Tall buttercup, a native of central and northern Europe, has becomenaturalized in the United States and Canada, and in South Africa, Tasmaniaand New Zealand. In Canada and New Zealand it has become an economicallysignificant weed in cattle-grazed pastures. In this study we develop aCLIMEX model for tall buttercup and use it to project the weed's potentialdistribution under current and future climates and in the presence andabsence of irrigation. There was close concordance between the model'sprojection of suitable climate and recorded observations of the species. Theprojection was highly sensitive to irrigation; the area of potentiallysuitable land globally increasing by 30% (from 34 to 45 million km2) under current climate when a “top-up” irrigation regime(rainfall topped up 4 mm d−1 on irrigable land), was included inthe model. Most of the area that becomes suitable under irrigation islocated in central Asia and central North America. By contrast, climatechange is projected to have the opposite effect; the potential globaldistribution diminishing by 18% (from 34 to 28 million km2). Thisrange contraction was the net result of a northward expansion in thenorthern limit for the species in Canada and the Russian Federation, and arelatively larger increase in the land area becoming unsuitable mainly incentral Asia and south eastern United States.