You can put out geraniums as soon as threat of frost has passed and the soil has reached 60°F.
What Temperature Do Geraniums Prefer?
Geraniums are a forgiving plant when it comes to temperature. They will survive in temperatures ranging from 45-80°F. For optimum plant growth, you want to plant geraniums where the average nighttime temperature is 60-65°F and the daytime average is 70-75°F.
Colder Conditions: When geraniums are exposed to temperatures lower than 60°F the growth, begin to slow. At a drop to 55°F, the plant will flower later. Yet another drop to 50°F and the plant's growth will almost come to a complete standstill and maturity will be delayed even when brought back to higher temperatures. Older leaves will turn a reddish color.
Hotter Conditions: Temperatures of 85°F over extended periods can cause heat stress to the plant and will hinder growth and development.
Acclimating/Hardening off Geraniums
If you have overwintered your geraniums or propagated cuttings over the winter and are ready to plant them out of doors the same rules apply for frost and soil temperature. It is always a good idea to acclimate plants to the outdoors that have been thriving in a warm home all winter.
Acclimating plants to the outdoors is referred to as ‘hardening off', which simply means gradually exposing the plant to varying amounts of temperature and sunlight.
Over several days, take the plants outdoors and place in a shady spot for a few hours each day for a week. Begin to expose to sunnier locations and leave out for longer periods over the next week. Always bring the plant in at night to avoid freezing.