Read an interesting 3 year study on industrial hemp root zone conducted at the Clemson University Edisto Research and Education Center (EREC) near the town of Blackville, SC, in 2019, 2020, and 2021. Two of The Hemp Mine's cultivars, Luck and Sunset, were used in the study.
HortScience Volume 57, Issue 10 Author: Gilbert Miller
Abstract Delineating the depth and extent of the industrial hemp (Cannabis sativa L.) root zone assists with proper irrigation management and minimizes nutrient leaching. The objective of this 3-year field study was to measure root distribution and root length density of industrial hemp cultivars produced for cannabinoids grown under polyethylene mulch with drip irrigation. Root length density (RLD) was measured from 75-cm-deep soil cores obtained during vegetative growth, at early bloom, and at flower harvest. Cores were taken in-row and 15 cm to the side of each plant. In addition to root cores, the trench profile method was used during 2020 to observe in situ gross root architecture of a direct-seeded cannabinoid cultivar. RLD was significantly greater in the 0- to 30-cm soil depth and dropped dramatically below 30 cm; RLD was not significantly affected by cultivar. These findings suggest that the effective root zone depth for industrial hemp cultivars produced for cannabinoids is 0 to 30 cm and that the cultivars tested in this study do not differ in root system size or location.
Interest in the production of industrial hemp (Cannabis sativa L.) has increased dramatically in the United States since 2014. The Agricultural Act of 2014, Public Law 113–79 (the 2014 Farm Bill) reintroduced industrial hemp production in the United States through state pilot programs. The 2014 Farm Bill defined “industrial hemp” as “the plant Cannabis sativa L. and any part of such plant, whether growing or not, with a delta-9 tetrahydrocannabinol concentration of not more than 0.3 percent on a dry weight basis.” At this time, states with laws that allowed growth or cultivation of industrial hemp could create a pilot program or conduct research on the crop. The Agricultural Improvement Act of 2018, Public Law 115–334 (the 2018 Farm Bill) expanded production beyond the pilot programs. Under the pilot programs, U.S. industrial hemp acreage reported by states increased from zero in 2013 to more than 146,000 acres in 2019 (Mark et al., 2020). This trend has been driven by the burgeoning consumer demand for cannabinoid products (Jelliffe et al., 2020; Sterns, 2019). Based on U.S. Department of Agriculture (USDA) 2021 survey results, 39.5% of hemp grown in the open in the United States was dedicated to floral hemp production, 31.3% to fiber, 20.4% to grain, and 8.7% to seed (USDA, National Agriculture Statistics Service, 2022). Key cultivation techniques and recommended cultivars for the production of hemp grown for fiber and grain are provided in the literature, particularly for production in Europe, China, and Canada (Amaducci et al., 2015; Government of Alberta, 2020; Laate, 2012; Parvez et al., 2021). There is limited peer-reviewed, published literature providing recommended cultivars and production practices for cannabinoid cultivars (Caplan et al., 2019; Mark et al., 2020). Several state universities have issued guidelines for industrial hemp flower production. All emphasize that additional research is required before scientifically based production recommendations can be made. A University of Georgia Extension publication (Coolong, 2020) states that much of the acreage of hemp grown for floral material is being produced using polyethylene mulch and drip irrigation with early spring plantings on black polyethylene mulch and late spring plantings on white-on-black polyethylene mulch. Clear skies and high temperatures can lead to excessive heat buildup on the surface of soil under black polyethylene mulch. The high heat can damage young plants and reduce germination of seeded crops. White-on-black polyethylene mulches can lower bed temperatures significantly and improve transplant survival and increase germination and survival of seeded crops. Although the exact date to switch from black polyethylene mulch to white-on-black polyethylene mulch can vary from year to year due to seasonal environmental conditions, a rule of thumb has been to switch to white mulch in the middle of June (Johnson, 2021). Although adequate research has not been conducted to determine optimal fertilizer rates for hemp flower production, empirical evidence from farms in the southeastern United States suggested that ∼100 to 150 pounds per acre (112–168 kg·ha−1) nitrogen should be sufficient with 50 pounds per acre (56 kg·ha−1) nitrogen applied preplant and 50 to 80 pounds per acre (56–90 kg·ha−1) applied through fertigation (Coolong, 2020). Mylayarapu et al. (2020), state that as much as 200 pounds per acre (224 