Year of Award


Document Type


Degree Type

Master of Science (MS)

Degree Name


Other Degree Name/Area of Focus

Ecosystem and Conservation Sciences

Department or School/College

W.A. Franke College of Forestry and Conservation

Committee Chair

Dr. Cara R. Nelson

Commitee Members

Dr. Ben Colman, Dr. Len Broberg


University of Montana

Subject Categories

Environmental Health and Protection | Natural Resources and Conservation | Plant Biology | Toxicology | Weed Science


Invasive plants can negatively impact native grasslands by changing their species composition, productivity, and function. Managers commonly use herbicides as a control method; however, this practice can lead to secondary invasion by other non-native invasive plants, unless measures are taken to promote natives. Because of this, managers often seed native plants after spraying herbicides. There is evidence, however, that chemical control of invasive plants may reduce the effectiveness of subsequent seed-addition treatments, but there is currently little quantitative information on optimal timing between spraying and seeding or on variation in herbicide sensitivity among native plants commonly used in seed mixes. I conducted an investigation of the magnitude and duration of effects of two commonly used herbicide active ingredients, picloram and aminopyralid, on performance of ten native grassland plants at the seed stage. I separated timing of herbicide applications by 0, 3, 6, 9, and 11 months before seed addition to potted soil in the greenhouse and then recorded rates of germination and germinant biomass after six weeks. Additionally, I installed seventy-two one-m2 plots at a nearby field site where I tested the effects of fall and spring-treated plots on seed performance after a spring seed addition. In the greenhouse experiment, the effect of timing on seed performance was significant for seven of 10 species, and the effect of herbicide was significant for all species. Four species had a 100% reduction in germination throughout the 11-month greenhouse trial, while there were significant among-time-period differences in germination for six species. In general, the herbicide impact on germination rate and biomass was more severe for picloram than for aminopyralid. In the field experiment, herbicide application significantly reduced seed performance for three of four species in spring-sprayed plots, while the effects of herbicide treatments were not significant in fall-sprayed field plots. Separating herbicide applications and native seed additions by as much time as field conditions allow may improve the germination rates and size of seedlings of some seeded species. Results from the greenhouse and field studies combined indicate that herbicides can have strong adverse effects on germination, but that the actual effects in field settings will be based on complex interactions between species traits, field conditions (including soil type), and management choices (season, herbicide used, and timing of seed addition after spraying). Thus, site-specific trials will ultimately be the best method for making inference to particular target sites.



© Copyright 2017 Christine McManamen