Bittersweet nightshade, Solanum dulcamara, is a member of the Solanaceae family. This family is commonly known as the nightshade or potato family and has over 2,500 species.
Bittersweet nightshade is native to Europe and Asia.
It was introduced to North America for ornamental and medicinal purposes and became widespread by the late 1800s.
It is now considered an invasive weed in most American states and Canadian provinces.
Its stems and berries have been used in herbalism to treat skin conditions such as herpes and eczema. However, the plant is toxic when eaten by humans, and its use is not recommended.
Bittersweet nightshade is a perennial, climbing vine. It grows in a wide range of habitats but prefers not to be in full sun. It can be found growing along hedgerows, forest edges, riparian zones and in forest understories.
Bittersweet nightshade may be confused with Asiatic bittersweet, Celastrus orbiculatus, which is also an invasive species and grows in PEI. Here are some distinguishing features of bittersweet nightshade that may help you positively identify it:
The stem is herbaceous when immature, and becomes woody with maturity. Stems can grow to over 3m tall.
Leaves are heart-shaped and arranged alternately.
When leaves are crushed they emit an unpleasant smell.
Clusters of flowers extend from the stem.
Flowers are blue-violet, star-shaped, with protruding yellow anthers.
Flowers bloom from May to September.
Forms clusters of green, ovoid berries that are red when ripe.
Spread by birds who eat the berries and by pieces of stem and root that are moved by soil or water.
What it does in the ecosystem
Bittersweet nightshade thrives particularly well in riparian areas, where it becomes a serious threat to aquatic habitats. In watercourses this vine can form large floating mats that reduce flow rate, preventing fish passage and resulting in the collection of sediment and leading to small streams being choked off.
The toxicity of this species makes it an exceptionally unwelcome invader of local ecosystems. The compounds that cause this toxicity (solanine and dulcamarine) are toxic to humans and pets. Unripe berries and young leaves have the highest toxicity, although it is unsafe to consume year-round. If humans, pets, or livestock consume bittersweet nightshade, they risk gastroenteritis, dermatitis, phytophotosensitivity, and in some cases death.
Some birds and wildlife (like skunks) are immune to its toxicity and enjoy feeding on its berries. Unfortunately, this spreads it over long distances when the animal defecates. Bittersweet nightshade establishes quickly, as each berry contains around 30 seeds. It often grows in dense, fast-spreading clumps, preventing native plant growth. Bittersweet Nightshade is a climbing plant that can cover or shade out low trees, shrubs, bushes, and other plants. It can harm orchards, affecting fruit crops and harvesting activities. As mentioned, the plant is toxic, and as such it may not provide an adequate nutritional replacement for some native species where it invades.
Before selecting a control method, consider the size of the infestation, your available resources, and the amount of effort you are willing to expend. Often, multiple control methods are used simultaneously. Consider also the local ecosystem and what other organisms or ecological processes may be affected by management.
Begin by surveying the area in detail before management. This is ideally done before the berries form and can occur as early as mid-may when the flowers emerge. Generally, it is best to start management in an area with the least amount of plants present. If the plant is growing along a stream, it is best to begin upstream so that the berries are not continually spread downstream into your other management areas. Take care when surveying and managing invasive species to avoid trampling native species in the area, they are an essential component in suppressing the bittersweet and helping regenerate the area’s native plant community.
Management of bittersweet nightshade should be done early in the season to ensure berries are not dispersed by wildlife. Be sure to use appropriate PPE when managing toxic plants. This plant has been known to cause rashes in some people. We recommend wearing thick gloves and eye protection, as well as boots, a long-sleeve shirt, and pants.
If managing bittersweet nightshade in or within 15m of a wetland or watercourse on PEI, you are required to obtain a Watercourse, Wetland and Buffer Zone Activity Permit from the PEI department of Environment, Energy, and Climate Action. Applications should be submitted a minimum of six weeks before the proposed project start date to allow for processing. More information about permitting can be found at the following link Apply for a Watercourse, Wetland and Buffer Zone Activity Permit | Government of Prince Edward Island.
Hand-pulling the whole plant (including roots) or digging up the whole plant can be effective for small to medium-sized patches and is easiest with young plants. Special care must be taken to avoid snapping the fragile stem off from its roots. It is easiest to pull in wet soils, so management can be timed to occur after a rain. Using a shovel or spade to loosen the soil can be quite effective, but care must be taken to pat down the soil after disturbance to prevent regeneration.
Large areas can be cut first and then dug. If it is impossible to dig the area (ie: due to a rocky environment the plant is growing through) repeated cutting will weaken the plant if done consistently more than five times per growing season.
After clearing an area, patches can be covered with a black tarp which will suffocate any new growth. Weigh the tarp down with old garbage, wood, bricks, wood chips, dirt, or anything else heavy that will not puncture the tarp. This should yield results in approximately two years, although it may take longer. Checks should be done during the growing season to ensure the plant is not escaping around the edges. The tarp could also be covered with 8” soil and then replanted with drought-tolerant native species, although the tarp would remain permanently below the soil in this scenario. If burying the tarp, consider a biodegradable material like burlap, jute, or cardboard and newspaper.
Mowing has been found to be ineffective in the long-term management of the plant and may agitate the plants, causing increased and aggressive growth. Mowing will also spread around plant parts which can take root and regrow.
The PEIISC does not provide advice for chemical treatment options at this time. If using chemical controls, it is imperative that all local legislation and manufacturer’s instructions be followed during application. Chemical Control should never be used when close to a water body or watercourse. When managing invasive species near (within 15m of) a watercourse, you are required to obtain a Watercourse, Wetland and Buffer Zone Activity permit. The permit should be applied for six weeks before beginning work.
Disposal and Follow-up
It is essential to properly dispose of all plant parts. Entire plants can regrow from left-behind root fragments or from sections of stems that reroot at the node. Clean all equipment before leaving the site to prevent spread or contact with toxins. Repeated visits to the site after initial control efforts will be necessary to ensure management is effective. Replant the area with native plants if possible.