kg·ha−1) nitrogen may be required for hemp flower production in Florida with 30 pounds per acre (34 kg·ha−1) nitrogen applied preplant. It is also recommended that hemp flower production is best accomplished on raised beds, and fertigation is a preferred method of nutrient application when grown on polyethylene mulch beds. Hemp flower production plant spacing guidelines vary with plant populations ranging from 800 to 5000 plants per acre (1976–12,350 per ha). The plant population per acre will be determined by inter- and intrarow spacing. Plants for hemp flower production can be grown as dense as 3-foot (0.9 m) interrow and 3-foot (0.9 m) intrarow spacing or 8-foot (2.4 m) interrow and 4-foot (1.2 m) intrarow spacing (How to Grow Hemp in Florida, 2022). A Missouri hemp publication (Horner, 2019) suggests plant populations for hemp flower production ranging from 1500 to 4000 plants per acre (3705–9880 per ha). Plants are on a wide inter- and intrarow spacing that ranges from 3 feet (0.9 m) to 5 feet (1.5 m). For optimal bud production, the University of Vermont Extension suggests a 4- to 6-foot (1.2–1.8-m) spacing of plants within rows with 800 to 1800 plants per acre (1976–4446 per ha) (University of Vermont Extension, 2020). Cultivars suitable for cannabinoid production are generally shorter, bushier plants, whereas cultivars suitable for fiber use are tall plants (Mark et al., 2020). Cultivars grown for fiber-only are cultivated differently from cultivars grown for grain or dual-purpose use. Likewise, fiber and/or grain cultivars are cultivated differently from those grown for cannabinoids (University of Kentucky Center for Crop Diversification, 2015). The focus of plant breeding programs over the past 50 years focused on hemp fiber, seed, or grain production cultivars. A cultivar suitable for a specific environment and end-use application and scientific-based production practices are critical to successful hemp cultivation (Amaducci et al., 2015). Plant roots play a vital role in the acquisition of belowground resources, yet we have a limited understanding of how they function in natural soil environments (Smit et al., 2000). Plant growth, in all but a few natural ecosystems, is limited by water and nutrient availability. Soil resources are unevenly distributed, so the spatial distribution of the root system, the root architecture of a plant, will determine the ability of the plant to exploit soil resources (Lynch, 1995). Major agricultural investments in irrigation and fertilization provide a beneficial environment for growth and development of crop roots (Waisel et al., 2002). Fine roots are the main components of the root system through which plants absorb water and nutrients (De Silva et al., 1999). Although the diameter defining fine root varies with plant species, roots of <1 mm diameter are generally considered to be fine roots (De Silva et al., 1999; Wells et al., 2002). Limited information has been published on the root characteristics of industrial hemp and little published literature on the root architecture of cannabinoid cultivars. Although specific data quantifying the extent and depth of the hemp root system are limited, two authors stated that hemp roots grow deep into the soil, 90 to 200 cm (Chabbert et al., 2013; Citterio et al., 2003). Most, if not all, of these references refer to hemp cultivars grown for fiber. One field trial, using contrasting plant densities, did provide specific data quantifying the extent and depth of the hemp fiber variety Futura 75 (Amaducci et al., 2008). In this trial, roots were found to 200 cm soil depth, but the RLD was greatest in the first 10 cm of soil. Both agricultural and nonagricultural water use is increasing globally, as are concerns about groundwater contamination from agricultural leachate (Clothier and Green, 1994; FAO, 2017). Appropriate irrigation application and the reduction of root zone drainage must be based on accurate delineation of the extent and depth of crop root zones. The effective root zone (ERZ) depth is the depth of soil used by the main body of the root system for water and nutrient uptake under proper irrigation. Application of irrigation water should be limited to an amount that will penetrate only to the ERZ (Ross and Hardy, 1997). The objective of this research was to estimate the ERZ for cannabinoid cultivars grown under polyethylene mulch with drip irrigation. Materials and Methods Research site and cultural practices. This research was conducted at the Clemson University Edisto Research and Education Center (EREC) near the town of Blackville, SC, in 2019, 2020, and 2021. The 10-ha field was located at 33°21′N latitude and 81°19′W longitude at 93 m above mean sea level. Each year studies were conducted within the 10-ha field at sites where hemp had not been previously grown. The soil for each study was a