The popular plant known as the geranium is actually a tender annual belonging to the genus Pelargonium. These bedding plants are native to South Africa and thrive in warm, sunny environments, making them particularly sensitive to cold temperatures and frost. Successfully moving them from a protected indoor space or greenhouse to the garden requires careful attention to environmental conditions and a structured acclimation process. This transition ensures the plants establish themselves quickly and provide vibrant color throughout the summer season.
The Critical Timing: Understanding Frost and Temperature
The single most important factor determining when to plant geraniums is the complete absence of frost. A single unexpected freeze event can cause irreparable damage or death to the plant tissue. Gardeners should consult local historical data to determine the average date of the last expected spring frost, and then wait an additional week or two for a margin of safety.
Beyond the frost date, the overnight low temperature must consistently remain above 50°F (10°C) before the plants are placed outdoors permanently. Planting when temperatures are lower can cause “cold shock,” which severely stunts the plant’s growth and delays flowering. Sustained cold and wet conditions are highly detrimental and can lead to fungal issues.
The soil temperature is also an important metric, as roots struggle to expand into cold, wet soil, delaying the establishment of the plant. Waiting until the soil temperature warms to approximately 60°F helps ensure the geranium’s root system can properly explore the surrounding medium for water and nutrients. Premature planting significantly increases the risk of damage.
Preparing Geraniums for the Outdoors
Before being placed in the ground, geraniums must undergo a process called “hardening off,” which slowly acclimatizes the plants to the harsher outdoor elements. Plants raised indoors or in a greenhouse are accustomed to stable temperatures and still air, and a sudden move outside can lead to sun scorch, windburn, or growth check. This toughening process typically takes between seven and fourteen days.
The hardening off process begins by placing the plants in a sheltered, shaded location outdoors for just a few hours each day. A covered cold frame or a spot against a south-facing wall works well to protect them from direct sun and strong winds. Each day, the duration of time spent outside should be increased by one to two hours to gradually expose the foliage to the varying light and temperature fluctuations.
During the second week, the plants can be exposed to increasing amounts of direct sunlight and left out overnight, provided no frost is forecasted. This gradual exposure helps to thicken the leaf structure and increase the waxiness on the leaves, preparing them for the intensity of the summer sun. After this period, the geraniums will be robust enough for their final location in the garden or container.
Successful Transplanting and Location Needs
Once fully hardened off, selecting the optimal final location is important for a season of abundant blooms. Most common zonal geraniums require a position that receives full sun, meaning at least six hours of direct sunlight per day, though some ivy-leaved or regal cultivars prefer partial afternoon shade. The soil must be well-draining, as geraniums are prone to root rot if they sit in soggy conditions.
When transplanting, the planting hole should be dug twice as wide as the nursery pot, but the plant should be set at the same depth it was growing in the pot. Geraniums generally thrive in a slightly acidic to neutral soil pH, and incorporating organic matter, such as compost, will improve drainage in heavy soils. Proper spacing is also important to ensure good air circulation between plants, which helps to minimize the risk of fungal diseases.
Immediately after planting, the geraniums require a thorough initial watering to settle the soil around the roots and eliminate any air pockets. While they are relatively drought-tolerant once established, consistent moisture is necessary as they root into their new environment. Fertilizer application should be light, as high nitrogen levels promote leafy growth at the expense of flower production.