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Invasive Species—Best Control Practices Michigan Department of Natural Resources Michigan Natural Features Inventory 4/2012 Black and Pale Swallow-worts Cynanchum louiseae (Vincetoxicum nigrum) and Cynanchum rossicum (Vincetoxicum rossicum) Black swallow-wort is native to southwestern Europe, around the Mediterranean, and northerrn Europe. Pale swallow-wort is native to the Ukraine and parts of Russia. They were introduced in the US in the 1800s, though nei- ther was ever highly rated as a horticultural specimen. Both black and pale swallow-worts are highly invasive; they form dense monocultures in shade or full sun. Swallow-worts produce compounds with anti-bacterial and selective anti-fungal properties, and inhibit the growth of many pathogens. The roots contain haemolytic glycosides, which are toxic to mammals, including livestock. Swallow-worts are also distasteful to many insect larvae and toxic to some, including monarchs. It is not clear how often female monarchs lay eggs on swallow-wort, but when they do, the caterpillars die. In New York state, swallow-worts pose a particular threat in alvar, where they out-compete a number of rare plant spe- cies and disrupt nesting by grassland birds. In general, insect diversity and abundance is significantly lower in dense stands, with cascading effects on the entire food chain. Suzan Campbell, MNFI Identification Habit: Pale swallow-wort flowers: Black and pale swallow-worts are both herbaceous Pale swallow-wort’s flowers perennial twining vines with clear, watery sap. They com- range from pink to deep monly reach 1-2 m (3-6.5 ft) in length. burgundy in color. The narrow petals are longer than they Leaves: are wide. Both pale and black Swallow-wort leaves are swallow-wort flowers are held opposite, with smooth in clusters that attach at the margins. They are shiny dark John M. Randall, The Nature leaf bases. In Michigan, they green, and narrowly oval or Conservancy, Bugwood.org begin blooming in June. heart-shaped, with sharply pointed tips. The leaves are Fruits/Seeds: about 5-13 cm (2-5 in) long Both species develop milk- weed-like pods that split and 2-6.5 cm (1-2.5 in) wide. Suzan Campbell, MNFI when the seed is ripe. The Stems: flat, brown seeds are attached Swallow-wort’s twining stems are covered with downy hairs. to fluffy hairs that help them Leslie J. Mehrhoff, University Dried stems and empty seedpods persist into fall or longer. disperse by wind in the fall. of Connecticut, Bugwood.org Black swallow-wort flowers: Habitat Black swallow-wort flowers Both swallow-wort species thrive in a wide range of settings are small, dark purple, and and are shade tolerant. They occur along roadsides, in gar- have 5 petals covered in fine dens, old fields, pastures, forests, alvar and along the upper light hairs. The bluntly trian- edge of fens. They can tolerate brief periods of flooding but Leslie J. Mehrhoff, University gular petals are about as wide not prolonged inundation. of Connecticut, Bugwood.org as they are long. Similar species species have short underground rhizomes but they do not appear to give rise to new plants with any frequency. How- Milkweeds ever, root crown fragments can regenerate when damaged The related native milkweeds or broken. (Asclepias spp.) are upright, Some seeds are polyembryonic; a single seed can contain rather than twining vines. two or more embryos, and give rise to multiple seedlings. Although they also have This condition is more common in pale swallow-wort than opposite leaves and similar black swallow-wort. Seed usually lands within a few meters seedpods, their flowers are far of the parent plant but may be carried several hundred more elaborate and they meters by gusts of wind. Little information is available on usually have milky sap. Suzan Campbell, MNFI the development of seed banks. Dogbanes Although seedling survival is high, plants initially grow The related native dogbanes slowly and do not reproduce for several years. It can take a (Apocynum spp) are also up- number of years for a single plant to mature, produce seed- right—not twining vines. Like lings and form a small population. swallow-wort, they have nar- In general, plants in full sun produce more flowers, seed- row seedpods, but their bell- pods and seeds. In high density stands of pale swallow- shaped flowers are arranged wort, seed densities of up to 32,000 seeds per square meter in clusters at the stem tips have been documented. rather than along the stems at Suzan Campbell, MNFI the base of the leaves. Plants in larger colonies (> 80 plants) tend to be more robust and produce more fruits per plant than single plants Oriental bittersweet or those growing in small groups (< 9 plants). Some of this The invasive non-native vine effect can be explained by density; intertwining plants Oriental bittersweet resem- shade out other vegetation more effectively. However, other bles swallow-wort but can factors clearly play a