Family: Lamiaceae

Origin, Description & Uses
Marrubium vulgare, commonly known as horehound or white horehound, is a perennial herb in the mint family native to Macaronesia, Europe, the Mediterranean region, and parts of Asia extending to the Central Himalayas. It has gray-green, wrinkled leaves covered in soft hairs and produces small white flowers clustered along square stems. Horehound has a long history of use in traditional herbal medicine and is commonly grown for herbal teas, cough remedies, and lozenges. The plant is drought tolerant and can thrive in dry, disturbed areas, making it attractive to some gardeners interested in medicinal or hardy landscape plants. In Hawaiʻi, it has become naturalized on Lānaʻi and Hawaiʻi Island.

Risks & Threats
Although valued as an herbal plant, Marrubium vulgare is considered a high risk species in Hawaiʻi because it is naturalized and possesses traits associated with invasive behavior. The species produces abundant seeds, tolerates harsh and dry conditions, and can spread into disturbed sites, grasslands, roadsides, and open natural areas. These characteristics raise concerns that horehound could compete with native vegetation and contribute to the degradation of tropical island ecosystems, especially in dry and lowland habitats already vulnerable to invasive species. Because of these risks, Plant Pono recommends choosing a low risk or native alternative when suitable alternatives are available.

High Risk Traits:

• Elevation range exceeds 1000 m, demonstrating environmental versatility
  
• Naturalized in areas with tropical climates
  
• Widely naturalized, including Hawaii and Lanai islands
  
• Disturbance-adapted weed with negative impacts to agriculture and the natural environment
  
• Other Marrubium species are weeds
  
• Generally unpalatable to grazing animals
  
• Tolerates many soil types
  
• Forms dense cover that excludes and inhibits other plants
  
• Reproduces by seeds
  
• Hybridizes with Marrubium supinum
  
• May reach maturity in one year
  
• Fruit attach to animals, vehicles, and clothing and also dispersed by water and contaminated agricultural produce
  
• Seeds survive passage through guts of horses
  
• Prolific seed production
  
• Seeds may persist in the soil for 7-10 years

Low Risk Traits:

• Unarmed (no spines, thorns or burrs)
  
• Medicinal uses
  
• Grows primarily in full sun
  
• Not reported to spread vegetatively
  
• Cultivation and fire provide effective control
  
• Herbicides may provide effective control