Lobelia, a genus encompassing both annual and perennial species, thrives when its specific environmental needs are met. Understanding the ideal temperature and humidity from the plant's perspective is key to promoting vigorous growth, prolific blooming, and overall plant health.
For most Lobelia erinus (the common annual bedding variety), the ideal temperature range for active growth and flowering is between 15°C and 21°C (60°F and 70°F). At these temperatures, metabolic processes like photosynthesis and nutrient uptake operate at peak efficiency. This results in strong cell development, compact and bushy growth habits, and the continuous production of its characteristic vibrant flowers. Temperatures consistently above 24°C (75°F) can cause the plant significant stress. The heat accelerates transpiration and respiration rates, leading to water loss and energy depletion. This often manifests as wilting, halted flower production, and leggy, weak growth as the plant struggles to cope. Conversely, Lobelia is highly sensitive to frost and freezing temperatures, which will cause immediate and severe damage to its tender foliage and stems.
A crucial aspect often overlooked is the benefit of a nighttime temperature drop. For Lobelia, a slight decrease of a few degrees at night is highly beneficial. It mimics its natural conditions and helps reduce respiratory loss, allowing the plant to conserve energy. Furthermore, some perennial lobelia species, such as Lobelia cardinalis (Cardinal Flower), require a period of cold dormancy to thrive in subsequent seasons. This process, known as vernalization, is triggered by prolonged exposure to temperatures near or just above freezing. Without this cold period, the plant may not flower as robustly or may have a shortened lifespan.
Lobelia prefers a moderate to high humidity environment, generally around 50-70%. From the plant's perspective, humidity is directly tied to transpiration—the process of water movement through the plant and evaporation from its leaves. In ideal humidity, the stomata (pores on the leaf surface) can remain open for efficient gas exchange (taking in CO2 and releasing O2) without risking excessive water loss. This balance supports steady growth. In conditions of low humidity, the plant will close its stomata to conserve water, which subsequently halts photosynthesis and can stunt growth. However, consistently saturated air (very high humidity) presents a significant danger, as it creates the perfect environment for fungal pathogens like powdery mildew and botrytis to colonize the leaves and stems.
These two factors do not act in isolation. Their interaction is critical. High temperatures combined with low humidity will cause extreme moisture stress, requiring constant watering to prevent collapse. Conversely, high humidity coupled with cool temperatures and poor air circulation will almost certainly lead to fungal disease and root rot. The goal is to maintain a stable environment where the plant is not forced to constantly open and close its stomata, allowing for consistent metabolic function and health.