role. grow much longer. Its leaves Both pale and black swallow-wort form associations with are alternate, with toothed beneficial arbuscular mycorrhizal fungi (AMF). These associa- margins, rather than opposite, tions result in improved nutrient cycling. Root colonization with smooth margins. It is James R. Allison, rates by mycorrhizae are higher in swallow-wort than in woody, not herbaceous. Georgia DNR, Bugwood.org native species growing in the same area. Similarly, four times Dogwood species more AMF spores are present in soils with swallow-wort Young native dogwoods can than in uninvaded soils nearby. be mistaken for swallow- Swallow-wort seedling survival rates are higher in soil with worts before they are large AMF than sterilized soil. Seedling densities of up to 5,000 enough to branch as their per square meter are common in large infestations, and can opposite leaves have a similar reach 10,000 per square meter. Dense swallow-wort vegeta- shape. They are woody, how- tion facilitates the exposure of seedling root systems to the ever, not herbaceous. They are AMF network and results in enhanced nutrition. In general, also easier to distinguish in plants are taller, with more leaves and greater biomass fruit or flower. when growing in the large populations where AMF thrive. Quick check AMF species are often species host specific. While general- Swallow-worts ALWAYS: ists can form associations with many host species, some • have clear, watery sap species can only form associations with a few related host species. Swallow-wort encourages the increase of general- • have opposite leaves ist AMF species at the expense of the more specialized AMF • twine around each other or a support or trail along the species that native plants need to grow and thrive. ground—they do not stand upright without support. Both black and pale swallow-wort appear to be allelopathic. Reproduction/Dispersal Swallow-wort root extracts can cause root length reduc- Swallow-worts reproduce by seed. They are able to self pol- tions of 40% in the related butterfly milkweed, for example. linate although sexual reproduction is more common. The Similarly, they have a negative effect on shoot growth in role of vegetative reproduction is not entirely clear. Both common milkweed. Allelopathic effects could be amplified in larger, denser populations. also be helpful in assessing spread within an area, as new The potential for swallow-wort mediated changes in soil plants will not have them. chemistry or mycorrhizae should be considered in any plans Documenting occurrences for restoration following swallow-wort control efforts. In order to track the spread of an invasive species on a landscape scale, it is important to report populations where Planning a control program they occur. The Midwest Invasive Species Information Net- Resources for invasive species control invariably fall short of work (MISIN) has an easy-to-use interactive online mapping the actual need, so it is important to prioritize sites for treat- system. It accepts reports of invasive species’ locations from ment and plan carefully. Assessing both the scope of the users who have completed a simple, online training module problem and any available resources is a critical first step: for the species being reported. It also offers the potential for • Map known populations. Are swallow-worts widely batch uploading of occurrence data for any invasive species. distributed throughout the region? Or are they just Herbaria also provide a valuable and authoritative record of beginning to appear? Are one or both species present plant distribution. The University of Michigan Herbarium’s in the area? database can be searched online for county records of • Identify potential dispersal pathways; do infestations lie occurrence, for example. When black and/or pale swallow- in the path of road-mowing crews that might spread it wort are first encountered in a county where they had not further? been known previously, specimens should be submitted to • How is it distributed? Is it sparsely scattered in otherwise the Herbarium to document their presence. Check the “On- native vegetation? Does it cover large expanses of low line Resources” section for links to both of these resources. quality habitat? Does it occur in both open and shaded sites? Control • Are volunteers available to assist with control? Controlling large populations of swallow-wort requires a Given this information, develop a strategy for control: multi-year effort. It is important to keep isolated plants or small infestations from expanding into the large popula- 1. Prioritize high value sites where success can be tions in which plant vigor and productivity are enhanced. achieved for treatment. 2. Choose appropriate control methods, given site condi- Multiple strategies should be employed to achieve these tions and available resources. ends; techniques that are effective in grasslands and old 3.

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