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Allelopathy is when one species of plant inhibits the growth of another species of plant by producing harmful chemicals.

It is thought to be an important factor in the successful invasion of many non-native plants. Plants in the native range of these invaders often have defenses against the chemicals to help buffer their negative effects.

Spotted Knapweed
© 2005 Louis-M. Landry

spotted knapweed
University of Idaho Archive,
University of Idaho,

Spotted Knapweed

Centaurea stoebe subsp. micranthos
a.k.a. C. maculosa
Spotted Knapweed is native to Europe and Asia. It was introduced to the United States in the late 1800s, probably by contaminated plant seed or in ship ballast. The plant is powerfully invasive because of its allelopathic properties. It secretes a chemical compound, called catechin, that kills other plants. This reduces competition and allows for more knapweed to grow. The effect of catechin is greatest on native plants. Invasive European species that grow in knapweed’s native range are resistant to catechin. They can benefit from the decreased competition fostered by the knapweed. As a result, the invasion of knapweed can convert native plant communities to communities of non-native weeds.
Garlic Mustard
David Cappaert, Michigan State University,

Garlic Mustard

Alliaria petiolata
Garlic Mustard is native to Europe and was first documented in North America by a botanist on Long Island, New York, in 1868. It was likely introduced for food or medicinal purposes. It quickly colonizes forest stands, where it competes with native understory plants for light and space. A single plant can produce thousands of tiny seeds, which can be transported long distances by humans and wildlife species. In native forests, some hardwood trees depend on specific fungi found in the understory for their growth and survival. The unique chemistry of Garlic Mustard suppresses the growth of these fungi. With this partnership now weakened, these valuable trees cannot grow and spread, resulting in dramatic changes in our forest communities.

Pale Swallow-wort
Leslie J. Mehrhoff, University of Connecticut,

Black Swallow-wort

Monarch Butterfly
Swallow-worts may have harmful
effects on the native Monarch
Butterfly. They can compete with
and displace Common Milkweed, a
major food source. Swallow-worts
also attract and stimulate female
monarchs to deposit their eggs on
them even though the larva cannot
develop on these plants.

Pale and Black Swallow-worts

incetoxicum rossicum and V. nigrum a.k.a. Cynanchum rossicum and C. louiseae
Pale Swallow-wort is native to Ukraine and southwestern Russia. Black Swallow-wort is native to southwestern Europe. They were brought to North America as ornamental garden plants but escaped into the wild in the mid- to late 1800s.

Less than common in their native ranges, swallow-worts are quite successful in New York state. Their seeds are easily dispersed by wind and by attaching to animal fur. Swallow-worts are typically most invasive in natural areas in the state. There they can compete with native plants for light and other resources. Once established, they spread rapidly and can completely cover native vegetation. Swallow-worts grow in a variety of situations and can be problematic in horticultural nurseries, perennial crops, pasture lands, and other disturbed areas. Chemicals they produce can negatively affect the survival of Monarch Butterfly (Danaus plexippus) larvae and resident soil microbial communities. The chemicals also may be toxic to wildlife and livestock that feed on swallow-worts.

Scientists are performing a variety of research on these allelopathic vines to learn how to control their spread. This research includes how to dispose of plants, especially their seeds. More long-term studies involve trying to understand the plants’ biology and ecology. This knowledge may help identify weaknesses in the plants’ structure or life cycle.

Swallow-worts are somewhat unique plants because some of their seeds have more than one embryo. This is called polyembryony. This feature results in the production of “sister seedlings” from a single seed. In the image, note the four “sister seedlings” emerging from one Pale Swallow-wort seed. This characteristic substantially increases the population densities of these plants.

Swallow-wort vines often form tangled dense thickets, hence the nickname “Dog-strangling Vine.”