Geraniums are tender perenniasl that are hardy to grow year-round in USDA Plant Hardiness Zones 8 through 10. The plants grow as an annual in cooler USDA zones. Transplant or replant container-grown geraniums in spring once there is no longer a risk of frost and the soil has begun to warm. The plants establish quickly and begin flower production early in the season when cared for properly. Geraniums prefer to grow in a root-bound environment and do not require transplanting during the summer growing season unless the soil becomes dry quickly after watering.
Select a planting container that is 12 inches in diameter and height, with bottom drainage holes. Set the container on a saucer filled with a layer of gravel to prevent the geranium from resting in water, as this creates an environment for root rot.
Fill the container two-thirds full with a well-draining potting soil. Create your own potting soil by mixing even portions of garden soil, perlite and peat moss. Mix in a slow-release fertilizer designed for container plants. No additional fertilizer applications are needed during the growing season.
Set your geranium into the new planting container so the root ball is at the same depth as previously planted. Cover the root ball with soil and gently tamp to hold it in place and eliminate air pockets.
Water the geranium after planting until water flows through the bottom drainage holes. Apply water several times a week during the growing season to keep the soil moist during hot summer months. Always water the soil and not the plant, as getting water on the leaves creates an environment for disease. Do not allow the soil to dry out, as this will decrease flower production.
Place the geranium container in a location that receives a minimum of six hours' direct sunlight each day. Geraniums grow well in full sun to partial shade as long they receive a minimum amount of sunlight.
Apply a fungicide to the geranium if the leaves form spots from a fungal disease. Treat the plant as soon as possible to prevent the disease from spreading.