Comparing Alternative Weed Management Practices to Black Plastic in CBD Hemp Production

Progress report for ONE20-366

Project Type: Partnership
Funds awarded in 2020: $29,993.00
Projected End Date: 11/30/2022
Grant Recipient: University of Maine Cooperative Extension
Region: Northeast
State: Maine
Project Leader:
Dr. John Jemison, Jr.
University of Maine Cooperative Extension
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Project Information


After an eight-decade hiatus, hemp production is back.  Farm diversity and profits can be increased if production costs can be controlled.  Approximately 2625 acres have been grown in Maine since 2017.  Hemp grown in the northeastern United States for cannabidiol (CBD)-rich flowers must be produced on wide (5x5) spacing to reduce disease due to high humidity.  No herbicides are labeled for hemp, and with the wide spacing, there is little crop shading to slow weed growth without using plastic mulch.  More than 19,000 ft2 of plastic is landfilled each year per acre of hemp produced. This waste and the fuel required to till and prepare hemp beds is unsustainable.  Planting hemp into no-tilled mowed or rolled/crimped cereal rye cover crops could reduce weed pressure and reduce cost.  Winter rye protects the soil from erosion, transpires water to dry soils in the spring, feeds microbes through root exudates, and inhibits weeds when rolled or mowed into a mulch.  In this 2-year, 2-location project, Sarah Hewitt of Victory Hemp Farm and I will evaluate CBD hemp performance in a no-till rolled/crimped or mowed rye mulch and assess possible allelopathy or nitrogen immobilization issues.  We will also evaluate novel stacked cultivation tools to see if intensive cultivation can reduce weed pressure. These systems will be compared to a tilled, standard black plastic mulch production system.  We will assess plant growth/development, flower and total plant yield, weed biomass, and cannabinoid content.  Results will be shared at grower meetings and on a hemp production website.  

Project Objectives:

Through replicated research trials, we seek to explore novel means to manage weeds in CBD hemp production.  We will compare a standard black plastic mulch (control treatment) to rolled/crimped winter rye, sickle-bar mowed winter rye, and stacked cultivation tools.  This will be done in two experiments: one conducted with Sarah Hewitt of Victory Hemp (VHF) in Union Maine, and the other at the University of Maine Whitter Center Rogers farm (UMRF). 


The project seeks to assess the effect of: 1) tillage on CBD hemp growth and development; 2) allelochemicals and nitrogen immobilization slowing plant growth and development; 3) rye cover crop destruction method (roller crimper or mowing) on continuity and thickness of cereal rye mat; 4) weed management treatments on weed density by species; and 5) weed density and plant stress on THC and CBD levels in the plant.  Study results will benefit farmers by providing them with research-based information on the risks and benefits of managing weeds with rye cover crops, reducing tillage, and the effects of plant stress on the total cannabinoid content in CBD hemp. 


The Farm Bills of 2014 and 2018 legalized hemp production across the US, leading to renewed farmer interest in this crop.  Maine initiated its hemp licensing program in 2016, and producers have grown approximately 2500 acres of hemp, primarily for high-CBD flower, in the state over the past two years.  Most farmers transplant hemp into black plastic mulch on a 5x5 or 6x6 spacing.  There are no legal herbicides for use in hemp, and plastic mulch serves to both control weeds and warm the soil.  However, the input costs and waste generated to produce CBD hemp in the Eastern US is not sustainable.  Black plastic can only be used for one year; so, following harvest, almost 19,000 ft2 of plastic is landfilled for each acre of production.  Alternative weed management systems using cover crops are needed that can produce adequate biomass to protect the soil from erosion over winter, improve soil health, slow weed germination, and make eastern hemp production competitive with our western competition.


Due to lower relative humidity, western growers don’t have to rely on plastic as they can produce hemp at increased plant densities with less concern for plant disease.  Tightly spaced hemp reduces weed competition and increases flower yield per acre.  Maine growers who have not used black plastic mulch have been unable to control weeds, leading to increased plant stress and lower flower yield.  Maine’s hemp program director stated that poor weed control is our state’s number one hemp production issue.  Along with reduced yields, CBD plants under stress can have delta-9 THC levels spike above the legal limit of 0.3%.  With the new USDA interim final rule reducing the level to 0.3% total THC on a dry matter basis, more growers with stressed plants risk failing the federal THC standard.  At $15,000 to $18,000 per acre production costs, a crop not meeting the THC standard is an extreme economic risk to the grower. 


Possible alternatives to black plastic include plant-based cover crop mulches and timely cultivation.  Planting a cereal rye cover crop in the fall would protect the soil from erosion, feed soil microflora, and when mowed or flattened/killed with a roller crimper in the spring, the rye residue will form a mulch bed through which to transplant the CBD hemp.  Seed cost for a 2 bu/ac seeding is $16/ac.  Not tilling the soil will also save fuel cost, labor, and carbon dioxide emissions. The University of Maine has access to a roller crimper to use for this purpose.  Other farmers may not have access to this tool.  Simply mowing the rye with a cycle-bar mower could be an alternative way to form a bed.  We must also explore improved cultivation methods. 


Research is needed to understand how CBD hemp will perform transplanted into a rye cover crop mulch.  Growers need to know if hemp will be affected by either nitrogen immobilization or allelopathic chemicals that might leach out of the rye mulch.  Practice using novel stacked-cultivation tools is also needed. 


Click linked name(s) to expand
  • Sarah Hewitt - Producer


Materials and methods:

This project will support two replicated field trials:  one conducted in Union Maine on Sarah Hewitt’s VHF, and the other will be conducted at the University of Maine Witter Center - Rogers Farm (UMRF) in Stillwater Maine .   Each study will be a randomized complete block study: the Union location will have 4 treatments:  black plastic mulch as the control, roller/crimped rye mulch, sickle-bar mowed rye mulch, and stacked cultivation with 4 replications; the UMRF location will have 5 treatments (those previously mentioned plus a weed free treatment) also with 4 replications.  Each study area will be approximately 0.5 acres in size.


