1

This publication is part of a series that provides an overview of the 

presence of invasive plant species monitored on an extensive systematic 

network of plots measured by the Forest Inventory and Analysis (FIA) 

program of the U.S. Forest Service, Northern Research Station (NRS). 

Each research note features one of the invasive plants monitored 

on forested plots by NRS FIA in the 24 states of the Midwestern and 

Northeastern United States.

United States Department of Agriculture

An Assessment of Oriental Bittersweet in Northern U.S. Forests Research Note NRS-251

Forest Service

Northern Research Station

Figure 2.—Dried oriental bittersweet for sale. 

Photo by Leslie J. Mehrhoff, 5503508 from Bugwood.org

Figure 3.—Oriental bittersweet in flower. Photo by 

Leslie J. Mehrhoff, 5501274 from Bugwood.org

Background and Characteristics

Oriental bittersweet (Celastrus orbiculatus; also known as Asian 

bittersweet) is a woody vine that is native to China, Korea, and Japan 

that was introduced as an ornamental around 1860 (Kaufman and 

Kaufman 2007). The vines can climb into trees where they can girdle 

stems and branches (Fig. 1). Additionally, the dense growth can shade 

out vegetation and break stems due to the weight. Birds, small animals, 

and humans are key vectors related to the spread of this species. 

Nurseries also aid in its spread. The showy fruit are sold for floral 

arrangements, crafts, and decor (Fig. 2; Czarapata 2005, Kaufman and 

Kaufman 2007, Kurtz 2013). 

Accurate identification is important as the native American bittersweet 

(Celastrus scandens) closely resembles oriental bittersweet. Some key 

differences are that the flowers and fruit of American bittersweet occur 

at the ends of the branches whereas those of oriental bittersweet are at 

the leaf axil. Additionally the capsule colors differ. Oriental bittersweet 

has yellow capsules and American bittersweet has orange capsules. 

Oriental bittersweet has higher shade tolerance (Kaufman and Kaufman 

2007).

Description

Growth: large vines can reach over 60 feet in length and over half a foot 

in diameter; leaves are 2 to 5 inches long, round or oval, simple, alternate, 

glossy, finely toothed.

Reproduction: almost always dioecious (male and female flowers on 

separate plants), five petaled flowers in clusters (Fig. 3); green fruits 

mature to yellow-orange capsules in fall, which open to show red berries.

Habitat: open woodlands, roadsides, urban areas.

Growth conditions: prefers sun to part shade.

Control: various chemical and mechanical methods; repeated 

monitoring is necessary due to resprouting (Czarapata 2005, Kaufman 

and Kaufman 2007).

Growth Conditions and Range

Oriental bittersweet is fire tolerant, hardy to -38 °F, requires 100 or more 

frost free days, and has moderate drought tolerance. It is currently found 

in 25 states and three Canadian provinces (NRCS 2018).

Figure 1.—Oriental bittersweet girdling a tree. 

Photo by Leslie J. Mehrhoff, 5487327 from Bugwood.org.
2

FIA crews visited 4,981 forested Phase 2 (P2) invasive plots 

across the NRS region for the 2016 inventory. The 2016 

inventory encompasses plots measured from 2011 to 2016. 

These P2 invasive plots are a subset of the standard P2 plots 

where 40 invasive plant species (IPS1) (39 species and one 

undifferentiated genus [nonnative bush honeysuckle]2) 

are monitored. Various attributes are collected including 

the occurrence and coverage of IPS as well as the standard 

forest variables measured on P2 plots (e.g., tree diameter, 

height). Overall, 52.0 percent of forested3 plots have one or 

more of the monitored invasives present.

Oriental bittersweet is found throughout most of the region 

on 241 plots (4.8 percent) across 18 of the 24 NRS states 

(Fig. 4). Connecticut has the highest percentage of plots 

with oriental bittersweet (56.0 percent), followed by Rhode 

Island, where 30.8 percent of plots have oriental bittersweet. 

