Family: Hypericaceae

Hypericum perforatum is native to Europe, western Asia, and northern Africa. It was introduced to Hawai’i for ornamental and medicinal purposes. The common name is St. Johns Wort. This plant is listed as noxious weeds in several states. When buried in the soil, a seed from this plant can last ten years. A single adult plant can produce up to 100,000 seeds per year.

Description and Dispersal:

• Hypericum perforatum is a herbaceous perennial with yellow flowers with 5 petals. This plant has many branches and tiny olive shaped leaves.

High Risk Traits:

• Broad climate suitability (temperate to subtropical)
  
• Naturalized beyond native range
  
• Disturbance weed (roadsides, railways, pastures)
  
• Agricultural weed (reduces carrying capacity)
  
• Congeneric weeds common globally
  
• Toxic to animals (photosensitivity, poisoning)
  
• Tolerates wide range of soils
  
• High viable seed production (84–94% germination)
  
• Hybridizes naturally
  
• Self-compatible / apomictic (no pollinator needed)
  
• Vegetative reproduction (clonal via roots)
  
• Generates in <1 year
  
• Seeds dispersed by animals and humans (sticky capsules)
  
• Intentional human introduction (ornamental, medicinal)
  
• Prolific seed output (up to 200,000/m²)
  
• Persistent seed bank (≥9 years)
  
• Tolerates fire (fire promotes growth)

Low Risk Traits:

• No spines, thorns, or burrs
  
• Not unpalatable (livestock graze it)
  
• Low human toxicity/allergy risk
  
• No fire hazard
  
• Not shade tolerant
  
• No wind, water, bird, or gut-passage dispersal
  
• Well controlled by herbicides