Family: Amaranthaceae

Salsola tragus, commonly known as Russian thistle or tumbleweed, is a fast-growing annual plant native to Eurasia that has spread widely around the world. It forms a rounded, bushy plant with stiff, spiny branches and narrow leaves, often growing up to 3 feet tall. As the plant dries at the end of its life cycle, it breaks off at the base and tumbles with the wind, scattering seeds across long distances. Russian thistle has been used historically as emergency forage and in limited soil stabilization efforts, but it is generally not cultivated intentionally today. In Hawaiʻi, it is found growing in dry, open, and disturbed areas such as roadsides, fields, and construction sites.

Despite its familiar appearance, Russian thistle poses significant environmental and safety risks. It spreads rapidly, produces large numbers of seeds, and easily invades disturbed landscapes, where it can outcompete native plants. When plants dry out and accumulate, they create highly flammable fuel, increasing fire risk and allowing fires to spread quickly—especially in dry, windy areas. Large tumbleweeds can also block roads, damage fencing, and interfere with infrastructure. In Hawaiʻi, Salsola tragus is naturalized on Oʻahu, Molokaʻi, Kahoʻolawe, Maui, and Hawaiʻi Island, making prevention of further spread and active management especially important to protect native ecosystems and reduce wildfire hazards.

High Risk Traits:

• Highly effective seed dispersal – Mature plants detach and tumble, dispersing thousands of seeds over long distances, including along roads and open landscapes.
  
• Very high seed production – Individual plants can produce tens of thousands of seeds, enabling rapid population buildup.
  
• Thrives in disturbed environments – Readily colonizes roadsides, construction sites, fallow fields, grazed lands, and burned areas.
  
• Drought tolerant – Well adapted to arid and semi-arid conditions; persists where native vegetation is stressed.
  
• Rapid growth and early reproduction – Can complete its life cycle quickly, allowing establishment before competing species.
  
• Fire hazard – Dead plants accumulate as highly flammable tumbleweeds, contributing to fuel loads, fire spread, and ember transport.
  
• Forms dense infestations – Can dominate open areas and suppress native or desirable vegetation.
  
• Agricultural and infrastructure impacts – Interferes with crops, fencing, roads, and drainage systems.
  
• Proven invasive history elsewhere – Widely documented as a problematic weed across western North America and other dry regions.
  
• Tolerance of poor soils – Grows in saline, compacted, or nutrient-poor substrates.

Low Risk Traits:

• Requires open, sunny sites – Less competitive in intact, shaded native ecosystems.
  
• Limited establishment in dense vegetation – Performs poorly where strong perennial cover is present.
  
• Seed longevity is moderate – Does not form extremely long-lived seed banks compared to some invasive species.
  
• Not toxic to humans – Though problematic, it is not a direct human health toxin.
  
• Control feasible with management – Early detection, vegetation cover, and disturbance reduction can limit spread.