Cynanchum louiseae, a species in the milkweed family, is also known as black swallow-wort,Louise's swallow-wort, or black dog-strangling vine,Cynanchum louiseae is a species of plant that is native to Europe and is found primarily in Italy, France, Portugal, and Spain. It is an invasive plant species in the northeastern United States, parts of the Midwest, southeastern Canada, and California.
Cynanchum louiseae is a perennial, herbaceous vine with oval shaped leaves that have pointed tips. The leaves are 3-4 inches (7.6-10.2 cm) long, and 2-3 inches (5.1-7.6 cm) wide, often occurring in pairs on the stem. The flowers have five petals that are star-shaped with white hairs. The flowers range in color from dark purple to black. The fruit of Cynanchum louiseae is slender, tapered pods that range in color from green to light brown, which are tightly packed with seeds attached to puffy fibers.
Cynanchum louiseae tends to grow in upland areas and is tolerant to variable light, salt, and moisture levels. In the United States, the vine is often found in abandoned fields, hedgerows, brushy areas, woodlands, river banks, transportation corridors, quarries, agricultural fields, and gardens. In gardens, the plant is seen as a fast-growing weed.
Cynanchum louiseae emerges in the spring, and flowers during June and July. Cynanchum louiseae is self-pollinating, and seed pods form throughout the summer. The number of pods formed is directly linked to the amount of light the plant is exposed to. If there is a lower level of light, then there are fewer seed pods compared to a plant exposed to a higher level of light. The seeds begin to be released by mid-August, and continue to be dispersed into early October.
Each seed is polyembryonic and contains about one to four embryos per seed, increasing Cynanchum louiseae's proliferation. Seeds use delicate fibrous "parachutes" in order to be dispersed by the wind over large distances. In addition to seeds for reproduction, Cynanchum louiseae also uses rhizomes as a method of propagation, meaning that the plant clones itself underground and produces new plants. After seed dispersal, the plant dies to the ground in the winter and returns in the spring.
The first group of Cynanchum louiseae in North America was recorded in Ipswich, Massachusetts in 1854. In 1864, a plant collector recorded that it was "escaping from the botanical garden where it is a weed promising to be naturalized". Cynanchum louiseae escaped from a garden in the Cambridge area of Massachusetts and naturalized in the surrounding states and is still spreading today. Massachusetts and New York classify the plant as an invasive species. Beyond the northeastern US, the plant has been reported in Wisconsin and California.
In the United States and Canada, Cynanchum louiseae is a threat to native species because it crowds them out. For example, it can completely replace a field of native goldenrod. Crowding out other species results in a reduced habitat for wildlife, which may become endangered because they can no longer find the correct habitat.
Cynanchum louiseae threatens the rare limestone pavement barren ecosystems by crowding out plants that the native wildlife needs to survive. It may also decrease bird presence in grasslands, which may in turn cause insect species populations to increase.
In Vermont, Cynanchum louiseae crowds out the endangered species Jessop's milk vetch. In Rhode Island, Cynanchum louiseae has been reported as reducing the effectiveness of electric fences, which may allow livestock to be put into danger or lost.
In addition, Cynanchum louiseae crowds out another species of milkweed that monarch butterflies use in order to reproduce. When the butterflies try to reproduce using the wrong host plant, the larvae do not survive. Thus, the spread of the plant threatens populations of monarch butterflies. Overall, Cynanchum louiseae reproduces very prolifically, and can easily take over various habitats in a short amount of time. Most of the possible implications of Cynanchum louiseae changing the structure of various ecosystems are yet to be known.
There are four methods of management that can possibly be used for the management of Cynanchum louiseae. These methods are chemical, manual, mechanical, and biological. Only the chemical, manual, mechanical methods are actually used in the United States and Canada. The biological method may be used in the future. Overall, early detection and removal is the best management.
The best chemical management of Cynanchum louiseae is through the use of systemic herbicides which prevent seeds from being viable. Garlon 4 (tridopyr ester) and RoundUp Pro (glyphosate) are the main systemic herbicides that are used to control the vine. The systemic herbicide is most effective when sprayed on the plant after flowering has begun. If the herbicide is used after seed pods have formed, it is less effective because viable seeds may still form. The most effective treatment using systemic herbicides is through a cut stem application, consisting of applying the chemical to the recently-cut stems of the swallow-wort vines.
Manual management is the removal of Cynanchum louiseae from the ground by digging up its roots so that the plant cannot reproduce. The vine has very shallow roots, making it relatively easy to uproot. Seed pods must be disposed of carefully, to avoid inadvertently spreading the seeds to new areas.
Mechanical management is the mowing down of Cynanchum louiseae. This method does not stop growth, but it does stop seed crops. No seed crop means no reproduction and no new generation.
Biological management in the use of the vine's natural enemies to stop the spread and diminish the population of Cynanchum louiseae. In the United States, Cynanchum louiseae has no natural enemies but in its native Europe, certain caterpillars, beetles, and diseases attack the plant. Researchers at Cornell University and the USDA have investigated the use of natural enemies as a way to control the plant. The use of natural enemies is controversial, because the implications of adding more non-native species to threatened areas is unknown.