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Cryptic invaders are inherently difficult to study due to morphological similarity with native lineages of the same species or genus. Wetland and riparian systems are particularly prone to plant invasions, and have been impacted by a number of widespread cryptic invaders such as Phalaris arundinacea (reed canarygrass). Here we combine molecular genetic analyses with species distribution modeling to assess the nativity of Phalaris and determine potential drivers of non-native Phalaris invasion in riparian areas across a large region of the semiarid northwestern USA. Based on our genetic analyses, we found that throughout our study region Phalaris is largely non-native, and no modern-day samples from our study region were of native North American origin. At least half of the four species distribution models suggested that non-native Phalaris invasion across the region was associated with warmer temperatures, more growing days, wetter summers, drier winters, higher nitrogen levels, shallower stream slopes, and at sites closer to roads and without a history of grazing. These findings can be used to determine the best locations for targeted monitoring. Furthermore, there is the potential for increased Phalaris invasion with climate change-induced temperature increases.