Family: Equisetaceae

Origin, Description & Uses:
Equisetum arvense, commonly known as common horsetail or field horsetail, is an ancient, fern-like plant native to much of the Northern Hemisphere, including Europe, Asia, and North America. It belongs to a very old lineage of plants that reproduce by spores rather than seeds. This perennial species produces distinctive hollow, jointed stems with whorls of slender branches, giving it a brush-like appearance. In spring, it sends up pale, cone-bearing fertile stems that release spores, followed by green, photosynthetic sterile stems. Common horsetail has been used in traditional herbal medicine for centuries and is sometimes grown as an ornamental in water gardens or naturalistic landscapes. Extracts of the plant have also been used historically as a mild diuretic and as a source of silica in herbal preparations.

Risks & Threats:
Although Equisetum arvense is not currently known to be naturalized in the Hawaiian Islands, it possesses several traits that raise concern for tropical island ecosystems. The plant spreads aggressively through an extensive underground rhizome system and can quickly form dense colonies that are very difficult to eradicate. Even small rhizome fragments can regenerate new plants, allowing it to persist despite repeated control efforts. In regions where it has spread outside its native range, common horsetail can invade gardens, agricultural fields, and disturbed natural areas, competing with desirable vegetation. Because of its persistence, rapid vegetative spread, and tolerance of a wide range of soil conditions, this species has the potential to become problematic if introduced to suitable habitats. For these reasons, planting common horsetail is discouraged, and gardeners are encouraged to choose low-risk or native alternatives for landscaping in Hawaiʻi.

High Risk Traits:

• Broad climate suitability, and elevation range exceeds 1000 m
  
• Naturalized
  
• Weedy and difficult to remove
  
• A serious agricultural weed
  
• Related Equisetum species have become invasive
  
• Toxic to cattle and other grazing animals
  
• Tolerates many soil types
  
• Forms dense stands that may exclude other vegetation
  
• May hybridize with other Equisetum species
  
• Spread by spores and vegetatively by rhizomes
  
• Can resprout if only aboveground vegetative material is removed

Low Risk Traits:

• Thrives mainly in temperate climates, so may only threaten higher elevation ecosystems in the tropics
  
• Unarmed (no spines, thorns, or burrs)
  
• Short-lived spores
  
• Herbicides may provide effective control