American bittersweet (Celastrus scandens) and invasive round leaf bittersweet (C. orbiculatus) are like identical twins: Only subtle differences tell them apart.
This is especially true after their leaves have expanded and before they develop flowers. In this in-between time, the vines are practically impossible to distinguish. Both have twining stems with alternate, toothed leaves. Leaf shape is generally round, eliptical or egg-shaped with pointed tips, but their shapes are variable and overlapping. Even the name "round leaf" isn't much help to detect the invasive species, because its leaves aren't always round.
In spring, though, two characteristics can help with ID: the way the leaves unfold or unfurl from the buds, called vernation, and how the flowers are positioned along the stem.
Vernation
Inside the bud and as they begin to emerge, the edges of American bittersweet leaves are rolled inward, toward the upper side of the leaf. The botanical term for this is involute. In contrast, the leaves of round leaf bittersweet are folded vertically along the midvein, so the upper sides of the blade face each other. This is a conduplicate pattern. Depending on weather, vernation may happen quickly. Early observations are helpful.
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| Left: American bittersweet leaves are involute -- they are rolled inwards in the bud and as they emerge. Middle and right: Round leaf bittersweet leaves are conduplicate -- intially, they are folded inward along the midvein. Photos by Lisa McIntire. |
Flower Position
Bittersweet vines typically flower in May and June. The flowers of both vines are about 1/4 inch (5 mm) across and greenish white to greenish yellow with five petals. Male and female flowers are on separate vines.
The difference is the flowers' positions. American bittersweet flowers are clustered only at the ends of stems, whereas the flowers of round leaf bittersweet grow from leaf axils, the areas where leaves join stems.
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| American bittersweet flowers are clustered at the ends of stems and branches. No clusters grow from the axils. These are male (pollen-producing) flowers. Photo by Peter Dziuk, (c) 2011, from Minnesota Wildflowers. |
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| Unlike American bittersweet, clusters of round leaf bittersweet flowers grow from leaf axils. These are female (seed-producing) flowers. Photo by K. Chayka, (c) 2013, from Minnesota Wildflowers. |
After flowering, male vines can't be identified to species using visible characteristics. Female vines, though, will develop fruits, and they are in the same positions as the flowers: American bittersweet at the ends of stems, round leaf bittersweet at the axils.
The spherical capsules are initially green, but as the season progresses their walls ultimately turn either orange (American) or yellow (round leaf). In later summer and fall, the capsules split open to reveal red arils, fleshy berry-like structures that enclose the seeds. American bittersweet capsules contain only one seed; round leaf bittersweet capsules contain five.
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| Left: Mature American bittersweet capsules are orange, later splitting to reveal red arils. Right: Mature round leaf bittersweet capsules are yellow. These have already opened. Both photos by Leslie J. Mehrhoff, University of Connecticut, Bugwood.org. |
Why Early ID Is Important
Round leaf bittersweet is originally from Asia, introduced here for its colorful fruiting vines. It's more aggressive than native American bittersweet and will girdle and smother the trees it grows on. Its dense growth can shade out any plants below. The result is lower plant biodiversity, with consequent effects on insects, birds, and other animals.
In Minnesota, round leaf bittersweet is listed as a noxious weed, and by law it must be prevented from spreading. Identifying the plant early, especially before it produces fruits, is important to contain its spread. More information is available from the Minnesota Department of Agriculture and the Minnesota Department of Natural Resources.
Despite control efforts, round leaf bittersweet still finds its way into Minnesota. Seeds are dispersed by birds, and some online sellers will ship seeds or live plants into the state. Mistaken identity can be a problem, too. What's sold as American bittersweet is sometimes round leaf. Look at any photos on the seller's website and compare them to the pictures above. Better yet, if you want American bittersweet, buy it from a reputable, local native plant nursery.If Round Leaf Bittersweet Is on Your Private Property
If you have a wreath, spray or garland made from real fruiting vines, keep it indoors. Don't toss it outdoors, and don't throw it in the trash. Both will aid its spread. If you want to dispose of it and there is no other round leaf bittersweet on your property, take it to a composting site that accepts noxious weeds (call first) or enclose it in a clear plastic bag and put it in the sun for several weeks to months to kill any viable seeds. Crushing the seeds with a hammer may also be effective.