Barnwell loamy sand with the following taxonomic class characteristics: well drained, fine-loamy, kaolinitic, and thermic Typic Kanapludults. The USDA-Natural Resources Conservation Service (NRCS) soil maps indicate that this soil type has a field capacity (FC) of 0.174 (cm3·cm3) and a permanent wilting point of 0.061 (cm3·cm3) for the top 30 cm of soil with an available water capacity (AWC) of ∼0.113 (cm3·cm3) (USDA, Natural Resources Conservation Service, 2021). In situ AWC tests using the procedures described in Starr and Paltineanu (1998) yielded values similar to the above NRCS estimates (Miller et al., 2013). Although the soil taxonomic classification for each study was a Barnwell loamy sand, soil texture determinations showed slight differences in the studies of 2019, 2020, and 2021. The soil in 2019 and 2020 showed sand prevailing to a depth of 45 cm. Sand and clay content, respectively, was on average 77.8% and 15.9% in the top 30 cm and 50% and 45% from 30 to 75 cm. The 2021 soil texture determinations showed sand prevailing the entire 75-cm profile. Sand content was on average 80.4% in the top 30 cm and 64.7% from 30- to 75-cm soil depth. Clay content was ∼18% in the top 30 cm in 2021 and 34% from 30- to 75-cm soil depth. Abruzzi rye (Secale cereal L.) mixed with crimson clover (Trifolium incarnatum L.) were planted each year in mid-November. Before winter cover crop establishment, the field was tilled with bent-leg shanks (WorkSaver’s Terra Max, Inc., Litchfield, IL) to a depth of 45 to 50 cm. Each year the winter cover crop was plowed down in mid-January. During early March, preplant fertilizer was applied: 40 kg·ha−1 of nitrogen (N), phosphorous (P), and potassium (K). Each year, either black or white polyethylene mulch film (0.019 mm thick, 154.4 cm wide; Guardian AgroPlastics, Tampa, FL) and drip irrigation tubing (Aqua-Traxx®; Toro Ag Irrigation, El Cajon, CA) were laid 1 week after preplant fertilizer application and ∼30 d before transplanting. The width of the raised beds covered by polyethylene mulch was ∼0.76 m, bed center height ∼15 cm, bed shoulder height ∼10 cm. The drip tape had an emitter spacing of 0.3 m with a flow rate of 1.14 lph per emitter at 69 kPa and was applied at ∼4 cm soil depth. There were some differences in cultural practices and planting dates each year. During 2019 there were two planting dates: 15 Apr and 25 Jun. The 15 Apr planting was on black polyethylene mulch film and the 25 Jun planting was on white-on-black polyethylene mulch film. In-row plant spacing for both 2019 plantings was ∼3 m. The distance between rows was 2.4 m for the 15 Apr and 25 Jun planting. There was one planting date for 2020, 11 May. The 11 May 2020 planting was on black polyethylene mulch film, 3.6 m in-row plant spacing and 2.4 m between rows. The 11 May 2021 planting was on black polyethylene mulch film, 3.6 m in-row plant spacing and 1.8 m between rows. The in-row plant spacing in each study exceeded the in-row spacing guidelines published (Horner, 2019; How to Grow Hemp in Florida, 2022; University of Vermont Extension, 2020). This was done to capture the root metrics for a single plant and reduce, if not eliminate, root intrusion from other hemp plants. Details for each study are included in Table 1. Table 1. Study details. Irrigation and fertigation are total amounts from transplant to root core date.
All cultivars tested in each study are included in Table 2. All cannabinoid-type hemp plants were propagated in a greenhouse at the Clemson University EREC. Cuttings were taken from “mother plants,” dipped in 0.1% Indole-3-butyric Acid (Take Root®; Schultz Company, Bridgeton, MO), and planted into a soilless mix, Pro-Mix FLX (Premier Tech Horticulture, Quakertown, PA). All cuttings were grown in 72-star cells, with a depth of 7.62 cm and top width of 3.96 cm (Greenhouse Megastore, Danville, IL). An automated-mist irrigation system provided water for a duration of 1 min, hourly from 0700 to 1100 HR with greenhouse relative humidity maintained above 90%. Greenhouse heating and cooling was programed to heat when the greenhouse temperature dropped below 18.3 °C and cool when the temperatures exceeded 26.6 °C. Clones were grown under AgroBrite T5 Fluorescent Grow Lights (Hydrofarm East, Fairless Hills, PA) with lights providing up to 20,000 lumens and an 18-h photoperiod from 0600 to 2400 HR. Duration from cutting to field planting was ∼36 d. Cannabinoid-type hemp cultivars included Cannabis sativa L. Cherry Citrus, Cherry Blossom, Cherry Wine, BaOx, Therapy, and T1, which were acquired from South Carolina Department of Agriculture–certified hemp producers. Two cultivars, Sunset and Luck, were obtained from The Hemp Mine, Fair Play, SC. Seed for the cultivar Suver Haze was provided by Oregon CBD (Independence, OR) and was direct seeded in the field in Studies 3, 4, and 5.