The project will be initiated in fields with standing pea, oat, and vetch green manure cover crops.  In early September, cover crops above-ground biomass will be sampled, and then mowed and incorporated into the field.  Treatment areas will be laid out and either oats or Aroostook rye will be broadcast sown at 2 bu/ac and rolled to ensure seed-soil contact following the model used in Mallory and Molloy (2017). 

  •  Each field plot area was identified in the late summer at Victory Hemp and the University of Maine farm.  The ground was worked on 6/6/2020 and the seed was spread onto the field area with a spin spreader at 200 lbs/ac seeding rate.  We sowed Peas Oats and Vetch with a grain drill on the Rogers Farm field on 6/10/2020.   The ground was very dry at Victory Hemp farm, and the germination was quite poor.  The Rogers Farm production was much better.  It was mowed and turned under at the Rogers farm on 9/10/2020 and our cover crops (winter rye and peas/oats) were sown at 2 bu/ac.  Due to equipment issues, we worked the soil and broadcast seed and rolled it on 9/29/2020 at Victory Hemp. This time germination was good.  I was unable to sample the biomass at Victory Hemp due to new Covid restrictions, but i was able to sample Rogers farm on 10/30/2020.  Biomass revealed that we produced a total of 42 g/m2 for the winter rye, and 43 g/m2 for the peas and oats.  As of now, the winter rye stand is good, and the peas and oats are still alive but barely.

Ms. Hewitt will start her CBD seedlings from feminized seeds in late April for planting in June for both studies.  This will be done in her greenhouse at VHF.  All soil-applied fertility treatments will be purchased in the early spring and weighed out in late April or early May for spreading on individual plots.  In late May or early June 2021 and 2022, all preplant soil amendments (crab meal, etc) ill be spread on the winter-killed oats, incorporated, and the soil will be prepared for transplanting.  Black plastic will be laid in the appropriate plots with a bed layer.  Prior to spreading the same treatments on the rye treatments, two biomass samples will be collected from each rye treatment to assess production.  The two rye treatments will be rolled / crimped or mowed.   Additional strips of rye will be planted at both locations to ensure that the roller/crimper is well adjusted and effectively processing the rye residue.  Three measurements on mulch thickness will be made prior to transplanting to assess thickness and continuity of mulch.  This will address project objective #3.  Transplants, started late April will be transplanted in three row plots – five plants per row in plots 25 feet long and 20 feet wide.  We will allow 20 feet between plots to allow for the cultivating equipment to properly align to the plants in the rows. 


Transplants for the RF location will be brought back to Stillwater and planted within a week.  Same procedures and measurements will be followed at UMRF with one additional treatment – the weed free hand cultivated treatment.  This treatment will be hand weeded once a week through the season. 


Cultivations with the stacked system presented previously will be done on a bi-weekly basis from week 1, 3, 5, and at early flower initiation.


Plant measurements

During the vegetative period, we will measure plant height and width at each cultivation timing.  With this data we will be able to assess how much the lack of tillage slows plant growth and development (meeting objective #1). We will use a SPAD meter (a protocol to be developed in summer 2020) to obtain a non-destructive assessment of leaf chlorophyll content (indirect measure of nitrogen status of the plant).  We will measure chlorophyll content (of the most recent mature leaf (MRML)) of five plants to assess treatment differences at each cultivation timing.  On the final cultivation, we will sample 10 leaves from the guard rows for nutrient analysis.  This will address objective #2 above.  We will both rate each plot for overall weed density (1-10 scale) before each cultivation and use a small quadrat to measure weed biomass at flower initiation and just prior to harvest.  Plants will be clipped at the ground level, sorted by species, dried and weighed.  This will address project objective #4.  Three plants per plot will also be randomly selected and rated for disease or insect damage. 



The interior three plants of the interior row of the three-row plot will be used to assess overall plant biomass. Each of the three plants will have height and diameter measurements taken, and then the plant will be cut at the base at 4-inches above the ground and total plant fresh weight will be measured for each.  Plants will cut up, bagged and returned to the university where we will separate each plant into bole, leaf, stem and flower and weights will be taken for each plot.  Six flower buds will be randomly selected for cannabinoid content and submitted to Proverde lab for analysis.  This will address project objective #5. 


Data analysis

All data collected from each location will be subjected to analysis of variance with JMP statistics program.  Preplanned comparisons will be used to separate treatment differences.  With four treatments at VHF, we can assess the effect of tillage vs no tillage, black plastic (standard) vs all others, and cultivated vs. black plastic (standard).  At UMRF, we’ll add to that one additional question, weed free vs. black plastic (standard). 

Participation Summary

Education & Outreach Activities and Participation Summary

Participation Summary:

Education/outreach description:

As mentioned previously, weed management and reducing cost of hemp production were the top production issues last year based on surveys from the Maine state hemp program staff.  Interest in alternatives is high.  Each year, we conduct training programs at the Agricultural Trades Show in January in Augusta Maine.  Data collected each year from this study will be presented in January 2022 and January 2023.  We will also hold one field day at VHF in Union in late July or early August just before or around flower initiation.  Growers will have a chance to see the roller/crimper and we’ll have a strip of rye at VHF to demonstrate the roller/crimper.  Trial results will be summarized in a report each year and a two-year overall summary will be prepared and emailed to all current licensed hemp growers.  This will also be put on a soon-to-be developed University of Maine hemp webpage. 

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.