Family: Amaranthaceae

Amaranthus spinosus, commonly known as spiny amaranth or spiny pigweed, is a fast-growing annual herb that is widespread across the tropics and subtropics. It is easily recognized by the sharp spines found at the base of its leaves, which make it less palatable to animals and a nuisance to people working in infested areas. This species thrives in disturbed soils, roadsides, pastures, and agricultural lands, where it can quickly form dense stands that compete with crops and other vegetation. Its prolific seed production and tolerance for a wide range of growing conditions allow it to spread and persist once established.

In Hawaiʻi, spiny amaranth has naturalized throughout the main islands—Kauaʻi, Oʻahu, Molokaʻi, Lānaʻi, Kahoʻolawe, Maui, and Hawaiʻi Island—as well as in the Northwestern Hawaiian Islands on Mokupāpapa (Kure Atoll) and Kuaihelani (Midway Atoll). Its ability to spread widely across such diverse environments demonstrates its adaptability and invasive potential. While it is sometimes used as a leafy green in other parts of the world, in Hawaiʻi it is primarily regarded as a weedy species that reduces the quality of pastures and disrupts native and agricultural ecosystems. Effective management requires early detection and removal before it sets seed, as once established it can be difficult to control.

High Risk Traits:

• Broad climate suitability and elevation range exceeds 1000 m
  
• Thrives in tropical climates
  
• Widely naturalized including all the main Hawaiian Islands
  
• Crop weed
  
• Other Amaranthus species are invasive
  
• Leaf axils with a pair of divergent spines
  
• Allelopathic
  
• Can be poisonous to cattle or other grazing animals
  
• Host of crop pests and pathogens
  
• Allergenic and cause of hay fever
  
• Tolerates many soil types
  
• Reproduces by seeds
  
• Hybridizes with other Amaranthus species
  
• Capable of self-pollination
  
• Annual growth habit
  
• Seeds dispersed by wind, water, in animal droppings, and as a seed contaminant
  
• Prolific seed production
  
• Seeds able to form a persistent seed bank

Low Risk Traits:

• Animals may consume plants, despite spines and toxic properties
  
• Relatively Shade-intolerant
  
• Not reported to spread vegetatively
  
• Herbicides may provide effective control