Date of Award
8-1-2025
Degree Name
Doctor of Philosophy
Department
Plant Biology
First Advisor
Gibson, David
Abstract
Hemp (Cannabis sativa L.) is an historically cultivated species re-emerging in modern agriculture due to its utility as a fiber, grain, and cannabinoid crop. While industrial hemp refers to cultivated varieties bred for specific agronomic traits and legally defined by low tetrahydrocannabinol (THC) content, feral hemp, often called "ditch weed", represents escaped or naturalized populations descended from historical cultivation. These unmanaged populations persist and spread in disturbed habitats across North America, particularly in the Midwestern U.S., raising ecological and agronomic concerns due to their potential for genetic contamination, invasive behavior, and interactions with native species and agricultural systems. The overall goals of this research were to 1) characterize the population dynamics and demographic drivers of feral hemp populations across diverse environmental gradients, identifying the biotic and abiotic factors that regulate persistence and spread in disturbed habitats within Illinois, 2) assess the impacts of feral hemp on native plant community composition, functional trait structure, and phylogenetic diversity, while evaluating how landscape features and climate influence habitat suitability and potential future distribution, 3) To evaluate the effectiveness of industrial hemp as a cover crop for weed suppression, focusing on the roles of fertility, seeding rate, cultivar type, and cover crop integration in shaping weed communities and hemp performance, and 4) investigate how weed competition and cultivar variation affect the physical and chemical traits of hemp, including biomass production, morphology, and cannabinoid concentrations, under variable competitive conditions. Feral hemp populations across Illinois exhibited complex demographic responses to both biotic and abiotic factors. Density-dependent processes such as herbivory and competition significantly influenced individual performance and population growth, while environmental variables including soil fertility, moisture, temperature, and topography shaped survivorship and reproductive success. Populations displayed diverse life history strategies, with both Type I and Type II survivorship patterns and strong size-dependence in survival, growth, and fecundity. Seed trait analyses confirmed high initial germination and viability but a rapid decline in seedbank potential, emphasizing a reliance of populations on continual annual recruitment. Feral hemp also significantly altered native weed community structure, reducing species richness and promoting trait convergence toward tall, fast-growing competitors with high specific leaf area and nutrient-acquisitive strategies. Phylogenetic and functional trait analyses supported the role of feral hemp as a driver of community assembly through both competition and environmental filtering. Despite this dominance aboveground, soil seedbanks retained high species diversity, suggesting legacy resilience in native communities. Habitat suitability models revealed that soil texture, organic matter, and climate variables, rather than topographic relief, are primary filters determining feral hemp distribution, with projections indicating potential range expansion under future climate scenarios. Industrial cultivars of hemp were found to suppress weed emergence effectively under favorable soil conditions, particularly when planted at intermediate densities or in conjunction with nitrogen inputs. However, performance was highly site- and cultivar-dependent, with poor soil conditions limiting hemp's establishment and competitive effect. Cover crops such as Trifolium repens and Secale cereale improved hemp establishment in marginal soils, suggesting synergy between hemp and other species in integrated weed management strategies. Further experimentation demonstrated that cultivated hemp cultivars outperform feral accessions in biomass, stem diameter, and fiber yield, with temperature and plant gender also significantly influencing performance. Weed competition reduced growth across all cultivars, but cannabinoid concentrations remained stable, indicating that chemical composition may be governed more by genetics and abiotic factors than by competition intensity. Collectively, these findings provide a comprehensive assessment of hemp's ecological behavior and management potential in both natural and agricultural settings. They highlight the nuanced roles of feral hemp as both a potentially disruptive colonizer and a component of novel ecosystems, while also revealing how industrial hemp may contribute to sustainable agriculture through competitive weed suppression and fiber production. This work informs policy, conservation, and cultivation practices at a time when hemp is once again becoming widespread across the U.S. landscape.
Access
This dissertation is Open Access and may be downloaded by anyone.