Increasing Yield and Carbon Sequestration of Hemp Production with Understory Companion Crops

Progress report for ONE19-329

Project Type: Partnership
Funds awarded in 2019: $20,224.00
Projected End Date: 05/31/2021
Grant Recipient: University of Vermont
Region: Northeast
State: Vermont
Project Leader:
Dr. Eric Bishop-von Wettberg
University of Vermont
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Project Information

Project Objectives:

Our objective is to examine different methods of ground management in hemp farming. Our experimental planting is designed to determine if procumbent forage crops like white and red clover or fenugreek can suppress weeds, provide soil nitrogen, sequester carbon and reduce erosion, at a price comparable to black plastic. By calculating input costs, we are modelling the expected monetary and environmental costs of different cultivation approaches to provide information for the growing number of hemp farmers in the Northeast.

We are also recording carbon capture data on the plots so that we will be able to identify and quantify the varying rates of carbon sequestered under each treatment regime. There is growing national and international interest in carbon sequestration, and hemp is among the most effective annual crops at sequestering carbon (Finnan and Doyle, 2013). If coupled with low input production approaches such as co-planting with forage legumes, carbon offset credits from existing carbon markets such as those in California or Quebec, or new ones in the US, are possible.


The acreage devoted to hemp (Cannabis sativa.) has increased rapidly in Vermont, with 2,000 acres in production in 2018 and 3,000 acres registered in 2019 (as of March 22, 2019). There is no standard method for growing hemp and only recently has research on potential management practices begun. Practices for growing fiber or grain hemp or recreational cannabis are poorly suited for CBD hemp.  Fiber and grain production practices are not attuned to quality production of CBD oils.  Cannabis production approaches, developed with the unique legal situation that has affected it in the past, are focused on indoor production.  Because field-scale production of CBD-rich hemp is now legal, more information is needed on the most sustainable practices.  Many farmers are growing thousands of plants using plastic mulch, which rips, tears, and a portion of which is ultimately left behind in a field. This plastic mulch has environmental consequences as well as a direct cost to the farmer. The primary reason for using such plastic mulch is that it is a widely held belief that to grow on a large-scale, plastic mulch is required to smother weed growth. This use of plastic mulch is usually paired with tillage. We hypothesize that strip tilling and companion cropping is more economical by means of reduced labor, greater water retention, effective weed suppression, and maintaining/ increasing soil health when compared to total field tillage and plasticulture.

Much of the existing agronomic literature in the Northeast on CBD hemp has been performed by Heather Darby at UVM Extension (e.g., Darby et al, 2017 a-f), who will advise on this project. Her work has examined planting date, seeding rate, variety selection, weed control options, and some of the different agronomic challenges under a limited range of production approaches. Many small growers have employed specialty crop production techniques for CBD hemp, such as high tunnels and black plastic groundcovers (Darby 2017g). As producers move towards larger scale planting, production means will have to shift away from high tunnels with transplants to direct seeded fields.  At small scales most producers opt for black plastic, which suppresses weeds but may remain in fields if it rips and may not be optimal for soil health. Forage legumes have the capacity to suppress weeds while providing organic matter and fixing atmospheric nitrogen to the soil. Costs will be an important consideration for different production approaches, as will nutrient management consequences, soil protection, and potential carbon credits.

This project aims to build and add on data on yield factors for industrial hemp. Our data on soil nutrient levels, weed management, carbon sequestration and yield variance from various mulches/companion crops will add to the body of knowledge that farmers can draw from, helping improve efficiency and sustainably. Hemp production methods on the thousands of acres, in Vermont and worldwide, could be affected by this research. With expanding production, the potential soil health benefits could be immense. This work will also provide production alternatives as growers develop nutrient management plans.

Furthermore, carbon offset credits are already being developed by leaders in more than 50 states, provinces or countries (including California).  We expect these opportunities to grow as the impacts of climate change continue to increase.  Hemp production could become more profitable if eligible for carbon credits. Information on carbon sequestration under different management approaches is critical for developing these markets and accurately quantifying farm-level credits.


Darby, H., Gupta, A., Cummings, E., Cubins, J., Emick, H., Post, J., Ruhl, L. and Ziegler, S., 2016. Industrial Hemp Weed Control Trial.

Darby, H., Gupta, A., Cummings, E., Ruhl, L. and Ziegler, S., 2017. Industrial Cannabidiol Hemp Report. Darby, H., Gupta, A., Cummings, E., Ruhl, L. and Ziegler, S., 2017. Industrial Hemp Fiber Variety Trial.

