Abstract
Hemp (Cannabis sativa L.) research and commercial production has recently experienced a global revival motivated by passage of laws reversing long-standing prohibitions and by expansion in markets. Collaborative research has been initiated in response to renewed interest in hemp production, such as the American Society for Horticultural Science Hemp Research and Extension Professional Interest Group (ASHS Hemp). Collaborators new to this crop have identified a lack of standard definitions, descriptions, and procedures for cohesive study specific to hemp production. Standards are necessary for synthesis of data gathered across research and industry programs. ASHS Hemp convened a workshop of hemp researchers and industry representatives to establish consensus on a minimum set of standards for research data and industry assessments. The resulting morphology and physiology standards developed at the workshop are presented here with a focus on plant height, flowering time, and crop quality. Plant height was defined as the vertical distance between the root crown at the soil surface and the stem node (or tip) of the apical meristem of the tallest branch. Plant height was importantly distinguished from stem length and canopy height, which may differ based on pruning and management of the plant. Flowering time was defined to indicate date of initiation of inflorescence development as the earliest day terminal flowering clusters appear visually. Flowering time was distinguished from solitary flowering behavior and floral maturity. Crop quality was determined to be a feature that should be established first by industry based on market standards and then subsequently adopted by researchers targeting outcomes in specific areas. A standard moisture content for dry flower, seed, and straw must be established. A moisture content of 10% to 12% was identified as a current standard for floral yield, whereas 8% was identified as a moisture content standard for seed crops. Bast-to-hurd ratio and decortication efficiency were fiber quality metrics identified for minimum standards, and thousand seed weight, protein content, oil content, and oil composition were considered for minimum seed quality standards. The hemp research community is well positioned to standardize genomic references and establish best management practices for production targets. These efforts would be assisted by the adoption of the proposed standard definitions, descriptions, and procedures decided by consensus at the ASHS Hemp 2022 workshop.
Hemp (Cannabis sativa L., <0.3% Δ9-tetrahydrocannabinol) research and industry efforts have been reestablished in the United States since the 2014 and 2018 Farm Bill authorizations (Agricultural Act of 2014, P.L. 113-79; Agriculture Improvement Act of 2018, P.L. 115-334) and preceded by global efforts in Asia, Europe, and Canada. The US Drug Enforcement Administration Schedule I designation of Cannabis limited most research conducted in the US during the 20th century to the exploration of the drug effects of plant-derived metabolites and mechanisms for eradicating illegal operations rather than the study of biological functions and development of agricultural commodities. Recently, research and industry efforts have demonstrated the potential for modern hemp production and processing for fiber, seed, and flower production (Mark et al. 2020). The development of basic knowledge and best management practices is ongoing with support from research programs and public–private partnerships. The efficient exchange of ideas and development of best practices will be facilitated by standard definitions, descriptions, and procedures to generate reliable information and data among collaborations.
Recent hemp production methods have been developed for three harvest goals: fiber, seed, and flower (Williams 2019). Hemp is a crop for which genetics and management practices vary in relation to the crop production goals. Fiber crops are planted mechanically at very high seeding rates (1.8–2.5 million per hectare). Seed crops are also planted mechanically at high rates (0.6–0.9 million per hectare), although target rate and harvest time may vary according to production goal, of which each of fiber, flower, seed, or multiple harvestable products may be attempted from this crop system. Crops primarily for flower production target 3000 to 50,000 plants per hectare and tend to require greater labor, management, and input intensity. Flower production has also been established in protected and controlled environments.
