The geranium, Geranium spp., is a relatively heat-tolerant flower that grows in USDA plant hardiness zones 4 to 8, with a few species cold-hardy to zone 3. Despite its reputation for handling heat well, the geranium grows better in certain temperatures.
The ideal temperature range for geraniums is between 60 and 75 degrees F, and temperatures slightly past that range on both ends–50 to 60 and 75-80 degrees–can affect leaf color and slow production. The University of Massachusetts, Amherst, says temperatures lower than 50 degrees F and above 80 degrees F can suspend growth. The University of Minnesota Extension Office notes that indoor geraniums can tolerate slightly cooler temps, with 55 degrees F as a suitable nighttime temperature.
Germination temperature tolerance is very narrow. Auburn University advises that seeds initially need temperatures of 75 to 78 degrees F for the first three days and 70 to 72 degrees F after that.
Texas A&M’s extension office in Bexar County, Texas, reports that two cultivars of geranium tested in 2005 and 2006, ‘Strawberry Sizzle’ and ‘Violet,’ were able to stand up to the wilting heat of San Antonio summers.