Interspecific Crosses Yield Sterile Triploid Porterweed: A Sustainable Alternative to Invasive Nettleleaf Porterweed

Porterweed plants ( Stachytarpheta sp.), notably the nettleleaf porterweed ( Stachytarpheta cayennensis ), present both ecological benefits and challenges. While these plants attract diverse pollinators and are drought-resistant, nettleleaf porterweed is also an invasive species in the southeastern United States and Hawaii, threatening native ecosystems by outcompeting local flora and reducing biodiversity. Despite its invasive nature, it continues to be popular in nurseries because of its robust growth and flowering characteristics. Prior studies have demonstrated the prolific seed production of the species and potential for naturalization, emphasizing the need for sterile, noninvasive alternatives. This study explored the development of such alternatives through interploidy and interspecific hybridization by focusing on creating triploid hybrids that exhibit both male and female sterility. This was achieved by using interspecific and interploidy crosses between tetraploid nettleleaf porterweed and diploid dwarf red porterweed ( Stachytarpheta microphylla ‘Red Compact’). Growth, flowering, and sterility of the resulting hybrids were assessed in multiple controlled and field conditions. The results identified three triploid hybrids that exhibited desirable horticultural traits comparable to those of the invasive nettleleaf porterweed but, importantly, showed no viable seed or pollen production, confirming their sterility. These hybrids displayed robust growth, substantial flower production, and sterility across multiple environmental conditions, making them ideal candidates for replacing invasive species in landscape settings.