Coordinated research efforts and public–private partnerships have begun to generate and synthesize knowledge related to plant physiology, genetic variability, and crop management for hemp crops. These efforts should include the establishment of standards, descriptions, and procedures that facilitate collaborative knowledge and discovery. For well-established cropping systems, standards exist regarding research methodology and market designations (e.g., Vollbrecht and Schmidt 2009). Research methodology is standardized by consistent measurement and analytical procedures (e.g., Long-term Agricultural Research, Robertson et al. 2008; RosBREED, Iezzoni et al. 2020; corn crop decimal code, Groot et al. 1986). Well-established markets are standardized by the definitions of products and quality (e.g., cotton fiber length, fat content of milk). Often these standards are established by consensus of research, industry, and governmental entities. A crop decimal code (Mediavilla et al. 1998) and a phenotyping manual (Stansell and Osatuke 2021) have been proposed for hemp with variable adoption among research groups. The US Department of Agriculture (USDA) currently operates on the definition of hemp as an agricultural commodity distinguished from marijuana at the threshold of 0.3% Δ9-tetrahydrocannabinol (Agriculture Improvement Act of 2018, P.L. 115–334). The USDA and US Drug Enforcement Agency standards also describe the analytical testing procedure for cannabinoid analysis as required to execute laws and regulations (Congressional Research Service 2019). Established European, Canadian, or Chinese standards are also important sources for developing international standards (e.g., ASTM International Committee D37). Despite these existing efforts, research and marketing standards related to hemp production have yet to be clearly identified and widely adopted.
The ASHS Hemp hosted a workshop in August 2022 to discuss and propose minimum standards for plant morphology, physiology, and crop production. The goal was to motivate quality control and collaboration in research and industry sectors related to the production of hemp flower, fiber, and seed crops. Participants examined live hemp plants, and core topics were discussed to motivate consensus around minimum standards for research data and industry assessments. The resulting morphology and physiology standards from the ASHS Hemp workshop are presented here with a focus on plant height, flowering time, and crop quality.
Materials and Methods
Workshop format.
The ASHS Hemp 2022 workshop interactively explored the morphology and physiology of flower-type hemp plants and discussed morphology, physiology, and crop quality minimum standards for flower, fiber, and seed crops. There were 34 participants representing scientists, students, and industry representatives from the ASHS membership. The workshop format was a 2-hour guided discussion to define terminology and a minimum dataset around features of morphology, flowering, and crop quality related to hemp production and yield. To initiate discussion and gather insight into participant opinion, participants examined potted flower-type hemp plants upon arriving to the workshop with accompanying questions to answer online (Fig. 1). For example, a hemp plant with a stationary measuring tape was accompanied by the question prompt: “What height is the hemp plant (in cm)?” Participant responses were collected anonymously and reported to the group at the onset of topic discussion.
Hemp plants presented to workshop participants upon arrival with questions and electronic codes to provide answers for (A) height and (B) flowering. Photo credit: Zachary Brym.
Citation: HortScience 58, 7; 10.21273/HORTSCI17093-23
Topic discussion on height, flowering, and crop quality was guided toward consensus for terms and standards that required definition. The workshop moderator posed a specific statement or question to the group following discussion and cued participants to raise a blue card for “agree” or orange card for “disagree.” Disagreement was encouraged as an important mechanism of defining and describing target standards with the intent to reflect on how and why disagreements occurred. Those who disagreed with a statement were asked to explain their position and to offer specific ways that the statement or question could be modified to earn their agreement. This procedure was continued until disagreements were resolved and all participants raised a blue card. During discussion and consensus-building, participants were prompted in turn to provide a brief description of the specific methods they currently use to measure or evaluate the characteristic or feature being discussed with reference to existing standards.
Topics.
The topic of consideration for hemp morphology was hemp “height.” A plant with a stationary measuring tape was provided as an example (Fig. 1A). The measuring tape was placed at soil level and extended vertically at the main stem passing beyond the greatest extent of the plant canopy. Participants were asked: “What height is the hemp plant (in cm)?” Hemp ‘flowering’ was surveyed with three example plants (Fig. 1B). For each example plant, participants were asked: “What flowering development is hemp plant A, B, or C (in %)?” Production standards with respect to crop quality were discussed for hemp flower, fiber, and seed crop types through a reflection on existing and underdeveloped standards. In addition to discussion and consensus, participants were asked to provide references for “What methods do you reference as a hemp research standard?”
Reporting.
