It is widely believed that Acacia karroo Hayne trees compete with and negatively affect the production of grasses in their immediate surroundings. This perception formed as a result of observations by farmers of their veld as well as some research. The complexity of plant interactions in different conditions required that research be done to better explain the extent of grass and tree interactions. In response to this need, the effect of a range of Acacia karroo densities on grass production was tested in areas of differing rainfall. Results emphasize the complexity of such plant interactions but highlight rainfall as influencing the effect of tree density on grass production and grass quality. Study sites were chosen with significant differences in tree density and rainfall and this was found to be concomitant with significant differences in grass yield and crude protein (p < 0.001). Grass yield (mass produced) was negatively correlated with tree density (p < 0.001) and was affected by rainfall. High tree density reduced grass yield at the lower rainfall sites, but a reduction in yield was only recorded during dry conditions in the higher rainfall areas. By contrast, the crude protein content of grasses was positively correlated to tree density (p < 0.001). However, as with yield, rainfall played an important part in the strength of the correlation. Soil properties such as temperature (-) (p = 0.011), phosphorus content (+) (p = 0.006), calcium concentration (+) (p = 0.005), acid saturation (-) (p = 0.018), and soil pH (+) (p = 0.008) were also significantly correlated to tree density. Tree density enriched the soil, subsequently benefitting grass quality. The study sites investigated, had different plant communities and environmental conditions with Cradock, Kubusi Drift and Kei Mouth having unique communities and conditions while Adelaide and Cathcart could not be separated in either. Cradock was the driest site and the vegetation was dominated by both grasses and karroid shrubs, differing in pattern and process from the other sites. The Kubusi Drift site had high tree densities, to the extent that the trees altered the grass species composition. The effects of rainfall on grass yield and quality exceeded that of community composition differences.