Mugwort (Artemesia vulgaris) is an invasive perennial forb that is widespread throughout North America, though it is most common in the eastern United States and Canada. It is a weed of nurseries, turfgrass, vineyards, waste areas, forest edges, and roadsides. Mugwort spreads aggressively through an extensive rhizome system and will readily form large, mono-specific stands.
Mugwort is native to Europe and eastern Asia, where it has historically been used as a medicinal herb. Seed may have been first introduced to North America as early as the 16th century by Jesuit missionaries in Canada. It was also introduced throughout the continent as a contaminant in ship ballast and nursery stock.
Mugwort is a perennial with an extensive rhizome system. Shoots emerge during the spring, and flowering occurs from July to late September. A single plant can, depending on its environment, produce up to 200,000 seeds. The small seeds (~1mm in diameter) are largely wind dispersed. Seed production does not seem to be a major factor in the spread of mugwort populations, however, and some biotypes do not produce viable seed. Instead, mugwort spreads largely through vegetative expansion and the anthropogenic dispersal of root propagules. The root system is extensive though shallow (to 20 cm in depth), with numerous branching rhizomes up to 1 cm in diameter. Plants can regenerate from rhizome fragments as small as 2 cm (Klingeman et al. 2004).
The rarely-seen seedlings have oblong cotyledons without petioles. Adult stems are smooth and longitudinally ridged, with numerous axillary branches towards the upper portions of the plant. The stems become somewhat woody as they age. The leaves are alternate, densely covered with wooly, silver-white hairs on the underside, and slightly hairy on the upper surface. Leaf morphology is variable throughout the plant. The lower leaves are petiolate, with stipules at the base, and generally coarsely toothed and pinnately lobed. The upper leaves are sessile and lanceolate with smooth or toothed margins. The numerous ray and disk flowers are small (5 mm), green, and grow in racemes and clusters at the end of stems and branches. The foliage is aromatic and slightly pungent.
Mugwort is a problematic weed in nurseries, where small root fragments can easily contaminate nursery stock. It is also a major weed in turf grass, field-grown ornamental crops, and orchards. Stands of mugwort displace native species, and can delay or disrupt succession in natural ecosystems (Barney and DiTommaso 2003). Mugwort produces several terpenoid potential allellochemicals, and decaying mugwort foliage has been shown to inhibit the growth of red clover in laboratory experiments (Inderjit and Foy 1999). Mugwort pollen is a common cause of hay fever.
The dense root system of mugwort can make it difficult to control. Pulling is ineffective, and may even promote growth by leaving residual rhizome fragments in the soil. Mugwort tolerates mowing, and even sustained mowing over two years will not fully eradicate mugwort stands. The relatively shallow roots make mugwort vulnerable to repeated cultivation in agricultural systems, though this practice risks spreading root propagules.
Chemical control of mugwort can have limited effectiveness. Though non-specific broadleaf herbicides such as glyphosate or dicamba can effectively control mugwort, the rates required for adequate suppression are rarely economical (Bradley and Hagood 2002). For small infestations, multiple spot-treatments of glyphosate can be effective (Bing 1983).