How to Identify Oriental Bittersweet in Winter

The twining stems, yellow capsules and scarlet arils of Oriental Bittersweet are distinctive in winter.

Winter is an ideal time to look for Oriental bittersweet (Celastrus orbiculatus), an invasive vine that is spreading in Minnesota. Left unchecked, this aggressive vine can smother and strangle its hosts and shade out native vegetation. For those reasons, Oriental bittersweet is considered a noxious weed in several states, including Minnesota. The state’s Department of Agriculture includes it on the Prohibited-Eradicate list, which means all above- and below-ground parts should be destroyed.

What to Look For

Like native American bittersweet (C. scandens), Oriental Bittersweet, also called Asian Bittersweet, is a twining vine. Oriental Bittersweet vines are often tightly wrapped around their hosts, sometimes girdling them, whereas American Bittersweet vines are usually looser. In both species, buds are mound-shaped, 3-5 mm long (about an eight of an inch) and pointed at right angles to the stem. As the stems age they develop ridges and furrows in irregular diamond-shaped or striped patterns. Vines are either male (pollen-producing) or female (fruit-producing).

Clockwise from top left: A bud, tightly wrapping vine, and mature bark
of Oriental Bittersweet.

Male and immature vines are hard to identify to species, but mature female vines can be identified by the color and placement of their fruits. Oriental bittersweet capsules are yellow, opening in fall to reveal scarlet arils. Fruit clusters grow from the axils, the angles between the buds and the stem. 

In contrast, American bittersweet capsules are orange with darker arils. The capsules may fade to tan during winter, but typically they retain at least a few specks of orange. Fruit clusters grow at the ends of stems and branches rather than from the axils. 

Fruits of Oriental Bittersweet (left) and American Bittersweet (right). 

Hybrids Can Have Intermediate Characteristics

Hybrids of Oriental and American Bittersweet are uncommon, but of the few that have been found, at least one displayed intermediate characteristics. A hybrid female identified during one field study  produced clusters of flowers and fruits both at the ends of the stems and at the axils. Its capsules were light orange, intermediate between the yellow of Oriental Bittersweet and the darker orange of American Bittersweet. More information about hybrid research follows the references below.

Where to Look

Oriental Bittersweet grows in a variety of conditions. Typical habitats include fields, forest edges, and open woods, but even forest interiors can support this shade-tolerant vine. Removing Oriental Bittersweet from residential properties is as important as removing it from natural areas, because birds can spread the seeds wherever they fly.

How to Manage Oriental Bittersweet

Managing Oriental Bittersweet typically involves applying herbicides to the basal (lower) bark, leaves or cut stems. Seedlings can be hand-pulled, but they're hard to identify at that stage. Most of the references below include management advice. 

References

Oriental bittersweet. EDDMapS. Website accessed November 5, 2022.

Oriental bittersweet. Minnesota Department of Agriculture. Website accessed November 5, 2022.

Oriental bittersweet. Minnesota Department of Natural Resources. Website accessed November 5, 2022.

Differentiating Oriental and American Bittersweets. Minnesota Department of Agriculture. Website accessed November 5, 2022.

Asian Bittersweet. Woody Invasives of the Great Lakes Collaborative. Website accessed January 29, 2021.

 

Research on Hybrids

Hybridization between Oriental and American Bittersweet is suspected of contributing to the decline of the native species. One study (Pooler et al. 2002) found that crossing male Oriental Bittersweet with female American Bittersweet produced seeds that germinated sooner (with shorter dormancy) and seedlings that grew faster than American Bittersweet. Although these results hint at possible dominance of the hybrid vines, the study did not follow the hybrids into maturity.

A later study (Zaya et al. 2015) looked for naturally occurring hybrids across the eastern half of the U.S. to determine their prevalence and their potential impact on American Bittersweet. Using DNA analysis, the researchers found that 4-8% of 475 individuals sampled were hybrids. Only one female hybrid was confirmed among the sampled vines, and it displayed intermediate field characteristics. Flower clusters were produced both at the ends of stems and from the axils, and capsule color was light orange, intermediate between the yellow of Oriental Bittersweet and the darker orange of American Bittersweet. The hybrid capsules were smaller than either parent and produced few or no viable seeds. 

Two male hybrid vines were also confirmed among the vines sampled. When researchers examined their pollen grains, they found that more than 90% of them were smaller than the average for either parent, so they were judged to be inviable. Like the female hybrid, the male hybrids in this study could not reproduce as successfully as their parents.

The 2015 research suggests that the harm to American Bittersweet caused by hybridization isn’t from the hybrids themselves. Unlike some crosses between native and introduced species, hybrids of American and Oriental Bittersweet appear to be uncommon, and they do not successfully reproduce to form aggressive populations. Instead, the harm comes from wasting the reproductive potential of female American Bittersweet vines.  All the hybrids identified in the 2015 study resulted from Oriental Bittersweet pollen fertilizing American Bittersweet eggs. Because female flowers so pollinated could not then accept pollen from a male American bittersweet, they were not able to produce seeds that could maintain the native population.

