Family: Fabaceae

Origin, Description & Uses
Melilotus officinalis (yellow sweet clover) is a biennial herb native to Europe, western Asia (including the Himalayas), and parts of the Arabian Peninsula. It produces upright, branching stems with small yellow flowers arranged in slender spikes and is easily recognized by its sweet, hay-like fragrance when dried. This plant has been widely introduced around the world and is sometimes used as a cover crop, green manure, and forage plant because it fixes nitrogen in the soil and can improve soil fertility. It has also been used in traditional herbal medicine and as a nectar source for pollinators such as bees.

Risks & Threats
Although Melilotus officinalis is currently not known to be naturalized in the Hawaiian Islands, it possesses traits that are cause for concern and could detrimentally impact tropical island ecosystems if it were to establish. As a vigorous nitrogen-fixing species, it can alter soil nutrient levels in ways that favor fast-growing invasive plants over native Hawaiian species adapted to low-nutrient conditions. It can also form dense stands that compete with and displace native vegetation, particularly in disturbed areas. Additionally, the plant contains coumarin compounds that can become toxic to livestock when improperly cured, highlighting its broader ecological and agricultural risks. Given these concerns, caution is recommended, and choosing a low-risk or native alternative is encouraged whenever such options are available.

High Risk Traits:

• Broad climate suitability
  
• Grows in temperate to tropical climates
  
• Widely naturalized (but no evidence in Hawaiian Islands to date)
  
• Agricultural weed
  
• Environmental weed
  
• Other Melilotus species are invasive weeds
  
• May be allelopathic
  
• Improperly cured or dried plants may be harmful to animals
  
• Tolerates many soil types
  
• Forms dense cover that can exclude other vegetation
  
• Reproduces by seeds
  
• Some plants may be self-fertile (with low seed set)
  
• Can reach maturity in <1 year (but often >1 growing season)
  
• Seeds dispersed by water, internally by animals, as a contaminant of other crops and seed mixes and intentionally by people
  
• Prolific seed production
  
• Forms a persistent seed bank
  
• May tolerate fire and cutting at certain growth stages

Low Risk Traits:

• Unarmed (no spines, thorns, or burrs)
  
• Provides fodder for livestock (palatable despite reports of toxicity)
  
• N-fixing (improves soil fertility)
  
• Not reported to spread vegetatively
  
• Beneficial to bees and other pollinators
  
• Herbicides may provide effective control if removal is desired