Family: Cupressaceae

Origin, Description & Uses
Thuja ‘Green Giant’ (Green Giant) is a fast-growing evergreen conifer that originated as a horticultural hybrid between western redcedar (Thuja plicata) and Japanese arborvitae (Thuja standishii). It is valued for its dense, pyramidal form, soft rich green foliage that remains attractive year-round, and exceptional growth rate, often reaching 30–50 feet tall under favorable conditions. Green Giant is widely planted as a privacy screen, windbreak, noise buffer, or specimen tree in landscapes due to its vigorous growth, adaptability to a variety of soils, and relatively low maintenance requirements. It is also appreciated for its resistance to many common pests and diseases that affect other arborvitae.

Risks & Threats
Green Giant is considered a low-risk landscape tree and has not been shown to become invasive in Hawaiʻi or other tropical island ecosystems. As a cultivated hybrid, it produces few if any viable seeds and spreads only through intentional planting, greatly reducing the risk of naturalization. While its large mature size should be considered when selecting a planting location, it poses little threat to surrounding natural areas. When planted in appropriate settings with adequate space, Green Giant is an excellent choice for creating evergreen screens and windbreaks without contributing to the spread of invasive plants.

High Risk Traits:

• Environmental versatility: Tolerates a wide range of soils and climates (USDA zones 5–8).
  
• Shade tolerant: Can grow in full sun to part shade.
  
• Human dispersal: Intentionally planted extensively in landscaping and nurseries.
  
• Potential allelopathy: Other Thuja species produce growth-inhibiting chemicals (unknown for this hybrid).
  
• Potential fire hazard: Parent species (T. plicata) has flammable foliage.
  
• Potential for vegetative spread: Propagated easily via cuttings, though natural fragmentation is unconfirmed.

Low Risk Traits:

• No seed production: Hybrid is sterile; propagated only by cuttings.
  
• No naturalization: No evidence of escaping cultivation or becoming weedy.
  
• No reproductive spread: No seeds, no wind/water/bird dispersal, no persistent seed bank.
  
• No toxicity or allergies: Not toxic to animals or humans (unlike some relatives).
  
• No dense thickets: Forms hedges only in cultivation; no wild thicket formation.
  
• No climbing or smothering: Grows as an upright, non-vining tree.
  
• No major pests/diseases: Generally problem-free in cultivation.