Geraniums, cherished for their vibrant blooms, are a popular choice for gardens and containers. While known for their resilience, temperature significantly impacts their health, growth, and flowering. Understanding and managing temperature fluctuations is essential for cultivating thriving geraniums that provide continuous color and vigor throughout the growing season.
Optimal Temperature Range
Geraniums thrive within a specific temperature range. During the daytime, an ideal temperature for mature geraniums is between 70 to 75 degrees Fahrenheit (21-24°C). This warmth encourages active photosynthesis and vegetative development. Nighttime temperatures are also important, with a preferred range of 55 to 65 degrees Fahrenheit (13-18°C). This cooler period allows plants to recover and allocate energy towards flower production. Maintaining these optimal ranges promotes healthy foliage and prolific blooming.
Cold Temperature Effects and Protection
Geraniums are sensitive to cold temperatures. Growth slows considerably below 54°F (12°C) and ceases entirely below 41°F (5°C). Frost is particularly damaging, as geraniums are not naturally equipped to withstand freezing. Symptoms of cold damage include wilting leaves, which may turn brown, black, or become bleached. While plants might recover if only leaves are affected, severe frost can kill the plant if stems and roots freeze.
To protect geraniums from cold snaps, several strategies help. Bringing potted geraniums indoors before the first frost is effective, especially when nighttime temperatures consistently fall below 55°F (13°C). For plants in the ground or those too large to move, covering them with garden fleece, old bedsheets, or cardboard boxes provides temporary insulation during unexpected cold nights. Placing plants close to a house wall can also offer warmth from residual building heat. Elevating pots off the ground helps, as cold air tends to sink and collect at lower levels.
High Temperature Effects and Mitigation
Excessively high temperatures can stress geraniums. Temperatures consistently above 85°F (29°C) can slow or stop new growth and flowering. When temperatures exceed 100°F (38°C), geraniums may exhibit symptoms like wilting, bleached or chlorotic foliage, and reduced blooming. In extreme heat, leaves can become susceptible to sunburn. Heat stress can also cause flowers to cease production.
Providing afternoon shade protects plants from scorching heat, especially in intense summer sun. Moving potted plants to a shadier location or using shade cloths offers relief. Increasing air circulation around plants helps dissipate heat. Adequate and consistent watering is important, as geraniums require more moisture during hot spells to prevent wilting. Watering in the morning and evening helps keep the soil moist without promoting fungal issues.
Year-Round Temperature Management
Year-round temperature management ensures geraniums remain healthy and vibrant. As temperatures drop in autumn, typically when nighttime lows consistently fall below 55°F (13°C), transition outdoor geraniums indoors. This move should occur before the first hard frost to prevent cold damage. Geraniums overwintered indoors prefer cool, bright conditions, with daytime temperatures around 65-70°F (18-21°C) and nighttime temperatures around 50-60°F (10-16°C). They need ample bright light, such as from a south-facing window, to continue thriving.
During their indoor stay, reduce watering significantly, only when the soil surface becomes dry, to prevent root rot in cooler conditions. Air circulation is important indoors to prevent fungal issues. As spring approaches and outdoor temperatures become consistently mild, typically after the last frost has passed, gradually reintroduce geraniums to outdoor conditions. This hardening-off process involves exposing them to increasing periods of outdoor light and temperature over several days before permanent placement. Proper watering tailored to temperature fluctuations and good air circulation, both indoors and outdoors, will support continuous plant health and vigorous growth.