Family: Myrtaceae

Agonis flexuosa (willow myrtle) is a graceful evergreen tree native to southwestern Australia, recognized for its drooping, willow-like branches and narrow, aromatic leaves that release a peppermint-like scent when crushed. In spring and summer, it produces clusters of small white flowers that attract pollinators, followed by woody seed capsules. Growing up to 35 feet tall, it is widely planted as an ornamental or shade tree and has several popular cultivars with colorful foliage, including burgundy and variegated forms. The species tolerates drought, wind, and poor soils, making it a resilient and low-maintenance choice for landscaping.

In Hawaiʻi, willow myrtle is not known to be naturalized, but it has traits that could allow it to spread and potentially threaten native ecosystems. In parts of Australia, it has naturalized outside of its native range, and its dense canopy and fast growth can suppress understory plants. The tree is also capable of producing large numbers of viable seeds and establishing in disturbed areas. While not currently invasive in Hawaiʻi, careful consideration should be given before planting, and non-invasive or native alternatives are recommended to protect the Islands’ biodiversity.

High Risk Traits:

• Environmental Weed: Naturalized and invasive in Victoria, Australia, threatening native heathlands and woodlands.
  
• Host for Pests: A confirmed host for myrtle rust (Uredo rangelii), posing a risk to other vulnerable species.
  
• Fire Adaptations: Increases fuel loads and resprouts vigorously after fire, altering natural fire regimes.
  
• Soil Tolerant: Thrives on a wide range of well-drained to poorly-drained soil types.
  
• Effective Dispersal: Seeds are wind and water-dispersed due to a membranous wing.
  
• Human Cultivation: Widely planted in horticulture, providing a constant introduction pathway.
  
• Resilient: Tolerates mutilation, fire, and grazing by resprouting from the stump.

Low Risk Traits:

• Not Toxic: Poses no toxicity risk to humans or livestock.
  
• Controllable by Grazing: Readily eaten by animals, which can help manage its spread.
  
• Limited Dispersal Vectors: Seeds are not bird-dispersed or adapted for external animal dispersal.
  
• Not a Structural Parasite: Does not climb over or smother other vegetation.
  
• Not an Agricultural Pest: No known impact on agriculture or pasturelands.
  
• No Physical Defenses: Lacks spines, thorns, or burrs.