Soil solarization as a tool to control weeds and soilborne pathogens in tree seedling nurseries in the Pacific Northwest

Project Overview

Project Type: Research and Education
Funds awarded in 2016: $247,329.00
Projected End Date: 11/30/2019
Grant Recipient: Oregon State University
Region: Western
State: Oregon
Principal Investigator:
Dr. Jennifer Parke
Oregon State University


  • Additional Plants: ornamentals, trees


  • Education and Training: demonstration, extension, on-farm/ranch research
  • Pest Management: soil solarization
  • Soil Management: soil microbiology, soil physics
  • Sustainable Communities: sustainability measures

    Proposal abstract:

    Nursery and greenhouse crops represent Oregon’s leading agricultural commodity, with an annual farm gate value of $745 million (2012 data). Approximately one-third of the crop value ($245 million) is from field-grown trees. Pacific Northwest nurseries also produce 86 million bareroot conifer seedlings for reforestation, with an annual value of $37 million. The growth and quality of tree seedlings for both landscaping and forestry are reduced by competition with weeds and activity of soilborne plant pathogens. Soil fumigation, once a routine practice for disinfesting soil, is no longer viable for most nurseries due to buffer restrictions, cost, or environmental concerns, although methyl bromide and chloropicrin are still used in forest seedling nurseries. Herbicide options for use on broad-leaved trees and conifers are limited, necessitating the costly practice of handweeding estimated at $900 to $3,380 per acre per year. Novel strategies are needed to disinfest nursery soil of weeds and soilborne plant pathogens. Soil solarization has been used for decades to disinfest soil in “warm” climates. Solarization employs solar radiation to heat the soil under a transparent plastic film to achieve temperatures detrimental to soilborne pathogens and weed seeds. Recent advances have made it more feasible to solarize soil in cooler regions. Polyethylene film treated with an anti-condensation coating allows greater penetration of solar radiation than regular, uncoated film (Funahashi and Parke unpublished). Many plant pathogens and weed seeds are sensitive to heat. For example, soil solarization for 2-4 weeks killed Phytophthora spp. buried at 5 cm and 15 cm (Funahashi and Parke, 2015). Common nursery weeds susceptible to soil solarization include Canada thistle, common groundsel, lambquarters, pigweed, shepherd’s purse, and smartweed (Cohen and Rubin, 2007). Preliminary observations in an Oregon wholesale nursery indicate that Mazzard cherry tree seedlings grown in soils solarized in 2014 were significantly larger in 2015 than those grown in adjacent, nonsolarized soil. Weed populations in fall (2014) and spring (2015) were also significantly reduced in solarized beds.  In 2015, we compared regular and anti-condensation polyethylene films of two thicknesses for solarization efficacy.  The highest soil temperatures were achieved with the anti-condensation films, and the thinner film (1.4-mil) worked nearly as well as the thicker film (6-mil).  Fall weed emergence was reduced from an average of 22.7 weeds per sq. ft. to <0.4 weeds per sq. ft. For the last three years, Parke’s group has conducted soil solarization trials with the anti-condensation film in OR, CA, and WA in collaboration with 25 different wholesale container nurseries. Soil temperatures sufficient for disease suppression and weed control were achieved in 36 of 42 trials. The purpose of the proposed research is to determine if soil solarization under Pacific Northwest conditions is feasible for tree nurseries that grow seedlings on raised beds. Research is proposed to optimize conditions for effective soil solarization, including determination of the minimum duration and optimal soil moisture required for effective solarization. On-farm trials will include continuous measurement of soil temperatures and soil moisture at different soil depths in relation to solar radiation and ambient air temperatures. Soil assays will include testing solarization effects on the soilborne pathogens Fusarium, Cylindrocarpon, Pythium, Agrobacterium, Verticillium and on mycorrhizal fungi. Weed populations (density and species composition) will be determined in the fall, spring, and summer following solarization. Plant growth will be measured one year after solarization by determining the size and grade of a subsample of seedlings in each solarization treatment. Outreach will consist of several different activities. Each year, a field day will be held at one of the three participating nurseries where growers can observe differences in weed populations and plant growth. Equipment for bed formation and mulch-laying equipment will be demonstrated and soil temperature and weed count data will be shared via printed handouts. YouTube videos that show results and teach concepts will be posted (see examples of other nursery projects at Presentations to growers will be made at the Farwest Show and smaller regional meetings such as Oktoberpest and the Shadetree Grower Group. Articles will be published in nursery and forestry trade journals (The Digger, American Nursery, NMPro, and Tree Planter’s Notes) and in peer-reviewed journals. At least one paper will provide an economic analysis of soil solarization in comparison to current methods of control. The final outreach component will be development of a web-based predictive model to enable growers to determine the feasibility and length of time necessary to disinfest soil with solarization. The model will potentially be added to an existing W-IPM Centers-supported IPM decision support system, at The model will be simple in that nursery managers and other users need only input a location and start date. A predictive model for container nurseries is currently being developed with support from the W-IPM Center; we propose to adapt this for use in Pacific Northwest bareroot nurseries with on-farm trial data collected in years 1 and 2. The predicted outcomes of the project will include knowledge on the feasibility of utilizing soil solarization for weed and pathogen control and practical advice and a web-based tool for optimizing soil solarization in the Pacific Northwest.

    Project objectives from proposal:

    1. Determine if soil solarization is an effective and economically feasible way to control weeds, soilborne plant pathogens and improve tree seedling growth in Pacific Northwest nurseries. (June 2016-November 2018)
      1.1. Establish on-farm trials at 3 commercial nurseries: test effects on weeds, indigenous pathogens, and subsequent plant growth
      1.2. Document economic costs and benefits associated with solarization vs. current practices
      1.3. Host a grower field day(s) at least once annually
      1.4. Produce a video with collaborating growers describing solarization results at their nurseries

    2. Optimize soil solarization for Pacific Northwest conditions. Determine optimal solarization duration by evaluating the effect of soil moisture conditions and soil texture on achieved soil temperature. (June 2016-November 2018)
    2.1. Determine the range of optimum soil moisture contents that will achieve target temperatures lethal to certain weeds and pathogens
    2.1.1. Apply Hydrus numerical model to predict heat transfer in soils with different textures and moisture contents for raised bed systems.
    2.1.2. Conduct field experiment to measure soil temperatures in raised beds as a function of soil moisture or soil textures.
    2.1.3. Validate the Hydrus model with actual field data.
    2.1.4. Analyze soil temperature data in relation to biological data.
    2.1.5. Develop a movie of soil temperature and moisture data.
    2.1.6. Conduct controlled laboratory and greenhouse studies to determine the interaction between soil moisture and heat on weed and pathogen viability.
    2.2. Test the minimum period of solarization duration required to achieve the desired biological effects

    3. Develop a web-based grower-friendly model for predicting the length of time necessary for disinfesting bareroot nursery soil based on geographic location, soil moisture content, and start date. (October 2016-May 2019)
    3.1. Provide heat transfer model to allow prediction of soil temperature from soil surface temperatures/air temperature/solar radiation
    3.2. Provide soil temperature and thermal mortality curves necessary to generate a predictive model for solarization efficacy in raised bed systems.
    3.3. Interface with OSU Integrated Plant Protection Center modelers to adapt the predictive model for solarization of container nurseries to raised bed systems.
    3.4. Conduct grower workshops to demonstrate how to use the web-based model.

    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.