Family: Dicksoniaceae

Origin, Description & Uses
Dicksonia squarrosa, commonly known as harsh tree fern, rough tree fern, or wheki, is native to New Zealand. This fast-growing tree fern develops slender, dark trunks that can reach 20 feet or more in height and often spreads by forming dense colonies of multiple stems. Its arching fronds create a lush, tropical appearance, making it popular in landscape plantings and botanical gardens. In its native range, wheki is an important component of forest ecosystems and has also been used in traditional Māori culture for a variety of practical purposes. In Hawaiʻi, it is sometimes planted for ornamental value in shaded gardens and restoration-style plantings.

Risks & Threats
This species is naturalized in Hawaiʻi and possesses traits that are cause for concern. It spreads readily by spores and by producing new trunks from underground stems, allowing it to form thick stands that can crowd out native understory plants. Its ability to thrive in moist forest environments means it could detrimentally impact tropical island ecosystems, particularly in native wet and mesic forests where competition for light and space is intense. Dense growth of harsh tree fern may alter habitat structure and limit regeneration of native species. Because of these risks, Plant Pono recommends choosing a low-risk or native tree fern alternative for landscaping and restoration projects to help protect Hawaiʻi’s unique forest ecosystems.

High Risk Traits:

• Able to grow, and potentially invade, regions with cool, subtropical climates
  
• Naturalizing in Volcano, Hawaii (Hawaiian Islands)
  
• Reported to be aggressive in gardens, needing room to spread
  
• A native pasture weed in New Zealand
  
• Other Dicksonia species are invasive
  
• Potentially allelopathic
  
• Reported to be highly flammable; may increase fire risk in drier areas
  
• Shade tolerant (can establish under intact forest)
  
• Tolerates many soil types
  
• Forms dense stands and excludes other vegetation in native range
  
• Reproduces by spores and apogamous gametophyes (requiring no fertilization)
  
• Spreads vegetatively by underground runners
  
• Spores dispersed by wind, water, and intentional cultivation
  
• Prolific spore production
  
• May form a persistent spore bank
  
• Tolerates and will resprout after cutting or browsing damage

Low Risk Traits:

• May primarily threaten cooler, higher elevations of the Hawaiian Islands and other tropical island ecosystems
  
• Unarmed (no spines, thorns, or burrs)
  
• Palatable to ungulates and other browsing animals
  
• Non-toxic