The geranium, or Pelargonium genus, includes more than 200 species. Geraniums come with various colors of blooms, single and double flower types, upright to trailing-vine growth habits, different leaf patterns and with assorted scents. Whether you are planting geranium seedlings or transplants; knowing when to plant them is important, because geraniums do not tolerate cold temperatures.
Do not plant geraniums outside in the landscaping until the last chance of frost has passed in your area. Wait until the soil temperatures reach 60 degrees Fahrenheit. Planting geraniums in colder temperatures could result in injury to the plants in the form of red foliage and reduced growth, according to the University of Rhode Island. Plant the geraniums on a cloudy and cool day or in the late afternoon hours on a sunny day.
Geraniums prefer a planting site with up to eight hours of full sun. The plants will produce more flowers in sunny locations but early morning sun with late afternoon shade is suggested for most types of geraniums to prevent a condition referred to as "heat check." This means the plants will stop blooming during extreme summer temperatures and will not resume blooming until lower temperatures arrive. You can prevent this from occurring if you grow heat-tolerant geranium varieties.
Prepare the planting area as soon as you can work the soil in the spring. Enrich the planting bed by adding 1 inch or more of organic matter, such as compost, peat moss or aged manure. Mix this well into the top 6 or more inches of soil. Water the geraniums in the containers several hours before planting. Dig planting holes slightly shallower than the containers to prevent planting the geranium plants too deep, which could cause the stems to rot and the plants to die.
Remove the geraniums from the containers and place them in the holes. Backfill the holes, and then firm the soil gently around the plants without harming the stems. Saturate the area with water to settle the geraniums in place and to remove any remaining air in the soil. Apply a time-released granular fertilizer as directed on the packaging once in the spring or use a water-soluble fertilizer three times during the season. Allow the soil to dry on the top inch or so before watering.