Field crews did not observe this IPS on plots in Kansas, 

Minnesota, Missouri, Nebraska, North Dakota, or South 

Dakota.

For the 2016 inventory, oriental bittersweet is the eleventh 

most commonly observed invasive species after multiflora 

rose (30.5 percent), nonnative bush honeysuckles 

(19.4 percent), garlic mustard (11.2 percent), Japanese 

honeysuckle (8.0 percent), autumn olive (7.5 percent), 

Japanese stiltgrass (7.3 percent), Japanese barberry (7.1 

percent), black locust (6.5 percent), common buckthorn 

(6.4 percent), and reed canarygrass (5.6 percent). Additional 

information about the invasives monitored and county-level 

occurrence maps for the NRS region from 2005 through 

2010 can be found in Kurtz (2013).

Oriental Bittersweet Presence on Phase 2 Invasive Plots, 2016

1 Hereafter IPS may also be referred to as “invasive species”, “invasive plants”, or “invasives”.

2 Autumn olive (Elaeagnus umbellata), black locust (Robinia pseudoacacia), Bohemian knotweed (Polygonum xbohemicum), bull 

thistle (Cirsium vulgare), Canada thistle (Cirsium arvense), Chinaberry (Melia azedarach), common barberry (Berberis vulgaris), 

common buckthorn (Rhamnus cathartica), common reed (Phragmites australis), creeping jenny (Lysimachia nummularia), 

dames rocket (Hesperis matronalis), English ivy (Hedera helix), European cranberrybush (Viburnum opulus), European privet 

(Ligustrum vulgare), European swallow-wort (Cynanchum rossicum), garlic mustard (Alliaria petiolata), giant knotweed (Polygonum 

sachalinense), glossy buckthorn (Frangula alnus), Japanese barberry (Berberis thunbergii), Japanese honeysuckle (Lonicera japonica), 

Japanese knotweed (Polygonum cuspidatum), Japanese meadowsweet (Spiraea japonica), leafy spurge (Euphorbia esula), Louise’s 

swallow-wort (Cynanchum louiseae), multiflora rose (Rosa multiflora), Japanese stiltgrass (Microstegium vimineum), nonnative bush 

honeysuckle (Lonicera spp.), Norway maple (Acer platanoides), oriental bittersweet (Celastrus orbiculatus), princesstree (Paulownia 

tomentosa), punktree (Melaleuca quinquenervia), purple loosestrife (Lythrum salicaria), reed canarygrass (Phalaris arundinacea), 

Russian olive (Elaeagnus angustifolia), saltcedar (Tamarix ramosissima), Siberian elm (Ulmus pumila), silktree (Albizia julibrissin), 

spotted knapweed (Centaurea stoebe ssp. micranthos), tallow tree (Triadica sebifera), tree of heaven (Ailanthus altissima).

3 FIA defines forest land as land that is at least 10 percent stocked with trees of any size or land formerly having such tree cover and 

not currently developed for nonforest use. Generally, the minimum area for classification as a forest is 1 acre in size and at least 

120 feet in width. There are more specific criteria for defining forest land near streams, rights-of-way, and shelterbelt strips (USDA 

Forest Service 2016).

Oriental bittersweet infestation. Photos by 

Leslie J. Mehrhoff, 5487256 and 5487337 from 

Bugwood.org.

Figure 4.—Percent of Phase 2 invasive plots with oriental bittersweet, 2016. 

Percentages are rounded to the nearest whole number.
3

The percent cover of oriental bittersweet is 

shown in two figures, one that illustrates cover 

by state (Fig. 5) and a second that focuses on 

plot-level data (Fig. 6). It is important to use 

caution when looking at Figure 5 because in 

some states the overall averages are driven by 

a small number of plots. For the states with a 

low number of observances, Figure 6 is more 

informative since individual plot values can 

be assessed. Iowa is the state with the highest 

average percent cover of oriental bittersweet 

on plots (16 percent), however this is based off 

only two plots (Fig. 6). These maps, along with 

Figure 4, reveal important information related 

to the presence and abundance of oriental 

bittersweet in the NRS region. Over time 

these maps will allow us to assess changes 

in abundance and spread throughout this 

region.

