Family: Polygonaceae

Origin, Description & Uses
Rumex hypogaeus (syn. Emex australis), commonly known as spiny emex or three-cornered jack, is a low-growing annual plant native to southern Africa and parts of the Mediterranean region. It grows in a spreading rosette of broad, dock-like leaves and produces small greenish flowers that develop into distinctive hard fruits with three sharp spines. These spiny seed pods readily attach to shoes, animal fur, and vehicle tires, helping the plant disperse over long distances. In some regions the young leaves have occasionally been eaten as a cooked green similar to spinach, but the plant is far better known worldwide as a troublesome weed in agricultural fields, pastures, and disturbed areas.

Risks & Threats
In Hawaiʻi, spiny emex is naturalized and possesses traits that are cause for concern. The plant produces large numbers of durable, spiny seeds that spread easily and can persist in the soil, allowing populations to expand rapidly once established. Dense infestations may compete with crops, pasture grasses, and native vegetation, while the sharp seed capsules can injure livestock, wildlife, and people, and even puncture bicycle and vehicle tires. Because of its ability to spread quickly and dominate disturbed habitats, this species could detrimentally impact tropical island ecosystems if left unmanaged. For these reasons, planting or intentionally spreading this species is discouraged, and gardeners and land managers are encouraged to choose low-risk or native plant alternatives instead.

High Risk Traits:

• Invasive weed: Naturalized in Australia, US (CA, HI), and elsewhere.
  
• Agricultural pest: Causes major crop/pasture losses; declared noxious in Australia.
  
• Environmental weed: Invades riparian zones, alluvial flats, and seabird rookeries.
  
• Spiny burrs: Sharp spines (5-10 mm) aid dispersal and can cause injury.
  
• Allelopathic: Releases chemicals that inhibit growth of other plants (e.g., wheat).
  
• Toxic to livestock: Contains oxalic acid, can poison sheep.
  
• Broad soil tolerance: Grows in sandy to heavy clay soils.
  
• High fecundity: Produces >1,000 seeds per plant; seed bank persists >7 years.
  
• Self-compatible: Can reproduce without a partner.
  
• Multiple dispersal modes: Contaminates grain/wool; spread by water, vehicles, machinery, and animals (internal & external).

Low Risk Traits:

• Shade intolerant: Requires open, disturbed sites; does not invade shaded areas.
  
• Non-climbing herb: Prostrate growth habit; does not smother vegetation.
  
• No vegetative spread: Reproduces by seed only; no underground storage organs.
  
• Herbicide sensitive: Controlled by dicamba, sulfonylureas, etc.
  
• Biocontrol available: Successfully controlled by weevils in Hawaii.