The vibrant flowers commonly called geraniums are botanically classified as Pelargoniums. Growing these annuals from seed offers a cost-effective path to a wide selection of varieties unavailable as nursery starts. Success hinges on precise timing and creating the optimal environment for the seeds and subsequent seedlings.
Essential Preparation Before Planting
Calculate the ideal sowing date, which should be 10 to 12 weeks before the last expected spring frost in your region. This early indoor start allows plants sufficient time to mature and begin flowering shortly after being moved outside. Select high-quality, fresh seeds and pair them with a sterile, fine-textured seed-starting mix to discourage fungal diseases.
Geranium seeds possess a hard outer coat that can inhibit water absorption and lead to erratic germination. To overcome this natural dormancy, pre-treatment is highly beneficial. Scarification involves gently scratching the seed coat with fine-grit sandpaper to allow moisture penetration. Alternatively, soaking the seeds in room-temperature water overnight helps soften the coat and encourages more uniform sprouting.
The Process of Sowing Geranium Seeds
Once the seeds are pre-treated, thoroughly moisten the seed-starting mix until it is uniformly damp but not waterlogged. Fill clean, sterile seed trays or small planting cells, about two to three inches in size, with the prepared medium to prevent pathogens that can harm young sprouts.
Sow a single seed into each cell or pot to prevent overcrowding and minimize root disturbance during transplanting. Plant the seeds shallowly, covering them with only a thin layer of starting mix or vermiculite, typically about one-eighth of an inch deep. After sowing, water the trays gently using a fine mist to settle the covering material.
Managing Germination and Seedling Growth
Maintaining a consistently warm soil temperature is the most important factor for successful germination of Pelargonium seeds. The growing medium temperature must be held steadily between 70°F and 75°F, a condition often best achieved using an electric heat mat placed beneath the seed trays. This warmth encourages the rapid metabolic activity needed for the seeds to break dormancy and sprout, which can occur within three to 21 days.
To maintain high humidity and prevent the surface from drying out, cover the trays with a clear plastic dome or plastic wrap. As soon as the first seedlings emerge, immediately remove the covering to allow for air circulation and prevent damping-off disease. Once germinated, the air temperature can be slightly cooler, ideally kept around 70°F during the day and no lower than 60°F at night.
Seedlings require intense light for 12 to 16 hours daily to develop into sturdy plants, a requirement rarely met by a windowsill alone. Position the young plants four to six inches beneath fluorescent or LED grow lights to prevent them from stretching and becoming weak. Once the first pair of true leaves appears, begin watering from the bottom by placing the trays in a shallow pan of water for 30 minutes, allowing the medium to wick up moisture. At this stage, a diluted, half-strength liquid fertilizer can be introduced weekly to support vigorous growth.
Hardening Off and Transplanting Outdoors
The final stage involves gradually preparing the indoor-grown seedlings for the outdoor environment through a process called “hardening off.” This acclimation strengthens cell walls and reduces transplant shock by slowly exposing the plants to direct sunlight, wind, and fluctuating temperatures. Begin this transition seven to ten days before the final planting date.
Start by placing the seedlings in a sheltered, shaded outdoor location for only one or two hours on the first day. Over the following week, incrementally increase the duration they spend outside and the amount of direct sun exposure. The final transplant outdoors should only occur after all danger of frost has passed and nighttime temperatures remain reliably above 40°F. Geraniums thrive in full sun and require rich, well-drained soil.