References:

Pooler, MR, Dix, RL, and J Feely. 2002. Interspecific hybridizations between the native bittersweet, Celastrus scandens, and the introduced invasive species, C. orbiculatus. Southeastern Naturalist 1(1): 69-77.

Zaya, D.N., Leicht-Young, S.A., Pavlovic, N.B. et al. 2015. Genetic characterization of hybridization between native and invasive bittersweet vines (Celastrus spp.). Biol Invasions 17, 2975–2988. https://doi.org/10.1007/s10530-015-0926-z

How to Identify Common Buckthorn in Winter

A female Buckthorn displays a persistent leaf, fruits and terminal thorn that help with winter identification.

For an explanation of the terms used here, see a previous post, “How to Identify Deciduous Trees and Shrubs in Winter.”  A two-page guide to Buckthorn ID is free to download. 

Common Buckthorn, Rhamnus cathartica, is an invasive shrub or small tree that was introduced to the U.S. in the late 1700s as an ornamental and a hedge plant. Thanks to its prolific fruit production, it soon spread to a range of habitats, from forests and savannas to prairies and wetland edges. According to Bell Museum herbarium records, it has been present in Minnesota since at least 1937, when it was found at a barge terminal on the Mississippi River in Minneapolis.

This aggressive species degrades habitat in many ways. Typical of many invasive plants, established stands of Buckthorn shade out native species and reduce diversity. Their nitrogen-rich leaves are quickly consumed by soil invertebrates, including invasive earthworms that deplete the duff layer of organic matter. The resulting bare, mineral soils erode more easily and are more quickly colonized by additional Buckthorn or other invasive plants, such as Garlic Mustard (Alliaria petiolata).

Everything green in this understory is Common Buckthorn. The photo was taken in November.

 

Because Buckthorn retains its leaves late into fall, it is easy to recognize by the green layer it forms under a leafless canopy. Even after it sheds its leaves, however, there are easy ways to recognize this species. In fact, winter can be an ideal time to map and manage Buckthorn. This post shows how to identify the plant by its twigs, buds, bark and fruits.

“Buckthorn” Describes the Ends of the Twigs

Buckthorn's common name comes from the pair of curved buds at the ends of the twigs. They resemble a buck's hoof, and because a thorn is often found between them, the plant is called "buck-thorn." All buds are covered by multiple, dark brown scales. Lateral buds are mostly subopposite. 

Bark is Warty to Flaky with an Orange Inner Layer

Buckthorn saplings have silver or brown bark with light, horizontal, warty lenticels. As the trunk grows, its bark turns dark brown and flaky. Inner bark is bright orange. The bark of cherries and plums can look like Buckthorn, but none have bright orange inner bark or brown, subopposite buds.

Females Have Blue-Black Fruits – Lots of Them

Common buckthorn has separate male (pollen-producing) and female (fruit-producing) plants. Females produce hundreds to thousands of globose, dark blue to black, berry-like fruits that are clustered at the nodes. The fruits may stay on the plant through winter. Ingesting them causes nausea and vomiting, so this plant is also called purging buckthorn. 

Buckthorn Look-alikes

Glossy Buckthorn (Frangula alnus), another invasive, prefers wetter habitats than common buckthorn. Its twigs are not tipped with thorns. Buds are alternate and in winter are covered by rust-colored hairs rather than scales. Bark of younger trunks is speckled. Inner bark is yellow.

Left: A twig of Glossy Buckthorn showing alternate buds and a terminal bud covered with rust-colored hairs.
Right: Lower stems of Glossy Buckthorn showing speckled bark. The largest stem is about 5 cm (2 inches) in diameter.

Alder-leaved  Buckthorn (Rhamnus alnifolia), a native shrub typically found in moist or wet soils in forested parts of Minnesota, rarely reaches more than 3 feet tall, and its stems are typically no more than 1 inch in diameter (Smith 2008). Its buds are clearly alternate and lack thorns.

References

Buckthorn. Minnesota DNR. Website accessed January 12, 2021.

Buckthorn Management. Minnesota DNR. Website accessed January 12, 2021.

Buckthorn: What You Should Know, What You Can Do. Minnesota DNR. Guide EWR_395_17. (This is formatted as a printable guide. )

Common Buckthorn. University of Minnesota Extension Service. Website accessed January 12, 2021.

Common Buckthorn. Woody Invasives of the Great Lakes Collaborative. Website accessed January 14, 2021.

Common or European Buckthorn. Minnesota Department of Agriculture. Website accessed January 12, 2021.

Winter Control of Buckthorn (video). University of Minnesota Extension Service. December 2013.

Smith, W.R. 2008. Trees and Shrubs of Minnesota. University of Minnesota Press, Minneapolis.