Darby, H., Gupta, A., Cummings, E., Ruhl, L. and Ziegler, S., 2017. Industrial Grain Hemp Planting Date Trial. Darby, H., Gupta, A., Cummings, E., Ruhl, L. and Ziegler, S., 2017. Industrial Grain Hemp Variety Trial.

Darby, H., Gupta, A., Cubins, J. and Ruhl, L., 2016. Industrial Hemp Seeding Rate Trial.

Darby, H., Gupta, A., Cummings, E., Ruhl, L. and Ziegler, S., 2017. Industrial Grain Hemp Planting Date Trial. Darby, H., Gupta, A., Ruhl, L., Cummings, E. and Ziegler, S., 2017. Industrial Hemp Fiber Planting Date Trial. Darby, H., Gupta, A., Cubins, J., Ruhl, L. and Ziegler, S., 2016. Industrial Hemp Planting Date X Variety Trial. Gupta, A., Cubins, J., Ruhl, L. and Ziegler, S., 2017. 2016 Industrial Hemp Seeding Rate Trial.

Finnan, J. and Styles, D., 2013. Hemp: a more sustainable annual energy crop for climate and energy policy. Energy Policy, 58, pp.152-162.

Keith, H. and Wong, S.C., 2006. Measurement of soil CO2 efflux using soda lime absorption: both quantitative and reliable. Soil Biology and Biochemistry, 38(5), pp.1121-1131.

Kuhlgert, S., Austic, G., Zegarac, R., Osei-Bonsu, I., Hoh, D., Chilvers, M.I., Roth, M.G., Bi, K., TerAvest, D., Weebadde, P. and Kramer, D.M., 2016. MultispeQ Beta: a tool for large-scale plant phenotyping connected to the open PhotosynQ network. Royal Society open science, 3(10), p.160592.

Qian, P., and J. J. Schoenau. 1995. Assessing nitrogen mineralization from organic matter using anion exchange membranes. Fertilizer Research 40:143–148.

Qian, P., and J. J. Schoenau. 1996. Ion exchange resin membrane (IERM): A new approach for in situ measurement of nutrient availability in soil. Plant Nutrition and Fertilizer Sciences 2: 322–330.

Qian, P., and J. J. Schoenau. 2002. Practical applications of ion exchange resins in agricultural and environmental soil research. Canadian Journal of Soil Science 82: 9–21.

Qian, P., and J. J. Schoenau. 2005. Use of Ion-Exchange Membrane to Assess Nitrogen-Supply Power of Soils. Journal of Plant Nutrition 28:2193 -2200

Subler, S., Blair, J.M. and Edwards, C.A., 1995. Using anion-exchange membranes to measure soil nitrate availability and net nitrification. Soil Biology and Biochemistry, 27(7), pp.911-917.


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Materials and methods:

Our trial is taking place within a 20,000 CBD hemp planting in Addison County, VT.  Following Darby et al (2017a-f), we planted 3 foot plant to plant spacing in rows (i.e. 6 ft / plant), with 4-5 feet between middle of rows (i.e. plant to plant alley wise).  The farm uses alleys that are ~3ft wide with~2ft wide beds.  We used treatment plots of 45 plants, with 15 plants per row and three rows in each plot.  We had six soil-cover treatments, with three blocks of each treatment in a randomized position in each block.  In the summer before restarted we piloted six treatments including black plastic, mixed clover (re, a mixture of all three legumes, bark mulch, and a no ground cover null treatment.  Based on the poor performance of fenugreek, we used four treatments (black plastic, mixed clovers, hay mulch, and no ground cover) in 2020.  We also added a second farm with black plastic, each clover species separately, and no ground cover.

We measured a range of agronomic characteristics, following the reports from Darby and colleagues.  We measured hemp biomass, floral biomass of the hemp, height of the hemp and the forage legumes, and noted the identity and cover of weed species.  Furthermore, we measured a range of soil parameters. At planting we sent soil samples to the UVM soil testing lab to establish baseline conditions, including soil organic matter and nitrogen levels. We measured plant nitrogen levels and photosynthetic parameters, with a PhotosynQ MultispeQ (Kuhlgert et al., 2016).  We sent samples to Cornell for the CASH (comprehensive assessment of soil health) test at the end of the season.

