Fighting Invasive Bittersweet and Stiltgrass
First control, then restore. A forest edge tapers to protect the ecosystem at its interior.
Birds like Oriental bittersweet; that doesn’t make it good for the ecosystem.
Dear Avant Gardener, What is it with forest edges? As far as I can tell, they are good for one thing — bittersweet. In many years' war with this plant, I have wished for a magic weapon. I think I found one: Invasive grasses. Specifically stiltgrass and lady's thumb. These grasses have superpowers. Wherever I have yanked and pulled and extracted vines by the root, the grasses have propagated like mad. And they are outcompeting the vine. Amazing!
Looking into the woods in those areas, instead of a blanket of vines I see what appears to be orderly transition. The trees are not being murdered. The grasses beneath reach up to an appropriate height. Nice. As much as I am proud of what I have done to restore sanity to that enigmatic field-forest transition, I can't get past the feeling that I am not really moving things along in a positive direction.
My question: Is it ethical to battle one invasive with another invasive? I think I know what Dr. Kissinger would say. But what about Dr. Bronner and Dr. Scholl? And what about you? — Can of Worms in Dutchess County, NY
That’s frightening 😱, not ethical. We can’t apply a contemporary aesthetic to an ancient ecosystem, equating “orderly” and “appropriate height” with good. There’s nothing “nice” about the way stiltgrass takes over forest understory with a blanket of greenery. Stiltgrass is pernicious (see “Why” below).
Conversely, some vines that look “murderous” fulfill ecological functions. Your bittersweet is most likely the invasive Oriental species (Celastrus orbiculatus). There is also a non-invasive American bittersweet (Celastrus scandens), more common in the midwest. It grows more slowly and does not get as big.
So, how can you eradicate your stiltgrass before it destroys the entire forest floor? And how can you keep the Oriental bittersweet at bay?
Stop pulling out roots; do this instead.
The key to battling invasive plants is control and then restore, i.e., plant beneficial native plants in their place. To control the invasives:
Stop yanking out bittersweet — or anything else. That’s a counterproductive horticultural tradition and a hard habit to break. When you pull out plants by the roots, you expose seeds in the soil to light, causing them to germinate. If your soil’s “seed bank” is mostly invasive like bittersweet, stiltgrass, and lady’s thumb, that’s what will grow there.
Cut weeds to the ground before they seed. I found cutting Oriental bittersweet every few weeks eventually killed existing vines. In future years, you will still have to occasionally cut new vines that emerge after birds resting on your trees drop seeds in their poop. You can mow stiltgrass in late summer after it flowers but before seeds set.
Stiltgrass and lady’s thumb, like turf grass, can also be smothered. Cover it with six or more inches of fallen leaves. You can seed or plant directly into the decaying leaves.
When you expose or disturb soil, plant fast-growing native annuals to outcompete the invasives in your seed bank. For example, seeds are widely available online in bulk for Virginia wild rye (Elymus virginicus) and Partridge pea (Chamaecrista fasciculata), both native to the eastern two-thirds of the U.S.
Restoring the forest edge
Natural plantings transition gradually in height from field to forest, sealing the edge and preventing further disturbance of the forest interior. Shrubs are key. Consider planting early successional species native to your Northern Highlands ecoregion along your forest edge. These include, from taller to shorter, paper birch (Betula papyrifera), sassafras (Sassafras albidum), pagoda dogwood (Swida alternifolia), and summersweet (Clethra alnifolia). For more shrub suggestions, see my column, “Creating a Privacy Hedge: Part II.”
In your area, you can buy and get advice about native shrubs and groundcover plants at Catskill Native Nursery.
— The Avant Gardener
Why, How, Wow!
Why?
Japanese stiltgrass (Microstegium vimineum), also known as Nepalese browntop and Asian stiltgrass, replaces native vegetation in a wide range of ecosystems. In turn, the lack of native plants harms insect, bird, and other native species that evolved with those plants and depend on them.
Areas infested with Japanese stiltgrass have decreased biodiversity. In addition to the early-season plants that are typically crowded out by invasive species, late-season grasses, sedges, and herbs are also affected. Infested areas also have an increased occurrence of other invasive plants and decreased native wildlife habitat and can provide good habitat for invasive animals including the cotton rat which can further affect local wildlife.
Japanese stiltgrass is not preferred by grazers such as white-tailed deer, goats and horses, which adds to its ability to outcompete native, preferred vegetation. A 2010 study by Pisula and Meiners indicates that Japanese stiltgrass has allelopathic potential to inhibit seed germination. — New York State Invasive Species Information
Stiltgrass outcompeting native vegetation (source: Michael Ellis - Own work, CC BY-SA 4.0, Wikimedia)
How
The first step in ecological landscape restoration is identifying any invasive plants. Japanese stiltgrass resembles a small, delicate bamboo. It grows up to three and a half feet long. Its leaves are one to three inches long with an off-center mid-rib and are alternately arranged on the stalk. There are several similar-looking North American native grasses. Here’s how to tell the difference:
The unique line of silvery hairs found on the midrib of Japanese stiltgrass is a quick identifier. Unlike most native grass leaves which are rough in one direction when rubbed, Japanese stiltgrass leaves are smooth in both directions. — New York State Invasive Species Information
Note silvery midrib in stiltgrass leaves
Wow!
Forest edges naturally have more understory vegetation than the interior of the forest, where less light penetrates. Paper birch and pagoda dogwood are two of the many strikingly beautiful species that thrive at the forest edge.
Paper birch and pagoda dogwood at forest edge
Related Resources
Wonder what Kissinger would say? See if you can figure it out from this review of the 2020 Kissinger bio, The Inevitability of Tragedy.
Want to know whether your bittersweet is Oriental or American? Want to know the difference? See this USDA flyer.
Live elsewhere and want to find native shrubs? Nationally, you can buy online at Bower & Branch Audubon Native Shrubs
