dose may just kill the top of a plant, allowing it to resprout before the herbicide can be drawn down to the stem. Sometimes a mechanical method is just as fast and effective as a chemical method. Thousands of emerging garlic mustard seedlings can be destroyed quickly by hand or with a weed whip.

We have to prioritize. We can’t keep every invasive plant out of every natural area, so we have to look carefully at the impacts of the invasives on the native plant species at each site. We walk our borders, looking for entry points and trying to stop the invasion at the edges whenever possible. Often, the sooner an invasion is stopped, the easier it is to control. We have a watch list of the worst invasives for our region, and we try to stop certain species from becoming established in certain areas. We have learned the hard way that it is worth our while to be meticulous during the early stages of an invasion. We may return to a given site again and again and again to keep an invasive species in check. If it got there once, it can get there again, and usually there are remaining propagules, such as buried seed or root fragments, that allow a population to reestablish itself.

Sometimes when we set back the invasives, native species bounce back. However, at other times we are left with a bare spot wide open to colonization by the same or another invasive plant. We have begun collecting native plant seeds so we can quickly replant these areas with appropriate and desirable species.

Research on invasive plants and how best to treat them is growing rapidly. We check the Web each season to see if any new information on how to treat a given invasive is available. Several organizations, such as the Nature Conservancy, produce fact sheets with information on the life cycles of various invasive plants, and alternative methods for how to get rid of them. Researchers hold out hope that introduced insects may help keep some invasive plants, like garlic mustard and purple loostrife, in check. Ultimately, the best tool to protect our rich diversity of native flora may be an equally  rich diversity of strategies.