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

Oriental Bittersweet in Winter

The yellow capsules and scarlet arils of Oriental Bittersweet stand out in winter.

Oriental, Asian or Asiatic Bittersweet (Celastrus orbiculatus) is an introduced vine that can girdle or smother its hosts. Efforts to remove this invasive plant are complicated by its similarity to native American Bittersweet (C. scandens).

Fortunately, mature female vines of each species are noticeably different, especially in fall and winter. Open this updated post from January 2021 to learn how to tell them apart. A two-page ID guide is free to download. 

How To Identify Deciduous Trees and Shrubs in Winter

Even after deciduous trees and shrubs shed their leaves, most can be easily identified using their winter characteristics. Buds, bark and overwintering fruits are reliable clues to identification. With a little practice, they're as useful as leaves to pin down a plant's name.

Look at Twigs

If they’re within easy reach, twigs have many features that are helpful for winter identification. The size, shape and arrangement of buds and leaf scars are reliable clues. Using green ash (Fraxinus pennsylvanica) as an example, here's what to look for. 

Depending on the species, buds may be covered with no scales, one scale, or several scales. Bud shape may be long and narrow, short and wide, or somewhere in between. Leaf scars and vascular bundle scars may be conspicuous or so small they are difficult to see without magnification. In any case, the combination of characteristics is unique for a species, as shown in the photographs below. 

Box Elder (Acer negundo) twigs are maroon in winter, often with a blue-gray, waxy coating. Surfaces with that coating are said be glaucous. The twigs turn green in spring but may still be glaucous.

Buds scales are covered with short gray hairs that are easiest to see with a magnifying lens. There are two buds per node, so Box Elder has opposite buds. If the twigs and buds are out of reach, look at the branching pattern. That, too, will be opposite.

Leaf scars are narrowly V-shaped. The opposing V’s meet on each side of the twig, forming a point.

Inside each leaf scar are three vascular bundle scars (arrows, left). One is directly below the bud and two are at the tips of the V, one on each side. 

The ends of Black Walnut (Juglans nigra) twigs are covered with short, gray or white hairs. Buds are similarly hairy, and they are longer than wide. Lateral buds are alternate – one per node.

Beneath each lateral bud is a heart-shaped or V-shaped leaf scar. Vascular bundle scars are dark brown. One semicircular bundle scar is at the bottom of the heart and two smaller bundle scars are at the top. The pattern reminds some of a face, with two eyes above and a mouth below.

The buds and leaf scars of butternut, Juglans cinerea, are similar, but the leaf scars are straight across the top instead of notched. 

Red Elderberry (Sambucus racemosa) doesn’t have twigs, but its coarse stems and branches are just as distinctive.

The lateral buds are opposite and covered with green, brown or red scales. Flower buds tend to be larger – almost as wide as they are long.

Below the buds are triangular or shield-shaped leaf scars with five large vascular bundle scars.

The stems and branches of Red Elderberry also have conspicuous “warts” on their bark. These raised areas are lenticels, eruptions of the bark that allow gas exchange with interior tissues. Many species have lenticels on their twigs or young stems, but few are as conspicuous as Red Elderberry. 

Look Closely at Bud Arrangement

Knowing the bud arrangement – opposite, subopposite, alternate or whorled – is especially helpful because it quickly narrows the choices for identification. If the twigs and buds are out of reach, look for the branching pattern. It will have the same arrangement as the buds. Be sure to look at several branches and twigs; there may be more than one kind of arrangement. Although that can be confusing, it’s useful information.

With the bud or branch arrangement determined, the next step is to find or recall a list of species that belong in that group. To remember species with an opposite arrangement, a helpful mnemonic is “MAD Cap Buck Horse.”

• M is for maples, genus Acer.
• A is for ashes, genus Fraxinus.
• D is for dogwoods, genus Cornus.
• Cap is for the family Caprifoliaceae, which includes honeysuckle shrubs, genera Lonicera and Diervilla.
• Buck and Horse are for Buckeyes and Horsechestnuts, respectively. Both are trees in the genus Aesculus. 

Other trees and shrubs also have an opposite arrangement. To the list above add Viburnums (Viburnum species), Elderberries (Sambucus species), burning bushes (Euonymus species) and buffaloberries (Shepherdia species).  Amur Corktree (Phellodendron amurense), an invasive tree, also has opposite buds and branches. There are several more genera in this group. A thorough guidebook will point them out.