We are currently working with our farm partner to accurately track expenditures related to this effort using a custom-designed form in Google Sheets. This approach allows our farm partners to easily enter data as costs (or effort) are incurred, and offers the University research team ongoing, real-time access to expenditure data throughout the lifecycle of the trial plots. Expense reporting is maintained in separate worksheets for each of the test plots. Itemized details for field inputs (seeds – hemp and cover crops, fertilizers, mulch and black plastic), labor (planting, field maintenance, harvesting and processing), and equipment will be entered into the worksheets. The form is accessible for entering and evaluating plot-level data in both field (via cell phone or tablet) and office settings. Following data entry, which is underway, plot-level summary reports will be generated and shared with our partners. Our farm partners will be consulted on the design of and delivery mechanism(s) for these information packets to ensure their utility.

To minimize the risk(s) associated with pursuing new opportunities (i.e. industrial hemp production), we need to better understand the economic and environmental impacts and / or benefits to scaling production from backyard to whole farm operations. While this experiment will not capture the full range of economic costs associated with industrial hemp production because of the relatively small size of the test plots (in particular, harvest and drying costs), our effort will go a long way towards quantifying likely expenses for a variety of production protocols. We may be able to estimate harvest and drying costs through our collaboration with Four Suns Farm (depending on their planned approach to their remaining acreage) or anecdotally through estimates for similar crops. Approaches to mechanical harvesting and drying are varied and oftentimes involve retrofitting existing farm machinery. While these stages of the production cycle represent a significant bottleneck to scaling industrial hemp production, estimating their costs extends beyond the intent of this effort.

Research results and discussion:

A preliminary analysis of a 2019 planting of hemp using the same experimental treatments we had proposed, but started before out start date, showed that black plastic treatments, the standard agronomic approach for CBD hemp, has roughly twice the biomass of the various legume undersowing approaches we aimed to pilot.  Consequently, our null hypothesis remains that standard practice is going to lead to the highest CBD hemp yields.  We will need a full growing season to verify this result, but for now preliminary data supports standard practices.

Processing of 2020 field data is still underway.  Our processing is behind due to COVID restrictions that have limited the number of students we can have in the laboratory to process samples.  However, early results suggest legume undersowing has equal yield to black plastic ground cover.  Soil carbon estimates are still pending.

A full economic analysis is underway based on our preliminary data.  Considering the importance of yield, we expect a full analysis to support our results.

Research conclusions:

Based on preliminary data we currently conclude that standard agronomic processes for CBD hemp, with black plastic, have equivalent yields to undersowing with clovers.  Since undersowing legumes may have benefits in terms of providing nitrogen and maintaining soil organic matter, it likely balancedany potential reduced yields and high seed costs.  Although, the high cost of companion and cover crop seeds remains a challenge, we have come to believe they are worth the investment in this case, if carefully managed.

Participation Summary
2 Farmers participating in research

Education & Outreach Activities and Participation Summary

3 Consultations
2 Curricula, factsheets or educational tools
1 On-farm demonstrations
3 Webinars / talks / presentations
1 Development of a whitepaper for the new ARS center at UVM to link it to hemp research needs in Vermont

Participation Summary

4 Farmers
1 Number of agricultural educator or service providers reached through education and outreach activities
Education/outreach description:

Due to COVID, our outreach plan has been badly impacted.  We are unable to have farm tours, have had conferences scheduled.

Learning Outcomes

5 Farmers reported changes in knowledge, attitudes, skills and/or awareness as a result of their participation
Key areas in which farmers reported changes in knowledge, attitude, skills and/or awareness:

Our primary farmer partner gained experience in dealing with challenging clay conditions, and in providing adequate fertility for hemp.  We reached out to other farmers through the winter of 2020, despite COVID.  A second farmer performed a parallel trial, and three more considered it.

Seed quality, variable weather, and learning to grow hemp are all reported challenges. Seed costs, for hemp and companion legumes, are also a challenge.

Project Outcomes

3 Farmers changed or adopted a practice
6 Grants applied for that built upon this project
2 Grants received that built upon this project
$220,000.00 Dollar amount of grants received that built upon this project
5 New working collaborations
Project outcomes:

Our project currently finds that legume undersowing is equivalent to black plastic ground cover in yield, and has a number of other benefits for soil health.  

Our project has reached out to the growing number of hemp growers in the northeast, to the extent possible around COVID.

Assessment of Project Approach and Areas of Further Study:

Although we have been slowed by COVID and are still processing plants and analyzing data, we currently think legume undersowing and plack plastic are equivalent for CBD hemp production. As legume undersowing has other benefits, we support it as preferable practice.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.