ASHS 2024 Conference

Scientific research activities focused on production of high-quality Cannabis sativa continue to grow in both the industry and academic sectors. At the university level in the United States, the majority of the research uses hemp-type or low tetrahydrocannabinolic acid (THCa) cannabis cultivars due to federal restrictions. Only at the industry level and in states with some level of medical or recreational legalization can production research be carried out on high THCa or drug-type Cannabis. Dried female flower inflorescences are the harvested and saleable portion of the Cannabis plant. The quantitative traits of flowers are relatively clear and easily enumerated. To be considered superior quality, THCa concentration should be 25% or greater and the total terpene concentration should be 2% or greater on a dry weight basis (moisture content of 10-12%). While the average cannabis consumer values these characteristics, they also greatly appreciate many of the qualitative characteristics of cannabis flower, which include visual appearance or sometimes referred to as bag appeal, olfactory characteristics when opening the container of cannabis and while breaking apart the flowers, texture, and taste when smoking. These characteristics are more difficult to define and quantify. When conducting research, both quantitative and qualitative characteristics need to be of great consideration when the goal is to produce high quality Cannabis flower.

There is a growing demand for hemp-derived products but because of the crop's criminalization history, there is limited University-produced information describing best production practices. The research objective was to define the critical weed-free period (CWFP) for transplanted floral hemp. Field trials were conducted in 2022 and 2023 at an outlying NDSU Agriculture Experiment Station near Prosper, ND (46.57° N, 97.01° W), the Cornell AgriTech campus in Geneva (42.88°N, 77.01°W), and the Clemson University Coastal Research and Education Center (33.46°N, 79.55°W). Four cultivars were transplanted at NY with three cultivars (Bubbatonic, Sour Space Candy, and Quick Spectrum) in common with ND. Three cultivars were transplanted in SC with two cultivars (Cherry Wine and Bubbatonic) in common with NY. The CWFP treatments were weeded for 0, 1, 2, 4 and 6 weeks, and weed-free. Weeding was accomplished by manually hoeing and hand-weeding. Weed species varied at each location but were mainly annual weed species (both broadleaf and grasses). Plants were harvested after at least a 16-week period in the field and air-dried before removing leaf and floral biomass from stems. At SC, when averaged across cultivars and trial years, a significant increase in mean floral yield with 2, 4, 6, and season long weed-free intervals when compared to 0 weeks weed-free. In addition, a significant decrease in dry weed biomass existed when comparing the same weed-free intervals to the 0 weeks weed-free treatment. At NY, hemp biomass averaged across all cultivars was significantly affected by the duration of weed competition. Per plant yields were reduced >80% when weeds were allowed to compete almost season-long. Biomass production was maximized when weeds were suppressed for at least 6 weeks after transplanting. At ND, results were quite different due to timely rainfall and lack of rain. Under adequate to excellent moisture conditions, the stem diameter, stem number, plant weight:height ratio, and dry biomass yield responses were significantly different only when weed control was provided for one week. However, under extreme season long drought conditions, stem diameter increased 53%, weight:stem ratio increased 172% and dry biomass increased 201% when weeds were controlled the entire season. Results indicate that transplanted hemp is sensitive to competition and preventing weed establishment for several weeks was necessary to reduce competitive interactions. Results also suggested that the need for a weed-free period was exacerbated and most important when rainfall during the growing season was limited.

Current fiber hemp (Cannabis sativa <0.30% total THC) production systems rely on pre-emergent herbicides to manage weeds prior to canopy closure. Due to the rapid growth of fiber hemp, paired with the plastic nature of the crop, the need for weed suppression is most critical in the first month after planting. No-till systems with a rolled cover crop residue may be able to provide weed suppression without the use of herbicides, which can lower the cost of inputs, reduce the risk of negative environmental impacts associated with herbicide use, and improve soil health. Four cover crops - crimson clover (Trifolium incarnatum), hairy vetch (Vicia villosa), cereal rye (Secale cereale), and triticale (xTriticale)- were roller-crimped into a mulch, and fiber hemp was no-till drilled into the residue. Each cover crop plot was paired with two bare ground plots, one of which was weeded weekly, while the other remained weedy. This design was repeated in three locations in the coastal-plain region of North Carolina (Kinston, Clinton, and Goldsboro). Crop emergence, weed counts and biomass, and fiber hemp biomass and yield were measured in each plot. Cover crop biomass was measured before planting and after retting to determine whether this residue may be a potential contaminant during baling. Soil health parameters were compared among cover and bare plots. Hairy vetch resulted in greater hemp emergence in Kinston, but emergence was not affected by the mulches otherwise. Hairy vetch plots had less weeds than others in Clinton and Goldsboro. Soil respiration and potentially oxidizable carbon did not differ by treatment, with the exception of higher soil respiration in grass covers in Goldsboro. Cover crop biomass remaining after retting was higher in grass plots than legume plots, indicating a potential for bail contamination. These preliminary results indicate that fiber hemp can be grown in a no-till system.