Each year, all plots received similar programed daily irrigation. The goal was to provide ∼25.4 mm water per week. The daily drip irrigation program specified three 52-min irrigation cycles with each cycle delivering ∼1.21 mm water. In 2020 and 2021, each plot contained two Sentek TriSCAN EasyAg 50-cm soil water capacitance probes (Sentek PTY, Ltd., Kent Town, South Australia). Probes were located immediately adjacent to the drip irrigation tape, one at the drip tape emitter and one between the 30-cm spaced emitters. The volumetric moisture content (VMC) was recorded at 15-min intervals at 10, 20, 30, 40, and 50-cm depths. Following excessive rain events, if VMC exceeded FC in the top 30-cm soil depth, irrigation was temporarily reduced. The goal was to maintain VMC in the top 30-cm soil depth near but not exceed FC. Total amounts of irrigation, applied from transplant or seeding to root core date, for each study are included in Table 1.
During early March, preplant fertilizer was applied: 40 kg·ha−1 of nitrogen (N), phosphorous (P), and potassium (K). Fertigation began on the day of transplanting. Fertigation rates (N and K) per day were low initially, 0.56 kg·ha−1, gradually increased through the growing season, 1.12 to 1.68 kg·ha−1, and reached a maximum, 2.24 to 2.8 kg·ha−1. Total amounts of fertigation nutrients, applied from transplant or seeding to root core date, for each study are included in Table 1.
Root core sampling.
Using the decimal code for growth stages of hemp provided by Mediavilla et al. (1998), root cores were acquired at 1014 (vegetative growth), 2200 (early bloom), and 2202 (flower harvest). Root core samples were collected at ∼4 weeks after planting in 2019 and 2021, at early bloom in 2020 and 2021, and at floral biomass harvest in 2019. The direct-seeded Suver Haze initiated bloom earlier than the other cultivars tested in 2020 and consequently root cores for Suver Haze were taken 15 d earlier (Table 1). Although Suver Haze initiated bloom earlier than other cultivars tested in Study 3, root core data for all tested cultivars were combined to compare RLD and distribution at the early bloom growth stage.
A truck-mounted hydraulic soil sampling and coring machine (Model HDGSRPS; Giddings Machine Co., Windsor, CO) was used to take cores to a depth of 75 cm. Plastic tube liners of 4.45 cm in diameter and 91.44 cm in length were inserted in the metal coring tube. Core sample locations were positioned in the center of the row, 15 cm to the side of each plant. Hemp plants were harvested immediately before root core sampling, to allow for the truck-mounted coring machine to straddle the row and acquire the sample.
Each core was frozen to −4 to −2 °C, cut into five 15-cm segments, bagged, and maintained at −4 to −2 °C before processing. Roots were separated from soil by handwashing using a U.S. Standard Test Sieve (No. 18, 1 mm, 0.14 inches) following the method of Smit et al. (2000). Root samples were refrigerated at 8 °C in polypropylene vials with 60 mL by volume of 50% ethanol until analysis. Root measurements were performed with WinRHIZO Pro 2009 software (Regent Instruments Inc., Quebec City, Quebec, Canada) and an Epson STD4800 color scanner (Epson America, Inc., Long Beach, CA). Roots were spread in a 20 × 25-cm plastic tray containing a 2- to 3-mm-deep layer of water and scanned at a 400-dpi resolution with the TWAIN (Epson America, Inc.) driver interface active. Morphological information acquired from each image included total root length (cm), RLD (cm·cm3 of soil), surface area (cm2), root diameter (mm) at 0.25 mm increments up to 2.25 mm, and average root diameter (mm).
