Mar 6, 2019

Invasive Asian and Oriental Bittersweet

Vine has visual appeal but can choke off surrounding trees and plants

By Lawanda Jungwirth

You know how when you were a kid there were things in your house that were just always there but you didn’t give them much thought or ask any questions about them? When I was a kid, my mother had a plastic decoration with brown stems and little red berries with orange-y wing-like things surrounding the berries she put out each fall.

I never thought much about it, nor did it even enter my mind once I left home until about 20 years later when I spotted the exact same berries – real ones – growing on a vine on the back fence in the yard of the house my husband and I purchased. I was astounded the real berries looked just like the fake ones and that they were actually growing on my fence.

I hadn’t even known the plastic stem and berries were modeled after a real plant! I identified the vine  as bittersweet, but it wasn’t until many years later I first heard the term “invasive plant” and realized bittersweet was one of them. The vine was kind of growing like a monster and had completely covered the neighbor’s tree in the field behind the fence, so that should have been a clue. Once I realized what was happening, I cut the vine back year after year until we eventually moved and the battle became someone else’s. Or not.

When the Wisconsin Department of Natural Resources came out with its invasive species rule, NR40, in 2009, both Asian Bittersweet(Celastrus loeseneri)and Oriental Bittersweet (Celastrus loeseneri) made the list. Even though I had experience with the out-of-control bittersweet vine in my own backyard many years earlier, I had never once seen it anywhere else.

In 2017, my husband and I visited a farmer’s market in Stillwater, Minn., where we struck up a conversation at the local master gardener booth and they mentioned how bad the local bittersweet invasion had become. Still, I didn’t think it was a problem here in Wisconsin, although I had a bad feeling it would eventually march across our western border and invade the Badger State.

Then only two months later, I spotted the distinctive red berries on a vine climbing a telephone pole on the Terrell’s Island trail maintained by the Butte des Morts Conservation Club outside of Omro. I was as floored as I was the first time I saw it on my back fence. Then, of course, I saw it in another spot on the trail, climbing a tree. Invasive bittersweet had arrived at a natural area near me.

The bittersweet profile

Bittersweet vines invade forests, woodlands, fields and hedge rows and can grow in open areas or right in the middle of a forest. They can become so large and heavy that they pull down mature trees, usually after girdling their trunks and preventing nutrients from reaching the upper branches or by smothering them so that the shaded leaves can no longer photosynthesize. The tree eventually dies.

Stems may grow up to a diameter of six inches and can reach 60 feet in length. Glossy green leaves with shallowly toothed edges are two to five inches long and are round with a pointed tip. Small, five-petaled greenish yellow flowers appear in clusters of three to seven in late spring in the leaf axils – where the leaf attaches to the stem – and are inconspicuous.

The bright red fruits are abruptly noticeable in autumn just after the leaves fall, even though they are on the plants – although green – from mid-summer on. In fall the seed pots split open to reveal three red fleshy fruits, each containing one or two seeds. The split open seed pods are the orange, wing-like things I noticed as a child.

Birds and small animals are happy to eat the seeds and deposit them elsewhere to spread the invasion. Along with multiplying by seed, invasive bittersweet also spreads by underground roots which sprout new stems.

There is a non-invasive American bittersweet, but nursery plants may be mislabeled either accidentally or intentionally and landscapers may find themselves planting the invasive type. American bittersweet (Celastrus scandens)has fewer, larger clusters of flowers and fruits, which are found at the end of the stems rather than in the leaf axils. The native bittersweet’s leaves are less rounded than those of the Asian and Oriental plants’ and are nearly twice as long as wide.

Unfortunately, hybrids occur between the native and invasive bittersweets, which makes identification difficult. The bittersweet in the photos here seems to be a hybrid with fruits both at the ends of stems and in leaf axils.

Asian and Oriental bittersweet are “restricted” in Wisconsin by Rule NR40. This means you are not allowed to buy, sell, transport or introduce them. You are allowed to possess restricted bittersweet – permit it to continue to grow on your property - but are strongly encouraged to eradicate it.

The bittersweet end?

There are both chemical and non-chemical control methods for bittersweet. Physical removal by pulling or digging is the first method. Roots must be removed completely or they will resprout. If seeds are on plants when removal takes place, avoid transporting them unless you can ensure by use of tarps or old sheets that seeds will not be scattered where they can germinate.

The second method of control is by mowing. Obviously this works only for young plants. Cutting must begin in spring and should be done every two weeks throughout the growing season to exhaust root reserves. Mowing is often used before chemical treatments. Allow one month after the last cutting to let the canopy regrow before applying chemical pesticides to leaves.

Prescribed burning is not recommended unless it is integrated with other methods for a couple reasons. While spring burns will kill germinating seedlings, it will also suppress above-ground growth of other established plants, thus opening an area for bittersweet to jump in and invade. Use caution when burning vines that climb trees since the flames could climb the vine and carry fire to the tree crowns. A handheld propane torch is effective for scorching and killing bittersweet seedlings.

The final control method is by herbicide application. There are quite a few different chemicals that can be used and several methods of application. Instead of going into all of them here, refer to this publication from the University of Wisconsin-Extension:

Pay special attention to the timing of application, don’t apply chemicals on a windy day, don’t use herbicides of any kind near water, and follow all safety precautions.

The annual cutting back I did on the bittersweet on my backyard fence was counter-productive. I usually did it on a cool mid-summer day and all I was doing was, in effect, pruning to encouraging new growth. I didn’t know any better at the time, but now I do and now you do, too.

Cutting back bittersweet isn’t a control method!