Family: Asteraceae

Achillea millefolium, commonly known as common yarrow or milfoil, is a perennial herb native to temperate regions of the Northern Hemisphere, including parts of Europe, Asia, and North America. In the Hawaiian Islands, it has become naturalized, particularly on the island of Hawaiʻi, where it thrives in disturbed habitats such as roadsides, pastures, and open forests. This adaptability allows yarrow to establish itself in a variety of soils and elevations, contributing to its widespread presence in the region .

While Achillea millefolium is not currently listed among Hawaii’s most problematic invasive species, it is recognized as a non-native plant that has naturalized in the state. Its presence in disturbed environments suggests that, under certain conditions, it could potentially become invasive. Therefore, monitoring its spread and impact on native ecosystems is advisable to prevent any adverse effects on local biodiversity .

High Risk Traits:

• Naturalized where grown: It has become established in Hawaii (at higher elevations), Australia, and is a major problem in parts of New Zealand.
  
• Broad climate suitability: It shows wide tolerance to environmental conditions and thrives in USDA Zones 3-10, adapting to various soil types and climates.
  
• Garden/amenity/disturbance weed: Yarrow can invade and compete in gardens and cropland, frequently appearing in disturbed grasslands, open forests, hedgerows, pastures, and fields.
  
• Agricultural/forestry/horticultural weed: It is a major issue in some New Zealand crops and can cause off-flavored milk in dairy cattle.
  
• Environmental weed: In Australia's Snowy Mountains, it occurs along roadsides and disturbed areas, from where it can invade native vegetation and form monocultures that inhibit other plant growth.
  
• Congeneric weed: Several Achillea species are recognized as weeds, indicating a weedy tendency within the genus.
  
• Host for recognized pests and pathogens: It serves as an alternate host for chrysanthemum stunt virus.
  
• Causes allergies or is otherwise toxic to humans: Contact with foliage or sap can cause allergic reactions, and prolonged use may increase sun sensitivity.
  
• Shade tolerant: It tolerates shade despite preferring abundant sunlight.
  
• Tolerates a wide range of soil conditions: It can grow on many soil types, including sandy and unfertile sites.
  
• Forms dense thickets: It can form monocultures, suppressing other plant species.
  
• Geophyte: It is an herbaceous plant with underground storage organs, aiding its persistence.
  
• Produces viable seed: It reproduces from seeds.
  
• Hybridizes naturally: The species includes many hybrid variations and numerous cultivars developed through inter-species crossing.
  
• Does not require specialist pollinators: It is pollinated by a wide range of insects.
  
• Reproduction by vegetative fragmentation: Its extensively branched rhizomes enable rapid colony formation through creeping roots.
  
• Short minimum generative time: It can flower within one year.
  
• Propagules likely to be dispersed unintentionally: Its spread has increased due to contaminated gravel and hay.
  
• Propagules dispersed intentionally by people: It is cultivated as a medicinal and ornamental plant.
  
• Propagules likely to disperse as a produce contaminant.
  
• Propagules adapted to wind dispersal: Its small seeds are primarily wind-dispersed.
  
• Propagules survive passage through the gut: Viable seeds can be dispersed in animal feces.
  
• Prolific seed production: Each plant can produce up to 4,000 seeds, with a potential maximum of 51,400 seeds per square meter.
  
• Forms a persistent propagule bank: Seeds can remain viable in the soil for several years (41% viable after 9 years, 16% after 3 years burial).
  
• Not easily controlled by herbicides: While some herbicides can reduce biomass, the recommended treatment did not always significantly reduce aerial biomass or flowering stems.
  
• Tolerates or benefits from mutilation, cultivation, or fire: Its rhizomes can regenerate from fragments and survive cultivation; it is very tolerant of frequent mowing.

Low Risk Traits:

• Not primarily suited to tropical or subtropical climates
  
• Does not produce spines, thorns or burrs
  
• Palatable to grazing animals: It has poor to fair forage value for cattle and fair to good for sheep; heads may be eaten by pronghorn, deer, and sheep.
  
• Not generally considered toxic to animals: Although it may contain toxic compounds, it is not widely regarded as poisonous.
  
• Does not create a fire hazard in natural ecosystems: It is listed as a fire-retardant ground cover.
  
• Self-incompatible: It requires cross-pollination for reproduction.