Before trench excavation, penetrometer readings were performed at each plant location using a Dickey-John Soil Compaction Tester (Dickey-John, Auburn, IL) with a 1.9-cm tip. Penetrometer readings were recorded in pounds per square inch (PSI). Sentek TriSCAN EasyAg 50-cm soil water capacitance probes provided VMC amounts at time of penetrometer readings. A rectangular trench ∼2 m in length, 1.5 m wide, and 1.5 m depth was dug ∼0.3 m in horizontal distance from the hemp plant. Excavations were performed at three plant locations during 2020, 93 d after planting (DAP). Soil was removed using a water jet spray. This procedure was conducted to observe the gross root anatomy and determine taproot depth and major lateral root formation in ‘Suver Haze’, which was direct seeded.
Experimental design and data analysis.
This experiment used a randomized complete block design for all studies. All studies contained four replications of each industrial hemp cannabinoid cultivar. Each block contained plants designated for root core sampling and destructive harvest at vegetative, early bloom, or flower harvest. Measurements at each growth stage included RLD, average root diameter, and root surface area. All data were analyzed by analysis of variance with PROC MIXED and PROC GLIMMIX (SAS Institute Inc., Cary, NC), and means were separated with Fisher’s least significant difference.
Using <1 mm as the threshold diameter for defining fine roots, the bulk of the roots measured in this research would be considered fine roots (De Silva et al., 1999; Wells et al., 2002) (Table 3).
Fraction total root length <1 mm in diameter (all sample depths pooled) by cultivar by year and study number. P value of cultivar effects on % <1 mm.
RLD and soil depth.
There were no significant differences among cultivars in mean RLD (P > 0.05) in any study (Table 4). Although not significantly different from other cultivars tested in Study 3, early bloom growth stage, Sunset had the greatest mean RLD. ‘Cherry Wine’ had the least mean RLD occurring in Study 1, vegetative growth stage (Table 4).
Mean root length density (cm root·cm3 soil), all depths combined by cultivar. P value of cultivar effects on mean root length density.
The interaction of cultivar by depth was limited, with significant differences among cultivars occurring only at the 30- to 45-cm soil depth in Study 3 (P = 0.0091) (Table 5) and at the 30- to 45-cm soil depth in Study 4 (P = 0.0438) (Table 5). In Study 3, cores taken at early bloom growth stage, Luck, compared with other cultivars, had a greater RLD at the 30- to 45-cm soil depth. BaOx, in Study 4, cores taken at vegetative growth stage, had a greater RLD at the 30- to 45-cm soil depth compared with other cultivars. With all studies and all cultivars, RLD dropped precipitously at the 30- to 45-cm soil depth.
Mean root length density (cm·cm3) for cultivars by depth in studies 1 to 5.
RLD decreased with depth (P < 0.05) in all studies for all cultivars, with one exception (Table 6). In Study 1, vegetative growth stage, ‘Cherry Wine’ had similar RLD at 0- to 15-cm and 15- to 30-cm soil depths. With this exception noted, RLD measurements were greatest in the 0- to 15-cm soil depth for all studies and for all cultivars (Table 6). Hemp roots were found in the 60- to 75-cm soil depth in all studies, including the earliest sampling dates, Study 1 and 4, vegetative growth stage. Compared with the flower harvest growth stage, Study 2, and the early bloom growth stage, Studies 3 and 5, the vegetative growth stage had a greater percentage of total root length below the 30-cm soil depth. Although roots did extend as deeply as 75 cm in all studies, RLDs ranging from 99.86% to 77.04% were found within the top 30 cm of soil (Table 7). The distinct differences in RLD by depth were significant during vegetative, early bloom, and flower harvest growth stages.
Root length density (cm·cm3) by soil depth for cultivars in studies 1 to 5.
Percent total root length density (cm·cm3) by soil depth for cultivars in studies 1 to 5.
In Studies 3, 4, and 5, there were no significant differences among cultivars on mean RLD (P > 0.05) where Suver Haze was direct seeded, and other cultivars were vegetative clones (Table 4). In Study 3 and Study 5, both early bloom growth stage, 99.86% and 94.65%, respectively, of the total root length for ‘Suver Haze’ were found in the top 30-cm soil depth. Also, in Study 4, vegetative growth stage, 90.06% of the total root length of ‘Suver Haze’ was found in the top 30-cm soil depth (Table 7). Like vegetative clone cultivars, Suver Haze, although direct seeded, exhibited a propensity to establish the majority of the total root length in the top 30-cm soil depth.