Oriental Bittersweet Cover on Phase 2 Invasive Plots, 2016

Figure 6.—Average percent cover4 of oriental bittersweet on Phase 2 invasive plots, 2016. Approximate plot locations 

depicted. Percentages are rounded to the nearest tenth of a whole number.

Figure 5.—Average percent cover4 of oriental bittersweet on Phase 2 invasive 

plots, 2016. Percentages are rounded to the nearest tenth of a whole number.

4 Average percentage cover is calculated for plots based on subplot data for the portion of the plot that is forested. Each FIA plot 

consists of four circular 1/24-acre subplots located at the corners and center of an equilateral triangle that is 208 feet on a side.
4

Analysis in this section is limited to the 11 NRS states 

where oriental bittersweet is present on greater 

than 5 percent of plots, resulting in the analysis of 

1,752 plots (35.2 percent of the measured P2 invasive 

plots in 2016). Of the 1,752 plots, this destructive 

vine is present on 199. The 11 states in this analysis 

are Connecticut, Delaware, Indiana, Maryland, 

Massachusetts, New Hampshire, New Jersey, Ohio, 

Pennsylvania, Rhode Island, and West Virginia.

Data collected on sample plots suggest that oriental 

bittersweet is more common on plots near roads 

(Fig. 7; t-test; p<0.05). Several studies have highlighted 

the effect of roads on invasive distribution (Kurtz and 

Hansen 2013, Lundgren et al. 2004, Predick and Turner 

2008). Roads act as a conduit for seed dispersal and 

alter light and nutrient availability, as well as drainage. 

Vehicles traveling on roads carry propagules of many 

exotics which become dispersed along them. Wildlife, 

using these roads as corridors, also spread IPS along 

them.

Plots with oriental bittersweet have fewer seedlings 

(Fig. 8A; t-test; p<0.05) and trees 5 inches diameter at 

breast height (d.b.h) and greater per acre (8B; t-test; 

p<0.05). Plots with oriental bittersweet are also less 

forested (Fig. 9; t-test; p<0.05). However, there is no 

significant difference in the number of saplings per 

acre for plots with or without oriental bittersweet 

(Fig. 10; t-test; p>0.05).

Since the study is relatively new, with complete 

implementation across all of the NRS region in 2007, it 

is difficult to assess if the invasives are impacting tree 

cover or if they are establishing where there are fewer 

trees and less regeneration. Continued investigation 

is important because these plants can outcompete 

native species and without adequate understory 

regeneration to replace the aging overstory, the future 

of the forest remains in question. It will be important 

to continue monitoring how fragmentation and 

site characteristics affect the presence of oriental 

bittersweet.

Monitoring IPS offers information on the status, trends, 

distribution, and population size, and helps to detect 

new populations. These preliminary investigations 

are important as they suggest there is a difference 

between plots with and without Oriental bittersweet 

and future studies will help determine the effects 

of these species. IPS can affect property and timber 

value, biodiversity, habitat quality, and sustainability. 

Continued monitoring of IPS will help elucidate 

important factors related to the presence of these 

invasives as well as to evaluate the impacts of these 

species on ecosystems.

Characteristics of Plots with Oriental Bittersweet

Figure 7.—Average distance to the nearest road for plots 

where oriental bittersweet is absent or present, 2016.5

5 The error bars in Figures 7 through 10 show a 68% confidence interval for the observed mean. Within a figure, statistically significant values are noted by 

different letters (t-test; p<0.05).

Figure 8.—Number of seedlings (A) and trees ≥5 inches d.b.h. 

