Does Timing of Defoliation Affect Spotted Knapweed Seed Viability and Germination?

Project Overview

Project Type: Research and Education
Funds awarded in 2006: $62,600.00
Projected End Date: 12/31/2010
Region: Western
State: Montana
Principal Investigator:
Tracy Brewer
Park County Extension - Montana State University
Dr. Tracy Mosley
Montana State University Extension

Annual Reports


  • Additional Plants: native plants


  • Animal Production: range improvement
  • Education and Training: demonstration, extension, farmer to farmer, focus group, on-farm/ranch research, participatory research
  • Natural Resources/Environment: habitat enhancement
  • Pest Management: weed ecology


    Spotted knapweed (Centaurea stoebe), an aggressive competitor and ecologic threat, produces 25-35 flowers/head, 60 heads/plant and 5,000-40,000 seeds/meter squared annually. Prescribed sheep grazing, a tool used to control spotted knapweed, can offsets high costs of control and environmental concerns surrounding herbicides. However, spotted knapweed forms new flowers after prescribed sheep grazing is applied during the bolting or flowering stage. It is unknown if these new flowers produce viable seeds by the end of the growing season. The purpose of this two-year study was to determine the appropriate timing(s) or combination(s) of timings of defoliation on spotted knapweed to reduce viable seed production.

    Project objectives:

    Objective 1: Evaluate the effects of timing of spotted knapweed defoliation on: a) the number of buds/flowers present, b) the percent viability of filled seeds and c) the total number of viable seeds produced to determine the best timing of spotted knapweed defoliation to minimize viable seed incorporation into the seedbank annually.

    Objective 2: Combine results of this study with past research and existing grazing prescription protocols to refine the technique of using sheep grazing to control spotted knapweed.

    Objective 3: Educate ranchers, other landowners, livestock operators, county extension personnel, county weed district personnel, state and federal agency personnel, Montana State University researchers and concerned citizens about how properly timed sheep grazing to control spotted knapweed can reduce the amount of viable seed applied to the seedbank in a single growing season and can enhance the ecological integrity of the land.

    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.