Herbicide Study on Native Wildflower Seed Production is Underway

Inadequate and costly seed supplies and inconsistent establishment have limited the use of native species in revegetation projects. Wildflower seed producers need to gain scientific, technical, and practical knowledge for the successful production of native seeds. To fill this knowledge gap MSU-Bozeman and the BPMC evaluated broadleaf herbicide damage and integrated weed management practices for native wildflower seedlings in seed production fields and in a greenhouse setting.

Wildflower species, slender white prairie clover Dalea candida (DACA), blanketflower Gaillardia aristata (GAAR), fuzzytongue penstemon Penstemon eriantherus, silverleaf phacelia Phacelia hastata (PHHA), and prairie coneflower Ratibida columnifera (RACO) were evaluated for herbicide damage. The treatments consisted of trifluralin, linuron, imazapic, halosulfuron, and pendimethalin with a mechanical weeding component in the field study.

The greenhouse experiment was an herbicide screening evaluating chemical treatments, forb seedling height, and biomass production. A ranking table (Table 1.) displays relative treatment effects on seedling injury, with 1 the least affected (compared to the Control) and the greatest impact rated as 10.

The Gaillardia aristata was more sensitive to all the herbicides than the other three species tested. Imazapic was injurious to all species tested and halosulfuron caused significant damage to all species except Dalea candida. Additional results on herbicide effects on the number of leaves, seedling height, and biomass production are available upon request.

In addition to the greenhouse study, two field sites were established with the same wildflower species and herbicide treatments at Bozeman (latitude 45.679N, longitude 111.037W) and Bridger (latitude 45.294 N, longitude 108.916421W) to represent different Montana climates. Each treatment was subdivided into hand-weeded and non-weeded plots. Extensive results on wildflower injury, percentage cover, seed production, germination and viability, along with weed composition and cover are available upon request. Seedling injury varied by species with all combinations of trifluralin (Treflan™) and post-emergence herbicides causing reduced emergence in 3 of the 5 species. This is in direct contrast to the pre-emergence herbicide greenhouse study conducted at MSU-Bozeman 2 years ago.

Field testing herbicides represents a necessary step in understanding herbicide effects on wildflower species as environment is a large factor in efficacy.  Imazapic (Plateau®) resulted in excellent weed control, but high crop (wildflower) injury levels indicated this herbicide may not be a good choice for field production, also contrasting past research. We found the hand-weeded control plots showed significantly higher establishment rates than any of the herbicide treatment plots for all species. This implies a combination of management techniques may work better than herbicide alone. Graph 1. below shows the number of plants established at each site in the control plots compared to the plots where trifluralin and imazapic were used.  With herbicide included the number of plants is greatly reduced.

Graph 1.  Comparison of Mechanical Weed Control versus Herbicide Control for Plant Establishment.

A second field season in summer 2008 will elucidate useful broadleaf herbicides for seed production fields and determine effects of treatments on wildflower seed viability and germination.

By Jessie Wiese, MSU Graduate Student.

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Last Modified: 04/11/2008