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

Final 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


We assessed the potential of undersowing CBD hemp with clovers as an alternative practice to black plastic groundcover.  We found no significant difference in yields between black plastic and clovers.  However, the cost of clover seed greatly exceeded the cost of black plastic.  Although clovers can provide nitrogen, sequester soil carbon, and provide pollination services, these benefits are unlikely to outweigh the cost of clover seeds at a time when the profitability of CBD hemp in the northeastern US is generally low, and when farmers do not yet have widespread access to payments for soil carbon. 

Project Objectives:

Our objective was to examine different methods of ground management in hemp farming. Our experimental planting was 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 modelled the expected monetary and environmental costs of different cultivation approaches to provide information for the growing number of hemp farmers in the Northeast.

We also recorded carbon capture data on the plots so that we were 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.) increased rapidly in Vermont before 2020, with 2,000 acres in production in 2018 and 3,000 acres registered in 2019 (as of March 22, 2019). These numbers consequently fell in 2020 and 2021 as the CBD market crashed.  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 hypothesized that strip tilling and companion cropping could be 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 aimed to build 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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  • Benjamin Fisher (Researcher)


Materials and methods:

Our trial tookplace 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 used 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 we started (2019) 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 (Mountainview farm, Waitsfield VT).

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 2020 season.

We worked 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, plot-level summary reports were be generated and shared with our partners. Our farm partners were 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 could 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 tried able to estimate harvest and drying costs through our collaboration with Four Suns Farm.  They primarily focused on the lower-value smokeable CBD market, like many other Vermont producers who could not afford to invest in more expensive drying equipment.

Research results and discussion:

The 2020 data on flower bud weight and stem weight from Four Suns farm showed no differences across soil cover treatments.  Although it is possible that the small sample size did not provide enough power to detect differences, this suggests farmers can get equal yields using black plastic, clover undersowing, or hay. Using pairwise comparisons, the 2020 4 Suns harvest height and wet weight data resulted in hemp grown in the hay treatment being significantly heavier than hemp grown in all other treatments (black plastic, clover mix, control). The same analysis also showed that hemp grown in black plastic was marginally heavier than hemp grown in the clover mix treatment (p=0.0477). 

Both 2020 soil health data sets for 4 Suns Farm and Mountain View Farm resulted in no significant differences between treatments for total nitrogen and active carbon, a measure of soil carbon that is responsive to management. The lack of significance most likely stems from the fact that the fields were already high in nitrogen and active carbon, so any differences were most likely overshadowed by the already fertile soils. It is also possible that changes in total nitrogen or active carbon are not detectable in a single-season experiment. Lastly, the small sample sizes most likely do not give us enough power to detect any potential differences in total nitrogen and active carbon.


Table 1. Tukey HSD pairwise comparison results for wet weight, 2020 4 Suns Farm

Pairwise Comparisons

Treatment 1 – Treatment 2





Wet Weight (lbs.)





      Clover Mix-Black Plastic





      Control-Black Plastic





      Hay-Black Plastic





      Control-Clover Mix





      Hay-Clover Mix










Treatment 1 mean – Treatment 2 mean =  Estimate

Price Comparisons

Black plastic was compared to clover seed in terms of cost per acre, labor required, and soil benefits. Using the prices in table 2, the total cost for three rolls of 4x4000 ft black plastic would range from $327 to $762. For most clover seed at most 20 lbs. per acre would be needed ranging between $112-$360 for 20 pounds of seed. A disposal fee for the black plastic will be charged at the landfill each year, while clover seed may only need to be acquired in smaller quantities after the first planting to replenish beds, making it a competitive option.

In terms of labor required, using a tractor will most likely only require one person to lay plastic or to broadcast seed. Plastic mulch layer attachments for tractors cost around $2,000, while broadcast seeder attachments for tractors can cost between $500 and $1,000. Manual installation of black plastic is best done with at least two people. Black plastic will also need to be remove by hand or with a tractor. The undercutter tractor attachment needed to loosen the black plastic for removal can cost around $1000.  Manual broadcast seeding typically requires one person, depending on the size of the plot.