The taproot on each of the three direct-seeded ‘Suver Haze’ plants tested extended to a depth of 13, 10, and 20.3 cm, respectively. The taproot ended at this depth and ∼6 to 8 major root laterals of 1.3 cm in diameter developed. At close to ground level, each plant had ∼10 or more laterals ∼0.6 cm in diameter. Penetrometer readings at each respective maximum taproot depth was 25 PSI and showed no significant soil resistance (>250 PSI) until the 40 cm soil depth was reached. Volumetric moisture content in the 0- to 30-cm soil depth was slightly above FC at the time of penetrometer recordings.
Cannabinoid industrial hemp cultivars showed marked, consistent reduction in RLD below the 30-cm soil depth in this study. The drop in RLD below the 30-cm soil depth for all cultivars was dramatic each year and on all sampling dates. As the DAP increased the % RLD below the 30-cm soil depth decreased. In Studies 1 and 4, DAPs 36 and 31, respectively, % RLD below the 30-cm soil depth ranged from 6.9% to 22.96% RLD. At 85 DAP in Study 5, % RLD below the 30-cm soil depth ranged from 1.9% to 5.35%. The % RLD below the 30-cm soil depth at 108 DAP in Study 2, was comparable to Study 5, ranging from 1.09% to 8.71% RLD. These results indicate that the ERZ for cannabinoid industrial hemp grown on polyethylene mulch with drip irrigation is found in the 0- to 30-cm soil depth.
The trench profile results reinforced the root core results with numerous laterals developing close to ground level and no significant taproot extending below 20 cm with major root laterals emerging at this point. This result is consistent with other studies of crops grown on polyethylene mulch with drip irrigation (Machado and Oliveira, 2005; Miller et al., 2013; Oliveira Md and Calado, 1996).
The RLD measured in these studies (Table 6) significantly exceeded RLD measured in other studies at comparable DAPs. NeSmith (1999) studied root distribution for transplanted vs. direct-seeded watermelon and measured the greatest RLD of 0.4 cm·cm3 at the 10-cm soil depth at 77 DAP. Miller et al. (2013) determined an RLD of 1.82 cm·cm3 in the 0- to 15-cm soil depth for watermelon plants at 83 DAP. Using the industrial hemp fiber cultivar Futura 75, Amaducci et al. (2008) measured the greatest RLD, 4.6 cm·cm3, at the 15-cm soil depth, 114 DAP. In these studies, RLD was the greatest in the 0- to 15-cm soil depth and measurements ranged from 8.2 to 3.8 cm·cm3 at 85 DAP, 8.6 to 4.3 cm·cm3 at 86 DAP, and 7.5 to 4.8 cm·cm3, 108 DAP. These data indicate the hemp cultivars grown in these studies under the reported growing conditions produce a robust but relatively shallow root system.
Plants’ reliance on surface roots for soil water extraction has been well documented. Gardner (1983) pooled a large number of water extraction patterns for various crops and showed that water extraction dropped off substantially away from the surface. A crop root system growing in a uniform soil will tend to follow genetic patterns early in its growth and development (Coelho and Or, 1999). As the roots experience different soil environmental conditions, the growth and development patterns change in response to the soil conditions. Soil strength, water availability, aeration, nutrient supply, and other soil characteristics can affect the resultant root system architecture and root activity (Coelho and Or, 1999).
We expected to observe differences in RLD and root distribution between cannabinoid cultivars and expected to measure a greater percentage of total RLD below the 30-cm soil depth. We also expected the direct-seeded cannabinoid cultivar Suver Haze, compared with the cloned cannabinoid cultivars, to develop a deeper root system and have greater RLD at the lower soil depths. Nonetheless, there were no significant differences in RLD among the cannabinoid cultivars in this study and there were no significant differences in RLD among direct-seeded and cloned cannabinoid cultivars. In all studies, there was a dramatic drop in RLD below the 30-cm soil depth with as much as 99.86% and as little as 77.04% of total RLD found within the top 30 cm of soil depth. Irrigation management should endeavor to maintain water and nutrients in the 0- to 30-cm soil profile for industrial hemp cannabinoid cultivars grown on polyethylene mulch with drip irrigation.
