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The interaction between woody and grass species in semi-arid rangelands of central Argentina was assessed through the evaluation of the impact of woody species on grass growth over a 2-y period. The study comprised two morpho-physiologically different woody species and two cool-season perennial grass species highly preferred by domestic livestock. The study involved the assessment of total green length (TGL) and number of green leaves (NGL) per tiller in grasses growing beneath the canopy of woody species and in open rangeland areas. Soil temperature, pH and water, nitrogen, phosphorus and organic matter content were estimated at the same study sites. Precipitation during the study period was also recorded. Woody species induced higher TGL of grasses growing beneath their canopy when rainfall was near or above the long-term average for the study site and had no effect on NGL. Nitrogen and organic matter content of soils beneath the canopy of woody species were higher than values in open areas, while differences in the other soil parameters were scarce. Overall, the results of the present study suggest that woody species exert a facilitative effect on perennial grass species in semi-arid rangelands of central Argentina.