The garden geranium is favoured because of its easy care and brightly coloured blooms. However, plants of the genus Pelargonium do not thrive if kept in extreme temperatures. Although they may survive a light frost, geraniums are not cold hardy.
Although geraniums survive at a range of temperatures, for best growth and flower production, plants potted indoors are kept at a minimum daytime temperature of 18.3 degrees Celsius and placed by a window receiving maximum sunlight, ideally 10 to 12 hours per day. At night, geraniums produce best at a minimum temperature of 12.8C; at lower than 10C, leaves turn reddish and growth almost stops.
Because geraniums will not survive prolonged exposure to subfreezing temperatures, do not plant outdoors until safely past your last-frost date and when ground temperature has reached a minimum of 15.6C. For rapid production, geraniums prefer a minimum daytime temperature of 21.1C and a minimum night temperature of 15.6C. Remember, however, that at daytime temperatures of 26.7C or higher, geranium production will decline significantly.
Note that minimum temperature variation may exist among some species, such as Martha or Lady Washington geraniums (Pelargonium domesticum). Additionally, geraniums overwintered using dormant storage should be kept at 7.22 to 10.0C.