Geraniums are widely grown not only because they produce large, colorful flowers but also because they thrive on little water -- which is perfect for the busy or sometimes forgetful gardener. While geraniums grow well outdoors, they are just as suitable for growth in greenhouses. Growing geraniums in a greenhouse ensures the plants flourish year-round. Geraniums grown indoors require less fertilization and winter preparation than those grown outside.
Plant the geraniums in well-draining soil composed of one part sand and one part peat. Plants should be placed in clay or plastic pots with adequate drainage holes. Place a saucer underneath the plants to catch any excess water.
Position the geraniums in a location where they will receive full sunlight. If the geraniums are not in a location where they will receive 12 hours of sunlight, use fluorescent lighting to supplement natural light. Keep the greenhouse at a temperature of 65 degrees Fahrenheit during the day and 56 degrees Fahrenheit in the evening.
Water the geraniums lightly, once or twice per week. Geraniums do not need much moisture and should be kept on the dry side as the plants rot if they are too moist. To determine whether or not the plants require water, touch the soil. If it is dry, water lightly; if it is moist, don’t add any more water. Empty the drainage trays underneath the plants a few minutes after watering.
Pinch or prune the plants in the spring to encourage the plants to become bushier. Deadhead any dead flowers to encourage new flowers to bloom.
Fertilize the plants during their growth period, which for indoor geraniums varies depending on when the plants were propagated. Use a water soluble 20-20-20 fertilizer, mixing one tablespoon of fertilizer with 1 gallon of water.