Bittersweet (Celastrus spp.) is a vigorous deciduous vine known for its abundant and showy orange fruits that split in autumn to reveal red berries. Song and gamebirds relish the fruits as food, also acting to spread the seeds across the landscape. Oriental bittersweet (Celastrus orbiculatus) grows larger, faster and more aggressively than the American bittersweet (Celastrus americanus), causing ecological concerns.
Native to eastern North America, American bittersweet (Celastrus americanus) is a low, spreading, thicket-like shrub that has clambering and climbing stems. It has glossy green foliage in spring and summer followed by small clusters of orange, pea-sized fruits in early to mid-autumn. As the fruits dry, they split open to reveal a reddish fruit that are both ornamental and an important food source to songbirds, game birds and small mammals like squirrels and foxes.
The leaves of this species are oblong and a glossy green and the flowers and colorful fruits are usually in clusters of sixes irregularly distributed at the ends of the sprawling, viney branches.
This bittersweet can grow as tall as 60 feet upon woodland trees if soils are rich, moisture ample and sunlight plentiful. It can become quite large in size and width, potentially toppling weak trees that must support all the stems and foliage. Rarely will an impressive American bittersweet vine become so large that its stems with girdle the trunk of a tree upon which it grows. Its seeds are distributed by birds after they are eaten and dropped in excrement.
Oriental bittersweet (Celastrus orbiculatus) is a large, aggressive, sprawling vine native to eastern Asia first brought into North America in the 19th century as an ornamental plant. It produces grand numbers of orange-colored fruits in summer and autumn that also split open to reveal a reddish fruit. Just like the American bittersweet, these fruits are relished by wildlife, particularly birds, that scatter the seeds across the landscape.
Leaves of the Oriental bittersweet are slightly more rounded in their oblong shape, one key differentiating characteristic. Perhaps more easily seen to properly identify is the abundance of the flowers and fruits across the Oriental bittersweet vine. Evenly distributed in the areas at the bases of the leaves are clusters of 2 to 3 flowers or fruits.
In ideal growing conditions, this species will grow to heights of 60 to 100 feet, often spreading its dense cloak of foliage over trees and shrubs, killing them by shading out necessary sunlight. The U.S. Department of Agriculture has identified that the Oriental bittersweet is more efficient in photosynthesis, its ability to make food and grow. Moreover, more seeds are produced and its pollen is more viable when compared to that of the American bittersweet. Besides animals eating and distributing the seeds, Oriental bittersweet has been commercially grown and harvested for use in dried flower arrangements and autumn decorations. Disposal of these items into landfills allows the seeds to germinate in unmanaged areas.
American bittersweet is becoming less commonly encountered in its native range of eastern North America. Deforestation has diminished natural stands of the plant, but recently the aggressive and out-competing success of the Oriental bittersweet has led to habitat loss for remaining American bittersweet vines. Animals are attracted to the tasty and more abundant fruits of the Oriental species, thus increasing the distribution of the species into areas where the American species once thrived.
Scientists have observed that a hybrid is formed when these two different bittersweet vines grow and flower in proximity to each other. The hybrid combines the characteristics of the two species, but diminishes the genetic uniqueness of the American type in North America, effectively and progressively acting to eliminate its pure form in nature across the continent.