The workshop discussion and resulting consensus statements were documented by two moderator assistants. In addition, the workshop was recorded and transcribed (Supplemental Material S1). Moderator notes and transcription were cross referenced and summarized herein. The transcript with participant names redacted is available as supplemental materials.
Results and Discussion
Participants agreed overall with the intent of the workshop to develop research standards and further acknowledged a need for hemp research and industry development. Most workshop participants completed the interactive survey for height and flowering and a majority contributed actively to the discussion. An emergent outcome for the workshop was the beginning of a minimum data set for hemp activities in research and industry. The goal of this work was to encourage standard development in a way that builds cohesion and synthesis among research and industry efforts.
Height.
The consensus definition for “plant height” was the vertical distance between the root crown and the stem node (or tip) of the apical meristem of the tallest branch (Fig. 2). In practice, height is measured from the soil surface to the apical meristem of the highest branch above the soil. Several different possible height definitions were revealed in the survey results (mean 39.9 cm) with a majority of responses between 40 and 45 cm. Other survey responses included 0 cm and 15 cm. Part of the variation for the 40- to 45-cm range was determined to be measurement error, but also a critical distinction between height topping at the meristem node or the tallest part of the plant including the leaves (Fig. 2). Participants suggested that if the tallest leaves were included in the measurement of height, the result should be termed “canopy height” rather than plant height.
“Height” depicted as the vertical line between the horizontal levels based at the soil and apical node of the main or tallest stem. Notice the vertical distance for height does not include the leaves reaching above the apical meristem of the tallest branch. Photo credit: Luis Monserrate.
Citation: HortScience 58, 7; 10.21273/HORTSCI17093-23
Canopy height may be a more appropriate measurement for a plant population in field or greenhouse setting when considering the impacts of supplemental lighting of the crop in greenhouse settings or research on solar radiation and penetration in field settings and should be explicitly distinguished as a “canopy” metric. Plant height was also distinguished from “stem length” in that architecture of a hemp plant may not always include a main stem, especially if pruning or other structural damage is experienced. Tallest stem length may be important for plant physiology and phenotyping standards (Stansell and Osatuke 2021). In heavily pruned plants, the main stem may be challenging to define or may be missing entirely. The plant referenced for the workshop had been heavily pruned to just above the soil surface and allowed to regrow. As the main stem was pruned at the soil surface with regrowth of several main branches, the “main stem length” could then be defined as 0 cm with the regrown “main branch height” at 15 cm. Neither alternative height definition was satisfactory to the group, although they may be relevant to detailed investigation into plant architecture, management, and production.
Plant biomass, total branch length, and canopy volume were considered additional morphological features for a dataset standard with the expectation that these features correlate with plant height. Each of these features and their correlation may contribute to a deeper understanding of plant morphology and physiology. Pruning status should also be included in any description of plant morphology.
Flowering.
The critical research-relevant determination for “flowering time” or “date of flowering” was defined as a single point in time marking the visible initiation of inflorescence development corresponding to “terminal flowering” (Fig. 3). Most commonly, inflorescence development or the reduction of internode elongation, proliferation of axillary shoots, and formation of flower clusters, will be determined visually and reported as a point in time related either to some earlier plant or management milestone such as “days after planting,” especially in controlled environment cultivation, or by calendar date, because many cultivars flower in response to photoperiod. Flowering time is meant to represent a feature of phenology or the sequence of biological development in the plant (Kirchoff and Claßen-Bockhoff 2013; Sherry et al. 1993). Further plant assessment can then be defined as “days after terminal flower initiation” and a percentage of flowering plants in a population.
Visual key for determination of “flowering time,” for pistillate (or female) Cannabis sativa shown in this diagram as (A) “Not Flowering”—no visible florets or forked pistils, (B) “Solitary/Axillary Flowering—individual florets with forked pistil visible, (C) “Preterminal Flowering”—more than one floret produced but not a defined cluster, (D) “Terminal Flowering”—defined clusters of florets at shoot apex. A similar method may be used for staminate or monoecious hemp, although with slightly different visual cues. These phenological stages are reported as dates on first appearance. Photo credit: George Stack.