Amaducci, S., Zatta, A., Raffanini, M. & Venturi, G. 2008 Characterisation of hemp (Cannabis sativa L.) roots under different growing conditions Plant Soil 313 227 235 https://doi.org/10.1007/s11104-008-9695-0
Amaducci, S., Scordia, D., Liu, F.H., Zhang, Q., Guh, H., Testa, G. & Cosentino, S.L. 2015 Key cultivation techniques for hemp in Europe and China Ind. Crops Prod. 68 2 16 https://doi.org/10.1016/j.indcrop.2014.06.041
Caplan, D., Dixon, M. & Zheng, Y. 2019 Increasing inflorescence dry weight and cannabinoid content in medical cannabis using controlled drought stress HortScience 54 5 964 969 https://doi.org/10.21273/HORTSCI13510-18
Chabbert, B., Kurek, B. & Beherec, O. 2013 Physiology and botany of industrial hemp 27 47 Bouloc, P, Allegret, S & Arnaud, L Hemp: Industrial production and uses. CAB International Oxfordshire
Citterio, S., Santagostino, A., Fumagalli, P., Prato, N., Ranalli, P. & Sgorbati, S. 2003 Heavy metal tolerance and accumulation of Cd, Cr and Ni by Cannabis sativa L Plant Soil 256 243 252 https://doi.org/10.1023/A:1026113905129
Clothier, B.E. & Green, S.R. 1994 Rootzone processes and the efficient use of irrigation water Agr. Water Mgt. 25 1 12 https://doi.org/10.1016/0378-3774(94)90048-5
Coelho, E.G. & Or, D. 1999 Root distribution and water uptake patterns of corn under surface and subsurface drip irrigation Plant Soil 206 123 136 https://doi.org/10.1023/A:1004325219804
Coolong, T 2020 A preview of industrial hemp for flower production in Georgia Bulletin 1530 University of Georgia Extension. https://extension.uga.edu/publications/detail.html?number=b1530. [accessed 23 Mar 2022]
De Silva, H.N., Hall, A.J., Tustin, D.S. & Gandar, P.W. 1999 Analysis of distribution of root length density of apple trees on different dwarfing rootstocks Ann. Bot. 83 335 345
FAO 2017 Water pollution from agriculture: A global review Rome, Food and Agriculture Organization of the United Nations (FAO). https://www.fao.org/3/i7754e/i7754e.pdf. [accessed 16 Jul 2022]
Gardner, W.R 1983 Soil properties and efficient water use: An overview 45 65 Taylor, HM, Jordan, WR & Sinclair, TR Limitations to efficient water use in crop production. Am. Soc. Agron. Madison, WI
Government of Alberta 2020 Growing hemp in Alberta https://open.alberta.ca/dataset/033de9fb-ab1c-4018-940e-4143f4caec85/resource/9babce43-b6f6-422c-85cd-283ea1a56147/download/af-growing-hemp-in-alberta-2020-06.pdf. [accessed 6 Dec 2020]
Horner, J 2019 Missouri industrial hemp production https://mospace.umsystem.edu/xmlui/bitstream/handle/10355/83993/MX0073.pdf?sequence=1&isAllowed=y. [accessed 23 Mar 2022]
How to grow hemp in Florida A farmer’s guide 2022 https://greenpointresearch.com/how-to-grow-hemp-in-florida-a-farmers-guide/#:∼:text=Hemp%20is%20best%20grown%20on,aka%20boggy%2C%20wet%20soils. [accessed 23 Mar 2022]
Jelliffe, J., Lopez, R.A. & Ghimire, D. 2020 CBD hemp production costs and returns for Connecticut farmers in 2020 Zwick Center Outreach Report No. 66, University of Connecticut. http://www.zwickcenter.uconn.edu. [accessed 6 Dec 2020]
Johnson, G 2021 When to switch from black to white plastic mulch University of Delaware. https://sites.udel.edu/weeklycropupdate/?p=18243. [accessed 19 Jun 2022]
Laate, E.A 2012 Industrial hemp production in Canada https://open.alberta.ca/dataset/e28d1f22-2b27-4359-84e6-7c0c5f4f1c96/resource/00e5b6b3-28d1-4619-87cc-fb6d5f062142/download/ard-industrial-hemp-production-canada-2012.pdf. [accessed 6 Dec 2020]
Lynch, J 1995 Root architecture and plant productivity Plant Physiol. 109 7 13 https://doi.org/10.1104/pp.109.1.7