(B) per acre for plots with or without oriental bittersweet, 2016.5

Figure 9.—Percentage of the plot that is forested for 

plots with or without oriental bittersweet, 2016.5

Figure 10.—Number of saplings per acre for 

plots with or without oriental bittersweet, 2016.5

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Citation for this Publication

Kurtz, Cassandra M.; Hansen, Mark H. 2018. An assessment 

of oriental bittersweet in northern U.S. forests. Res. Note 

NRS-251. Newtown Square, PA: U.S. Department of Agriculture, 

Forest Service, Northern Research Station. 5 p. https://doi. 

org/10.2737/NRS-RN-251.

FIA Program Information

Bechtold, W.A.; Patterson, P.L., eds. 2005. The enhanced Forest 

Inventory and Analysis Program: national sampling design 

and estimation procedures. Gen. Tech. Rep. SRS-80. Asheville, 

NC: U.S. Department of Agriculture, Forest Service, Southern 

Research Station. 85 p. https://doi.org/10.2737/SRS-GTR-80.

Smith, W.B. 2002. Forest inventory and analysis: a national 

inventory and monitoring program. Environmental 

Pollution. 116: 233-242. https://doi.org/10.1016/S0269-

7491(01)00255-X.

References

Czarapata, E.J. 2005. Invasive plants of the upper Midwest: 

an illustrated guide to their identification and control.

Madison, WI: University of Wisconsin Press. 215 p.

Kaufman, S.R.; Kaufman, W. 2007. Invasive plants: a guide 

to identification and the impacts and control of common 

North American species. Mechanicsburg, PA: Stackpole 

Books. 458 p.

Kurtz, C.M. 2013. An assessment of invasive plant species 

monitored by the Northern Research Station Forest 

Inventory and Analysis Program, 2005 through 2010. Gen. 

Tech. Rep. NRS-109. Newtown Square, PA: U.S. Department of 

Agriculture, Forest Service, Northern Research Station. 70 p. 

https://doi.org/10.2737/NRS-GTR-109.

Kurtz, C.M.; Hansen, M.H. 2013. An assessment of multiflora 

rose in northern U.S. forests. Res. Note NRS-182. Newtown 

Square, PA: U.S. Department of Agriculture, Forest Service, 

Northern Research Station. 5 p. https://doi.org/10.2737/NRSRN-182.

Lundgren, M.R.; Small, C.J.; Dreyer, G.D. 2004. Influence 

of land use and site characteristics on invasive plant 

abundance in the Quinebaug Highlands of southern New 

England. Northeastern Naturalist. 11(3): 313-332. https://doi.

org/10.1656/1092-6194(2004)011%5b0313:IOLUAS%5d2.0.

CO;2

Natural Resources Conservation Service [NRCS]. 2018. The 

PLANTS database. Greensboro, NC: U.S Department of 

Agriculture, Natural Resources Conservation Service, Plant 

Data Team. Available at https://plants.usda.gov (accessed May 

11, 2018).

Predick, K.I.; Turner, M.G. 2008. Landscape configuration and 

flood frequency influence invasive shrubs in floodplain 

forests of the Wisconsin River (USA). Journal of Ecology. 96: 

91-102. https://doi.org/10.1111/j.1365-2745.2007.01329.x.

USDA Forest Service. 2016. Forest inventory and analysis 

national core field guide, vol. 1: field data collection 

procedures for phase 2 plots, ver. 7.1. Washington, DC: U.S. 

Department of Agriculture, Forest Service. www.fia.fs.fed.us/

library/field-guides-methods-proc/ (accessed March 22, 2018).

Additional Invasive Plant Information

Invasive and Exotic Plants: 

http://www.invasive.org/species/weeds.cfm

Invasive Plant Atlas of New England: 

http://www.eddmaps.org/ipane/

Invasive Plant Atlas of the United States: 

http://www.invasiveplantatlas.org/index.html

Midwest Invasive Plant Network: http://mipn.org/

Contact

Analyst: Cassandra Kurtz, (651) 649-5149; cmkurtz@fs.fed.us

Page 1 and 5 header: oriental bittersweet fruit. Leslie J. 

Mehrhoff, University of Connecticut, 5487275 from 

Bugwood.org.

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