Organic Control of Canada Thistle in Mulched Orchards

Project Overview

Project Type: Farmer/Rancher
Funds awarded in 2019: $8,920.00
Projected End Date: 02/28/2021
Grant Recipient: Two Onion Farm
Region: North Central
State: Wisconsin
Project Coordinator:
Christopher McGuire
Two Onion Farm


  • Fruits: apples


  • Pest Management: eradication, mulches - general, mulching - vegetative, weed ecology


    Most apple farmers raise dwarf trees because the dwarf trees generally provide a quicker return on investment, produce better quality fruit, require less labor, suffer less disease and are easier to spray.  However, dwarf trees have shallow root systems and compete poorly with weeds.  Many organic apple growers with dwarf trees rely on wood chip or bark mulch (by-products from local sawmills) to suppress annual weeds around their trees, but over time aggressive perennial weeds such as Canada Thistle can invade the mulched area.  Canada Thistle has deep roots and once established it is very difficult to eliminate using organic methods.  Repeatedly killing thistle shoots by mowing or other means can deplete thistle’s root reserves and gradually suppress or eliminate it.  Unfortunately, this repeated killing is time-consuming and difficult to accomplish on a commercial scale.

    We measured the cost and effectiveness of four methods of killing Canada thistle shoots in our apple orchard: hand-pulling, spraying with an organic herbicide, mowing with a string trimmer, and cutting shoots off with a hoe.  Based on previous research, we applied these four methods every three weeks during the growing season: a three week interval is needed to deplete the thistle's energy reserves stored in the roots.  We evaluated each technique in plots mulched with bark only and in plots mulched with both bark and cardboard. 

    Our main conclusions were:

    (1) All methods of killing shoots at three week intervals were extremely effective and completely eliminated thistle within two growing seasons. 
    (2) A layer of cardboard mulch underneath the bark mulch reduced thistle populations more quickly than bark mulch alone.  Installing cardboard took more time than it saved later in killing thistles, however, so it is probably worthwhile only if it can be done at times of the year when labor is plentiful.
    (3) Of the four shoot-killing methods, hoeing and string-trimming were least expensive.  Hand-pulling was expensive because it required a lot of labor.  Spraying with an organic herbicide was quick and effective, but expensive because of the high cost of the herbicide.

    Based on these results, we have implemented hoeing in our own orchard to kill established thistle patches.  We have shared results through email list serves, our website, a field day, conferences, and several printed publications and newsletters (some of which will only be printed after the end of the grant period).  Many farmers expressed interest in our results and implementing similar techniques on their own farms.

    Project objectives:

    1. Compare four methods of repeatedly killing Canada thistle shoots in our mulched apple orchard: hand-pulling, spraying with an organic herbicide, mowing with a string trimmer, and cutting shoots off with a hoe.
    2. Evaluate each method in plots mulched with bark only and in plots mulched with both cardboard and bark.
    3. Measure time required and effectiveness at reducing thistle populations for each treatment.
    4. Share results with other organic apple growers through our website, a field day, emails to grower list-serves, articles in grower publications, and conferences.
    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.