Two creeping perennial weeds – Canada thistle (Cirsium arvense) and field bindweed (Convovulus arvensus) – pose severe challenges to organic wheat, lentil, and other grain production in the Pacific Northwest (PNW) and Northern Great Plains (NGP). This project aimed to develop improved organic integrated weed management (IWM) for these two weeds, including biological (grazing, mites, pathogens), cultural (crop rotational sequence), and mechanical (mowing, tillage) tactics. Weed populations and their associated soil-root microbiomes were genotyped to evaluate genetic variables in weed response to control tactics.

Canada thistle collected from multiple sites in Washington, Montana, and North Dakota consistently hosted a core community of bacteria (28 taxa) and fungi (12 taxa) in the rhizosphere. An alfalfa hay crop (3 years) reduced Canada thistle and bindweed populations in a subsequent wheat crop as effectively as two years of intensive tillage (Noble sweep plow every 14-21 days), whereas various combinations of diversified annual crop rotations (cover and production crops), sheep grazing, and targeted tillage did not suppress these weeds. However, alfalfa reduced yields in lower-rainfall climates (14”/yr) due to soil moisture depletion, and the annual crop rotations yielded higher net economic returns.  Treatments had no effect on soil microbiome or soil health.

Inoculation with rust fungus (Puccinia punctiformis) in combination with mowing reduced Canada thistle stem density more effectively than fungus + spring and fall disking.