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History

close up of grayish leaf

Absinth wormwood has a long history of human use for its intoxicating and medicinal properties. It contains the chemical thujone, which affects the central nervous system. Extracts are reported to be useful for pain relief and stimulating appetite, and “wormwood” oil has been used to treat parasitic worms. This species is native to Europe and was intentionally introduced to North America. Its seeds were sold commercially in the U.S. as early as 1832, and just nine years later it was viewed by some as an established weed of roadsides. It now occurs in all southern Canada provinces and northern U.S. states.

Identification

grayish stems with numerous nodding flower heads

Absinth wormwood is a large statured perennial forb that can reach five feet tall. Its scent has been described as reminiscent of sage, but not as pleasant. Leaves
are two to five inches long and deeply lobed and dissected, and leaves and stems are covered with fine hairs that give the plant a grayish appearance (photo above). Flower stalks produce numerous nodding flower heads that are about 1/8 inch in diameter (photo, right).

Impacts

The ecological impacts and invasive potential of absinth wormwood are not well-documented. It commonly grows in disturbed areas such as roadsides, waste areas and overgrazed pastures where it can form dense stands (photo, below right). However, absinth wormwood does not generally form dense stands in undisturbed perennial plant communities. Absinth wormwood is described as unpalatable to fairly palatable for cattle, as it is reported to have a bitter taste. If cattle do browse the plant, it can give an unpleasant taste to their milk.

Habitat

growing on trailside

Absinth wormwood is most successful in areas with plentiful moisture. It lacks the interxylary bark on the roots that protects many Artemisia species from desiccation, which helps explain its relatively poorer performance on dry sites. While this species has been observed to die out on gravelly soils during drought, it can persist in ravines, ditches, and slopes with a north aspect during these same periods. Seedlings are not competitive with established perennial vegetation, and bare ground or disturbance is favorable for their establishment. This species grows most prolifically in fencerows, roadsides, gravel pits, abandoned fields, and overgrazed pastures.

Spread

Absinth wormwood reproduces only by seed. It is a prolific seed producer, and seeds remain viable in the soil for about four years. Seeds do not have specialized structures for dispersal by wind or animals, but they can float, allowing dispersal along waterways. Some sources mention the possibility of vegetative reproduction or weakly rhizomatous growth of absinth wormwood, but the original source of this information is unknown.

Management Priorities

In Montana absinth wormwood is listed as a noxious weed in four counties, and neighboring states have listed it. There are several control methods available. First, since seedlings are not competitive with perennial grasses, maintaining or establishing a vigorous perennial plant community is a priority for preventing establishment and limiting spread. Existing populations should be managed with a focus on preventing seed production. Herbicide is effective if applied at the appropriate time. Many products are available including those with the active ingredients picloram, aminopyralid, 2,4-D, dicamba, and glyphosate. Hand-pulling or digging may be effective for controlling small patches and young plants. Mowing prior to seed production can also help control absinth wormwood, but repeated mowing is necessary as plants will resprout and form new flowering stalks.

*all photos by Matt Lavin, Montana State University