Family: Cupressaceae

Origin, Description & Uses
Hesperocyparis macrocarpa, commonly known as Monterey cypress, is native to a small area of the central California coast. This fast-growing evergreen conifer is valued for its tall, wind-sculpted form, dense foliage, and tolerance of salt spray and strong winds. It can reach heights of 40–70 feet or more, with a broad, spreading crown and aromatic, scale-like leaves. Monterey cypress has long been planted as a windbreak, privacy screen, and ornamental tree in coastal landscapes. Its adaptability and rugged appearance have made it a popular choice in exposed sites, including parts of Maui.

Risks & Threats
Monterey cypress is naturalized on Maui and is considered high risk in Hawai‘i. It produces abundant seed, tolerates a range of conditions, and can establish beyond cultivation. These traits raise concern that it may spread into natural areas, where it can form dense stands that shade out native plants and alter habitat structure. In tropical island ecosystems—where many native species evolved without competition from large, fast-growing conifers—such changes can be especially disruptive. Because of its invasive potential and ability to impact sensitive ecosystems, planting Monterey cypress is not recommended. Gardeners and land managers are encouraged to choose low-risk or native alternatives better suited to protecting Hawai‘i’s unique environments.

High Risk Traits:

• Environmental weed (Victoria, Australia - serious threat to native vegetation)
  
• Naturalized beyond native range (California coast, Hawaii, Australia, New Zealand)
  
• Native/naturalized in subtropical regions (naturalized in Hawaii at 1219 m)
  
• Repeated introductions worldwide (Europe, South America, Africa, Asia, NZ, Australia)
  
• Toxic to animals (cattle abortions, fetal leucomalacia)
  
• Host for serious pathogens (Seiridium cardinale, Lepteutypa cupressi, Phytophthora spp.)
  
• Broad soil tolerance (planted individuals tolerate wide range)
  
• Hybridizes (with C. nootkatensis, C. lusitanica, C. arizonica)
  
• Wind & water dispersed
  
• Prolific seed production (abundant cones, ≤140 seeds per cone)
  
• Persistent seed bank (viable ≤4 years on tree, ≤10 years in storage)
  
• Fire-adapted (cones open with heat)

Low Risk Traits:

• Not an agricultural/amenity weed
  
• No spines, thorns, burrs
  
• Non-toxic to humans
  
• Shade intolerant (requires bare mineral soil, high light)
  
• No vegetative reproduction (seed only)
  
• Long generation time (first cones at 6–10 years)
  
• Low unintentional dispersal risk (no external attachment, not bird-dispersed, no gut passage, not a produce contaminant)