Saturday, January 30, 2021

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 orbicu...