Machado, R.M.A. & Oliveira, M.D.G. 2005 Tomato root distribution, yield and fruit quality under different subsurface drip irrigation regimes and depths Irrig. Sci. 24 15 24
Mark, T, Shepherd, J, Olson, D, Snell, W, Proper, S & Thornsbury, S Feb. 2020 Economic viability of industrial hemp in the United States: A review of state pilot programs, EIB-217 U.S. Department of Agriculture, Economic Research Service
Mediavilla, V, Jonquera, M, Schmid-Slembrouck, I & Soldati, A 1998 Decimal code for growth stages of hemp (Cannabis sativa L.) J. Int. Hemp Assoc. 5 2 65 68–74
Miller, G., Khalilian, A., Adelberg, J.W., Farahani, H.J., Hassell, R.L. & Wells, C.E. 2013 Grafted watermelon root length density and distribution under different soil moisture treatments HortScience 48 1021 1026 https://doi.org/10.21273/HORTSCI.48.8.1021
Mylayarapu, R., Brym, Z., Monserrate, L. & Mulvaney, M.J. 2020 Hemp fertilization: Current knowledge, gaps and efforts in Florida: A 2020 report SL476. Gainesville: University of Florida Department of Soil and Water Sciences. https://edis.ifas.ufl.edu/publication/SS689. [accessed 23 Mar 2022]
NeSmith, D.S. 1999 Root distribution and yield of direct seeded and transplanted watermelon J. Amer. Soc. Hort. Sci. 124 5 458 461 https://doi.org/10.21273/JASHS.124.5.458
Oliveira Md, R.G. & Calado, A.M. 1996 Tomato root distribution under drip irrigation J. Amer. Soc. Hort. Sci. 121 644 648 https://doi.org/10.21273/JASHS.121.4.644
Parvez, A.M., Lewis, J.D. & Afzal, M.T. 2021 Potential of industrial hemp (Cannabis sativa l.) for bioenergy production in Canada: Status, challenges, and outlook Renew. Sustain. Energy Rev. 141 5-6 110784 https://doi.org/10.1016/j.rser.2021.110784
Ross, E.A. & Hardy, L.A. 1997 National engineering handbook; Irrigation guide USDA Beltsville, MD
Smit, A.L., Bengough, A.G., Engels, C., Van Noordwijk, M., Pellerin, S. & van de Geijn, S.C. 2000 Root methods Springer New York, NY
Starr, J.L. & Paltineanu, I.C. 1998 Real-time soil water dynamics over large areas using multisensory capacitance probes and monitoring system Soil Tillage Res. 47 43 49 https://doi.org/10.2136/sssaj1997.03615995 006100060006x
Sterns, J 2019 Is the emerging U.S. hemp industry yet another boom-bust market for U.S. Farmers? Choices. https://www.choicesmagazine.org/choices-magazine/submitted-articles/is-the-emerging-us-hemp-industry-yet-another-boombust-market-for-us-farmers. [accessed 6 Dec 2020]
University of Kentucky Center for Crop Diversification Industrial hemp production Sept. 2015 http://www.uky.edu/ccd/sites/www.uky.edu.ccd/files/hempproduction.pdf. [accessed 6 Dec 2020]
University of Vermont Extension Northwest Crops & Soil Program Industrial hemp for flower production A guide to basic production techniques 2020 https://www.uvm.edu/sites/default/files/Northwest-Crops-and-Soils-Program/Articles_and_Factsheets/2020_Hemp_101.pdf. [accessed 23 Mar 2022]
U.S. Department of Agriculture, Natural Resources Conservation Service 2021 Web soil survey https://websoilsurvey.nrcs.usda.gov/app/WebSoilSurvey.aspx. [accessed 19 Jun 2022]
U.S. Department of Agriculture, National Agricultural Statistics Service 2022 National hemp report https://release.nass.usda.gov/reports/hempan22.pdf. [accessed 19 Aug 2022]
Waisel, Y., Amram, E. & Kafkafi, U. 2002 Plant roots Marcel Dekker New York
Wells, C.E., Glenn, D.M. & Eissenstat, D.M. 2002 Soil insects alter fine root demography in peach (Prunus persica) Plant Cell Environ. 25 431 439 https://doi.org/10.1046/j.1365-3040.2002.00793.x