From a physiological standpoint, temperature is a primary regulator of a carnation plant's metabolic rate. Carnations (Dianthus caryophyllus) are classified as cool-season plants, and their ideal temperature range reflects this. For optimal growth, flower initiation, and development, a consistent temperature between 10°C and 21°C (50°F and 70°F) is crucial. During the day, temperatures at the cooler end of this spectrum, around 15°C to 18°C (59°F to 64°F), promote strong, sturdy stem growth and vibrant flower color. At night, a drop in temperature to approximately 10°C to 13°C (50°F to 55°F) is highly beneficial. This diurnal temperature variation mimics the plant's natural environment and is essential for respiration management. Cooler nights slow down respiration, allowing the plant to conserve the energy (photosynthates) it produced during the day for growth and flowering, rather than burning it for basic maintenance.
Straying outside the ideal temperature range triggers stress responses that are detrimental to the plant's health and flowering capability. Prolonged exposure to temperatures above 24°C (75°F) accelerates the plant's metabolism excessively. This leads to soft, weak, and elongated stems (etiolation) as the plant stretches for cooler air. High temperatures also hasten flower development, resulting in smaller blooms with poor form and a significantly reduced vase life. Furthermore, heat stress can inhibit flower bud formation altogether. Conversely, temperatures consistently below 5°C (41°F) severely slow metabolic activity, stunting growth and delaying flowering. While carnations can tolerate a very light frost, prolonged exposure to freezing temperatures causes ice crystals to form within plant cells, leading to cell rupture and irreversible damage, manifesting as blackened, water-soaked foliage and flower death.
Humidity, which is the amount of water vapor in the air, directly influences the carnation's transpiration stream—the process of water movement through the plant and its evaporation from leaves. Carnations prefer a moderate relative humidity level, generally between 40% and 60%. At this humidity range, the stomata (pores on the leaf surface) can function efficiently. They remain open enough to allow for the uptake of carbon dioxide (CO2) necessary for photosynthesis, while the rate of water loss is manageable for the root system to replenish. This balance ensures turgid, healthy foliage and supports the high water demands of developing flower buds.
When humidity deviates from the ideal range, the plant's water relations are disrupted. High humidity, levels consistently above 70-75%, creates a significant problem. The saturated air drastically reduces the transpiration rate because the gradient for water vapor loss from the leaf to the air is minimal. This may seem beneficial for water conservation, but it has negative consequences. A slowed transpiration stream reduces the flow of nutrients from the roots to the shoots, potentially leading to nutrient deficiencies. More critically, high humidity creates an ideal environment for fungal pathogens like Botrytis cinerea (gray mold) and powdery mildew, which can quickly destroy foliage and flowers. On the other hand, low humidity (below 40%) accelerates transpiration, causing water to evaporate from the leaves faster than the roots can absorb it. This results in water stress, visible as leaf tip burn, wilting, desiccated flower buds (bud blast), and overall reduced plant vigor.
It is vital to understand that temperature and humidity do not act in isolation; they are intrinsically linked. Warmer air has the capacity to hold more moisture. Therefore, a high temperature combined with high humidity is particularly dangerous, as it creates a "greenhouse" effect that encourages disease. Conversely, high temperatures with low humidity will cause severe moisture stress. For the indoor carnation, the goal is to maintain the cool temperatures it prefers, which naturally helps to keep humidity at a moderate level. Strategies such as providing good air circulation with a small fan can help manage both factors by preventing stagnant, humid air pockets around the foliage and reducing leaf surface temperature.
