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Two species of alkaligrass, Nuttall’s alkaligrass (Puccinellia nuttalliana [Schuh.] Hitch.), a native, and weeping alkaligrass (Puccinella distans [Jacq.] Parl.), an introduced species from Eurasia, are found within semi-arid regions of the United States. Recently, land managers have become concerned over the ability of these two species to colonize a wide array of soil types, but have not been able to predict which sites might be at risk of invasion. Paired plots comparing site characteristics of infested versus very close, yet uninfested sites were sampled throughout the Grande Ronde Valley, northeastern Oregon. The results of this study indicate that Nuttall’s alkaligrass was most commonly associated with sodic soils (73%), whereas 85% of the sites infested with weeping alkaligrass were agriculturally ’normal’ sites. In general, Nuttall’s alkaligrass was positively associated with sodium; whereas, for weeping alkaligrass, competing vegetation was the only factor affecting establishment and abundance. Weeping alkaligrass showed traits typical of a resource generalist, establishing on a wide variety of sites and being strongly influenced by competing vegetation: whereas Nuttall’s alkaligrass appears to be more niche specific. Species that behave as resource generalists are more problematic than species linked to specific site characteristics because it is difficult to predict where they will establish. Therefore, a healthy crop cover on agriculturally productive sites is the best prevention against weeping alkaligrass establishment. For sodic sites, incapable of supporting a healthy crop, diligent weed control must be practiced to prevent the spread of either species of alkaligrass within agronomic systems.