If the plant doesn’t have opposite buds, then there are three other categories to choose from. Most of the remaining plants will have an alternate arrangement. Woody plants with a whorled arrangement are unusual. Catalpa (Catalpa speciosa), an introduced tree, has three buds and leaf scars per node. Native Bog Laurel (Kalmia polifolia), a low shrub of bogs, can have either an opposite or a whorled arrangement. Subopposite buds are unusual, too. Common Buckthorn (Rhamnus cathartica) has buds that can be subopposite, opposite or alternate.

Look at Bark

The bark of many trees and shrubs is distinctive enough to identify the species. The tricky thing is that bark changes with age. It often starts out smooth but develops more texture as the tree or shrub ages. Despite that change, the appearance of bark is a useful characteristic. The following questions highlight some of the features to look for.  

•  Is the mature bark ridged and furrowed? If so, how deep are the furrows? Do the ridges and furrows form any kind of pattern?
• Instead of ridges, does the mature bark have flat-topped plates, shaggy strips, or scales?
• Are lenticels present on younger bark? If so, are they round, linear, or both? If they are linear, what direction do they run?
• What color is the inner bark, the layer just below the surface? The brightly colored inner bark of some species provides a clue to their identification.

As with twigs, bark shows a lot of variety, both within and among species. The photographs below hint at some of that diversity.

Green Ash bark is brown to red when young, with white lenticels. Older bark is ridged and furrowed in a tight, honeycomb or diamond-shaped pattern.

The young stem on the far left is about 1 inch in diameter. The trunk of the mature tree is about 14 inches in diameter.

Young stems of Common Buckthorn (Rhamnus cathartica) have smooth, gray or brown bark with prominent, horizontal lenticels. Older bark is dark brown, rough and flaky. The photo at far left shows a clump of buckthorn stems at various ages. The largest one is about 3 inches in diameter.

One quick way to identify Common Buckthorn is by its bright orange inner bark. 

Black Cherry (Prunus serotina) has bark that changes drastically with age. On young trees, the bark is red-brown and smooth with horizontal white lenticels. The trunk at far left is about 4 inches in diameter.

In contrast, the bark of mature Black Cherry trees is scaly. Some say it looks like burned potato chips. The trunk at left is about 15 inches in diameter.

Look for Fruits

The fruits of most woody plants are shed before winter, but those of a few species tend to hang on. If they are present, their size, shape, color and texture (dry vs. juicy) and their placement on branches are important to note. As shown below, fruits can be easy and important characteristics for identification.

Bittersweet vines (Celastrus species) are easiest to identify by their persistent winter fruits. Invasive Oriental Bittersweet (C. orbiculatus), far left, has yellow capsules that open to expose fleshy orange arils. The clusters are found along the length of the stems.  In contrast, American Bittersweet (C. scandens), left, has orange capsules and red arils. Its clusters are found only at the ends of the stems.

Winged fruits are called samaras. Some samaras are in pairs, as in Amur Maple (Acer ginnala), far left. Other maples also have paired samaras.

Some samaras are single, as in Green Ash (Fraxinus pennsylvanica), left. Other ash species are similar. Some call the samaras keys, because a cluster of them resembles a bunch of keys on a ring.

Berries and berry-like fruits are typically juicy and either spherical or oblong.  Common Buckthorn (Rhamnus cathartica) has dark blue, almost black, berry-like fruits clustered at the nodes (far left). Inside each are 3-4 hard, seed-like stones (arrows, left). This type of fruit – juicy or fleshy with one or a few stones – is called a drupe. Berries are similar, but they lack stones and are juicy throughout.

Look for a Guidebook

Using a combination of buds, leaf scars, bark and fruits, anyone can identify trees, shrubs and woody vines in winter. All that’s needed is a guidebook, a reference that matches observed characteristics to names of species. Guides dedicated to winter identification are few, but there are some resources online and in bookstores that may be helpful.

• The LEAF Program from UW-Stevens Point is a K-12 forestry education initiative that offers many online resources. Under the link for Curriculum & Resources, choose LEAF Tree Identification Tools. The LEAF Winter Tree ID Key is available there as a downloadable PDF.
• Winter Botany: An identification guide to native trees and shrubs, by William Trelease. Dover, 1967. ISBN 0-486-21800-7.
• Fruit Key and Twig Key to Trees and Shrubs, by William M. Harlow. Dover, 1959. ISBN 0-486-20511-8.
• Winter Tree Finder: A Manual for Identifying Deciduous Trees in Winter, by May T. Watts and Tom Watts. Nature Study Guild Publishers, 1970.

Reviews of these references or suggestions for others are welcome. Please use the contact form at right to offer comments. Upcoming posts will feature winter identification of select groups of plants.