The average annual rainfall of 16 to 35 inches increases from west to east. Average monthly rainfall is lowest in January and February, and highest during May or June. After a midsummer depression, another peak is reached in September. Summer temperatures are high, with extremely high evaporation rates in the Laredo area. Soils of the South Texas Brush Country are clays and clay loams.
Soil reactions vary from alkaline to slightly acid.
Thorny brush is the predominant vegetation type in the region, including mesquite, acacia, prickly pear, and mimosa, among others. Areas of shallow soils and rapid drainage generally support this plant life. A grassland or savanna type vegetation which also occurs was somewhat more extensive in the 19th century and earlier, but long continued grazing and other factors have altered the plant communities to such a degree that ranches of the region now face a severe brush problem. The Brush Country has a greater diversity of animal life than any other in Texas. It is home for many near tropical species which abound in Mexico, many grassland species that range northward, and some desert species commonly found in the Trans Pecos.