Cannabis sativa is a known hyperaccumulator of heavy metals (HM), and testing of hemp medicinal products is required for HM contaminants including arsenic (As), cadmium (Cd), and lead (Pb). The objective was to evaluate how the timing of HM applications impacts hemp growth. A plant experiment was conducted where 40 Cannabis sativa ‘Wife’ hemp plants were grown in deep water culture systems in a growth chamber. Aqueous nutrient solutions including combined As, Cd, and Pb were applied directly into the hydroponic solution on a weekly basis, with HM concentrations of 0.0, 0.5, and 1.0 mg HM/L. During the first six weeks, plants were grown under long days to promote vegetative growth, and 15 plants were harvested at the end of this vegetative growth phase. The remaining 25 plants were grown under short days to initiate flowering, with harvest 6 weeks later. Total dry mass (P < 0.05), shoot dry mass (P < 0.05), and root dry mass (P < 0.01) were significantly affected by HM application timing. The total and shoot dry mass was the highest for the control group, followed by the plants that received HM at 0.5 or 1.0 mg/L during the reproductive phase, and the least growth occurred in plants that received HM at 0.5 or 1.0 mg/L during the vegetative phase. Root dry mass decreased as HM concentration increased (P < 0.05). Flower dry mass was not significantly affected by HM application timing or concentration. Results highlight how exposure to HM at 0.5 to 1.0 mg/L markedly reduce biomass accumulation, especially when exposure occurs early in the production cycle.

Nitrogen (N) fertilization plays a key role in determining the productivity and quality of horticultural crops and limited information is available on the N requirements of photoperiod-sensitive CBD hemp (Cannabis sativa L.). With the objective of providing recommendations on N fertilization to CBD hemp growers a field study was conducted in Pennsylvania to evaluate the response to N inputs of two photoperiod sensitive CBD hemp genetic resources: a clone named ‘FunDip’ (The Hemp Mine) propagated using rooted cuttings and ‘Sour Kush’ (Kayagene) propagated using feminized seeds. Both selections were planted mid-June on raised beds mulched with black polyethylene film and served by drip irrigation. Plants were established at 1.5-m in-row and 2.4-m between rows. After planting both CBD hemp selections were fertigated weekly with urea (46-0-0) using seasonal application rates equivalent of 84, 168, 252, and 336 kg/ha of N. An unfertilized control was used to account for the N available through the soil and to estimate the crop N use efficiency. Treatments were arranged according to a split plot experimental design with four replications. Nitrogen treatments were randomized within the main plots while hemp selections were randomized within subplots. Each experimental unit had a minimum of 12 plants, unfertilized border rows and in-row areas were used as buffer zones to avoid fertilizer cross-contamination between different N applications rates. Plant response to N inputs was evaluated conducting biometric assessments during the growing season and at final harvest. Representative plants were sampled to measure leaf and inflorescence, stem, and total plant fresh and dry biomass. Oven-dried plant tissue samples were analyzed for their total N content to estimate the plant N accumulation during the growing season. At every biometric assessment, soil samples were collected and analyzed for pH, EC, and nitrate (NO3-) content. A quadratic response to N inputs was observed in both selections. At final harvest, the total fresh plant biomass of both genotypes was maximized with the application of 252 kg/ha of N and declined at higher N rate. However, no increase in plant dry biomass were observed in plants fertigated with over 168 kg/ha of N in both genotypes. An accumulation of NO3-N and associated increase in EC was observed with the progression of the growing seasons especially in plots fertigated with over 84-168 kg/ha of N, in Sour Kush and FunDip, respectively, which suggest an excess of N was applied with N rates exceeding 84-168 kg/ha.

Successful field cultivation of hemp (Cannabis sativa L.) in Florida is restricted to summer months when rainfall is highest, as hemp is exceptionally sensitive to daylength. Given the prevalence of soils in Florida with poor nutrient and water holding capacities, controlled release fertilizer (CRF) could be ideally suited for outdoor cultivation due to its slow-release profile. To assess the effectiveness of CRF to support plant growth while minimizing water quality risks, the plant growth and biomass production of four hemp cultivars (‘Wife’, ‘Sunset Improved’, ‘FunDip’, and ‘Belle’) were evaluated in response to nitrogen and phosphorous availability from ten CRF formulations in Apopka, Florida during the summer of 2023. Plant growth was assessed post-transplant and monthly thereafter until harvest by measuring the height, widest width, and width perpendicular to the widest width of each hemp plant. At harvest, plants were severed at the soil surface and dried at 70 degrees Celsius for three days. For each plant, the total plant biomass and total flower biomass was measured. Significant differences in plant growth and biomass production were found among varieties; however, minimal differences were found in plant growth and biomass production within variety in response to varying CRF formulations. These results suggest that CRF can provide adequate levels of fertility for the growth and development of hemp cultivated outdoors in Florida, and that selection of hemp cultivar affects plant growth and yield.