Citation: HortScience 58, 7; 10.21273/HORTSCI17093-23
Most hemp germplasm is photoperiod sensitive, meaning the plant will transition to flowering after a certain photoperiod or length of diurnal darkness is reached (Petit et al. 2020; Stack et al. 2021; Zhang et al. 2021). As such, it is crucial that lighting conditions throughout the plant’s growing period, whether in a field or a controlled environment, are thoroughly described as context for reported flowering data. Photoperiod sensitive hemp is also an indeterminate plant, so the earliest sign of physiological maturity provides an anchor of reference for the date at which the phenological stage is recorded. The earliest flower development corresponds to plant maturity, with later inflorescence development influencing the behavior and timing of pollen dispersal and flower viability (Carlson et al. 2021; Spitzer-Rimon et al. 2019). Date of flowering should be clearly distinguished from floral maturity, development, yield, and cannabinoid content. A visual key for female plants is provided to help standardize research efforts, along with establishing a standard nomenclature (Fig. 3).
An alternate definition for flower development was proposed by Mediavilla et al. (1998) as an estimation of individual indeterminate development at “50% flowering.” However, this is difficult to determine practically for research on cropping systems, especially when crop populations are not genetically uniform, are highly dimorphic in male vs. female flowering behavior, or are a mix of dioecious and monoecious expression. The workshop flowering activity question took the perspective of Mediavilla et al. (1998) when participants were asked to score flowering development of a single plant as a percent. Strikingly, most participants answered 100% to signify that the plant was flowering, but “flowering time” could not be determined because the sample plant was already well past its transition to terminal flowering. The few participants who provided an answer of less than 100% were attempting to score the feature of “floral maturity,” which would be 100% at full maturity related to trichome color and transparency. Visual analysis of trichome color and transparency is one of the most common methods used by hemp cultivators to assess harvest time. It was mentioned that harvest will likely occur before 100% floral maturity at a timing related to peak cannabinoid and terpene development. In contrast, a second plant was mostly scored 0% for flowering development. It showed “solitary flowering,” but solitary or axillary flowers may appear early in the season as a behavior of day neutral regulation and may remain unchanged for weeks before the beginning of terminal flower clusters (Fig. 3).
Although much of the group discussion was focused on describing inflorescence development in individual pistillate plants (Fig. 3), there was discussion about rating staminate plants, monoecious plants, and dioecious populations that have nonuniform flowering. Because of the strong sexual dimorphism between male and female flowers and inflorescences, a rating key complementary to Fig. 3 may be developed for male and monoecious plants. A key for male and monoecious plants will likely have similar physiological stages for flowering time but will express those stages with different visual morphologies. Also, hemp is often evaluated at the scale of a population, rather than an individual plant. Participants briefly discussed rating scales for populations and the common method suggested was a determination for the proportion of the population that had flowered with a threshold of population proportion (e.g., 50%) that would mark a date of flowering.
Flowering time is a critical phenological milestone used to estimate harvest timing. The relationship between flowering and harvest timing will be influenced by crop type, genetics, and environment. In controlled environments, flowering time may be manipulated with supplemental lighting, with some time lag between lighting transition and the appearance of inflorescences. Flower fertility duration is another feature to consider for seed crops. Once established in practical context, hemp biology is expected to progress consistently between vegetative and flowering growth for a given genetic and environment combination. Commercially, an expected harvest time may be established from flowering time or lighting transition, but diagnostic efforts may also be completed for further calibration of yield and quality development. A definition of floral maturity should be provided with respect to harvest time, regulatory compliance, and targeted market.
Crop quality.
