Like all perennials, the geraniums (Pelargonium) have a set of circumstances under which they bloom and a seasonal period during which they bloom. Most gardeners, though, can get these tender perennials to flower continuously through the summer. Some even take plants indoors to bloom through the winter. The cultural methods they use are more appropriate to annuals than perennials, hence the term "grown as annuals." Pelargonium varieties grown as annuals include common zonal (P. hortorum), spectacular Martha Washington (P. domesticum) and trailing ivy (P. peltatum) geraniums.
Choose zonal or ivy geraniums for continuous bloom. Keep plants indoors until the ground warms to 60 degrees and all signs of frost are past. Introduce them to their summer home gradually to minimize shock that may stop flowering processes. Bring them in at night, then every few nights until nighttime temperatures stay above 50 degrees.
Plant geraniums in light, well-drained soil; geranium roots need water and air. Allow soil to dry before watering, then water deeply; do not allow plants to stand in water. Place them where they will receive at least eight hours of sunlight a day and be sheltered from strong winds. Hanging baskets must be checked and watered daily if necessary. To a geranium, the combination of sun and water says that it is early summer and time to bloom.
Fertilize geraniums conservatively. Work 4 lbs. of 5-10-5 garden fertilizer per 100 square feet of garden into the soil before planting and fertilize every three weeks with a half-strength solution of 10-10-10 liquid fertilizer or work in 2 lbs. of 5-10-5 dry fertilizer per 100 square feet every four weeks.
Deadhead geraniums continually, removing flowers as they begin to fade. Keep the plant tidy; remove dead or drying leaves to help the plant conserve energy. When branches suddenly bolt, or outgrow the rest of the plant, prune them back to about 4 inches long. The rest of the plant will resume flowering as the pruned branch starts new flowering branches.
Provide some shade for geraniums that are planted in full sun when summer heat sets in. Morning sun and a few hours in late afternoon will make up the eight hours needed without risking sun scorch or drought stress, both of which will slow flowering.
Bring geraniums in when nighttime temperatures begin to fall below 50 degrees. Reverse the hardening off process used last spring and put the plants in a south- or east-facing window. Supplement the less intense indoor light (and shortening seasonal sunlight) with a fluorescent light. Suspend it about 18 inches above your plants for three to four hours a night to make up the light deficit.