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Cover crops are used in vineyards to maintain soil structure, minimise soil erosion and optimise vineyard mechanisation, yet few data are available on water consumption and carbon dioxide assimilation (CO2). Whereas cover crop use is common in the grape growing regions of Germany it is uncommon in Argentina. To obtain some information on the suitability of a selected number of species in terms of carbon gain and water expenditure under the climatic conditions of Germany and Argentina, we first quantified evapotranspiration (EvT), CO2 assimilation and water use efficiency of Trifolium repens L (white clover) (TR), Festuca arundinacea Schreb (tall fescue) (FE), Sorghum sudanense (Piper) Stapf (Sudan grass) (SO), and Digitaria californica (Benth) Herr. (Arizona cotton top) (DI) in a field trial in Geisenheim, Germany. We then compared the performance of TR, FE and DI together with Sorghum halepense L (Johnson grass) (SH) in a pot experiment under the warmer climatic conditions of Mendoza, Argentina, where water supply could be controlled. In all cases bare soil served as a reference for soil water loss. Gas-exchange of plants and soil were measured with customised open system canopy chambers and an infrared gas analyser and water dynamics in the soil with soil moisture probes (field) or gravimetrically (pots). SO and TR were the two species that reached the highest degree of soil coverage in Germany and substantially reduced soil moisture in the first 0.40 m of depth. TR showed high EvT and CO2 assimilation rates in both climates when water supply was sufficient and maintained relative high rates when water status was reduced under cool conditions in Germany, yet performed poorly when high temperatures occurred concomitant to low water supply in Argentina. DI, being a C-4 species from arid regions, had difficulties to get established in the cool conditions of Germany and there was no clear tendency for more efficient water use than the other cover crops. However under the warm condition of Argentina, especially when water supply was reduced, DI displayed higher WUE which was entirely related to the maintenance of higher assimilation rates at substantially reduced EvT. FE being common in German vineyards had low assimilation rates combined with high evapotranspiration rates under Argentinian conditions and was very sensitive to water deficit in combination with high temperature. (C) 2013 Elsevier B.V. All rights reserved.