Markets define crop quality in ways that affect research priorities and relevant features to quantify, especially in breeding programs. As a general research approach, hemp yields should be reported on a dry weight basis normalized to an agreed standard moisture content. A 10% to 12% moisture standard was identified for dried flower following legal regulations for estimation of yield and quality in some states and countries. An alternate research standard is to report a temperature and duration for drying to define dry yield (e.g., 55 °C for 72 h). Temperature and duration for drying standards must be appropriate for chemical analysis because forced air drying may affect the concentrations of secondary metabolites of the flower (i.e., cannabinoid and terpene profile; Wang et al. 2016). Further qualitative metrics of interest to the flower hemp industry are olfactory, visual, and textural quality, although standardization of these qualitative features will require coordination and training. Additional features for a minimum standard data set for flower production would be “basal stem diameter” and “average internode length” that may be related to the proposed crop quality metrics.
Extending the conversation of crop quality to a fiber crop, participants determined “dry straw” biomass as the most relevant standard measurement. A standardized “dry” weight would be determined similar to floral biomass by percent moisture or drying procedure, but “dry straw” should also include a standard, or at least reported, retting process. Along with the controlled stem decay of retting, straw quality could also be affected by harvest time and plant maturity. Fiber crops are harvested commercially between male and female preterminal flowering times. Decortication is the processing required to separate the outer bast fibers from inner hurd fibers in dry straw. Fiber quality standard features include initial decortication efficiency, ultimate decortication efficiency, and bast-to-hurd ratio. To determine initial decortication efficiency, the weight of bast removed following a single decortication event is compared with the weight of bast when completely removed from the stem (Wang et al. 2018). Ultimate decortication efficiency relates to maximal mechanical effort to decortication and the remaining hurd that would require manual separation. The bast-to-hurd ratio results from complete decortication and a comparison of weights between the fractions. Given the challenges of standardizing the retting and decortication process, bast-to-hurd ratios may represent a reliable indicator of fiber plant phenotype related to crop quality. Additional crop quality features for fiber might include fiber length, fiber diameter, tensile strength, and characteristics of material following processing.
Seed quality, size, maturity, oil percentage, oil composition, and protein content were briefly discussed as important features to report for a seed crop. Harvestable maturity was defined when ∼50% of seeds on a plant are “fully developed” for more than 50% of plants in the field. Recall that hemp develops in an indeterminate fashion, so seeds at the bottom of the inflorescence will be more developed than seeds at the top. Seed yield should be reported as clean and normalized to 8% moisture content with a measure of percent purity (Clark et al. 1963; Parihar et al. 2014).
The proposed crop quality standards represent initial features in plant production and yield for hemp, yet the end use should be a key consideration to define additional important features for crop quality. Some flower crops will be harvested for smokable flower, and others will be processed as total biomass. Thus, specific crop quality parameters will be measured and prioritized based on different intended uses, supply chains, or market outlets. Some fiber crops will be processed primarily for bast and others for hurd yields, whereas seed might be hulled or pressed. To jumpstart a viable industry, the most rapid adoption scenario is for a hemp product to replace a feedstock in an existing processing supply chain or value-added product line. Many industries define standard marketing volumes or weights (e.g., bushel) and engage a regulatory body (e.g., USDA Agricultural Marketing Service) in quality standards and testing, such as through the United States Cotton Standards Act of 1923 (7 U.S.C. 51-65) for cotton. Such marketing standards have not yet been established for the hemp industry and would greatly advance the ability for research and industry coordination.
Conclusions
The revitalization of hemp research and industry efforts has lacked standard definitions, descriptions, and procedures related to plant morphology, physiology, and crop quality. The ASHS Hemp 2022 workshop sought consensus from participants to define attributes such as plant height, flowering time, and crop quality. Efforts continue to expand the scope of methods standardization, although the features contained herein represent a first attempt at developing a minimum reporting standard for modern hemp production and processing data. Additionally, a coordinated effort to define marketing and quality standards is of critical importance to the development of the hemp industry. This requires a level of organization of the industry that has yet to be realized and could be led by the USDA if codified by federal law as it has been for well-established crops. In the meantime, hemp professionals are encouraged to adopt these proposed standards at a minimum and to detail reported procedures in methods and metadata. The authors propose continued review and development of hemp standards for research and industry applications to motivate collaboration and consistency during the current adoption and innovation phase for hemp production.
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