Hay also seems to be a competitive contender compared to black plastic. For many farmers with large plots, hay is free or can be easily sourced from an adjacent farm for little cost. Hay is biodegradable, adds organic matter to soil, aids in water retention, and the process of growing hay sequesters carbon. However, it may be labor intensive to cut, dry, and lay the hay.

Table 2. Vendor, seeding/sizing, and price information for different clover based cover crops and black plastic

Cover Crop




Hemp Mix Cover Crop

High Mowing Seeds




Medium Red Clover

High Mowing Seeds






White Clover

High Mowing Seeds

5-9 Drilled

7-14 Broadcast





Medium Red Clover

Johnny’s Selected Seeds






New Zealand White Clover

Johnny’s Selected Seeds







Black Plastic



Sizing(1 mil)



Embossed Black Mulch


Johnny’s Selected Seeds











Embossed Black Mulch




Nolt’s Produce










Selection and pricing from April, 2021

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 or covering ground with hay.  Since undersowing legumes may have benefits in terms of providing nitrogen and maintaining soil organic matter, we thought it might balance high seed costs.  However, we did not observe these soil benefits in our analysis, perhaps due to the one-year timeframe of our student.  Furthermore, based on our economic analysis, the high cost of companion and cover crop seeds remains a significant challenge.  Given the low profitability of CBD hemp in 2021, we conclude that clover undersowing is unlikely to be profitable.

Participation Summary
2 Farmers participating in research

Education & Outreach Activities and Participation Summary

5 Consultations
2 Curricula, factsheets or educational tools
1 Journal articles
1 On-farm demonstrations
2 Online trainings
4 Webinars / talks / presentations
1 Other educational activities: Development of a whitepaper for the new ARS center at UVM to link it to hemp research needs in Vermont

Participation Summary:

4 Farmers participated
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 were unable to have farm tours.  von Wettberg presented at the online hemp UVM extension hemp conference and through the UVM online hemp course that is available beyond UVM. 
2021 Industrial Hemp Conference,

We did further outreach through a stakeholder group convened by the new UVM ARS center, which accounted for much of our outreach in the pandemic year of 2020.  Through the UVM ARS center, we contributed to the development of a white paper to complement the web outreach. 

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 2020 and 2021, despite COVID.  A second farmer performed a parallel trial, and three more considered it.  With low CBD hemp prices in 2021, our primary partner ceased growing hemp, and we struggled to find other farmers who remained in this sector.

Seed quality, variable weather, and learning to grow hemp are all reported challenges. Seed costs, for hemp and companion legumes, are also a challenge.  The largest challenge however is the difficulty in selling smokeable CBD.  Vermont is filled with barns of CBD that have no clear buyers.

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 may have number of other benefits for soil health.  However, soil tests did not verify these benefits.  These benefits are outweighed by clover seed being very expensive, at a time when hemp profitability is very low.

Our project has reached out to the growing number of hemp growers in the northeast, to the extent possible around COVID.  However, in the midst of this study, the hemp market stalled and acreage in production in Vermont declined substantially.

We do not find strong evidence for production practices as alternatives to black plastic.  Black plastic remains the most cost-effective groundcover management for Vermont and the Northeast.  

Assessment of Project Approach and Areas of Further Study:

Our project was badly impacted by both COVID and the challenges of the CBD hemp market.  In 2019 there was substantial grower enthusiasm for hemp, with expanding acreage in Vermont and the Northeast.  By 2021 acreage was declining and anecdotal reports of farmers unable to sell their harvest were widespread.  Our primary conclusion, that clover undersowing can have equivalent yields to black plastic but costs substantially more, does not help with the low profitability of hemp and generally low farm profitability.  As we found no soil benefits of clover, despite good reasons to expect such benefits, we do not have evidence to support alternatives to black plastic. 

Our outreach was badly impacted from COVID.  We were able to do some outreach, primarily through a new UVM-based ARS center, but it was really hard to show a field trial at a time when in-person visits are largely impossible.

We still have hope that payments for soil carbon may change the dynamics of high prices for cover crop and undersown crop seeds.  These payment systems are still in their infancy and are not widely available to farmers in our region, but are an area where we think future investigation is warranted.

Information Products

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.