The most profound difference in care stems from the plant's core biological strategy. Annual lobelias (like Lobelia erinus) complete their entire life cycle—germination, growth, flowering, and seed production—within a single growing season. Their objective is rapid, prolific flowering to ensure genetic legacy through seeds. Consequently, they direct nearly all their energy into continuous bloom production. Perennial lobelias (like Lobelia cardinalis or Lobelia siphilitica), in contrast, are built for longevity. Their goal is to survive for multiple seasons. They allocate a significant portion of their energy not just to flowering, but to developing and sustaining a robust root system (often a crown or rhizome) that will allow them to overwinter and regenerate the following spring.
While both types appreciate bright light, their tolerance for intense heat differs due to their life cycle length. Annual lobelias thrive in full sun to part shade but are notoriously sensitive to high summer heat. During hot spells, they often cease flowering, become stressed, and may require relocation to a cooler, part-shade location to survive the season. Perennial lobelias, with their established root systems, are generally more resilient. However, many perennial species are native to woodland margins or moist meadows, meaning they prefer and perform best in cool, consistently moist soil. They often struggle in hot, dry, full-sun exposures unless soil moisture is perfectly maintained.
This is a critical care distinction. Annual lobelias require consistently moist, well-draining soil. Their shallow root systems and constant flowering make them quick to wilt and suffer under drought stress. However, they also cannot tolerate waterlogged, soggy soil. Perennial lobelias have a much higher and non-negotiable demand for water. They are moisture-loving plants, with many species thriving in boggy conditions or even at the edge of water gardens. Allowing the soil of a perennial lobelia to dry out is a primary cause of failure, as it directly stresses the plant's survival organ—its perennial crown.
The fertilization strategy aligns with their energy goals. To support their marathon of blooms, annual lobelias benefit from regular feeding. A balanced, water-soluble fertilizer applied every two to four weeks encourages sustained flowering throughout the season. Perennial lobelias have different nutritional priorities. Their focus is on root health and preparing for winter dormancy. They typically require less frequent fertilization. A single application of a balanced or phosphorus-heavy fertilizer in early spring as new growth emerges is often sufficient. Over-fertilizing, especially with high-nitrogen formulas later in the season, can promote soft, late growth that is vulnerable to winter damage.
This care category is exclusive to perennial lobelias. As autumn approaches, a perennial lobelia’s foliage will naturally die back. The plant is not dying; it is entering a dormant phase, redirecting energy to its roots. Care involves ceasing fertilization, reducing watering, and, after the first frost, cutting the dead stems back to a few inches above the crown. In colder climates, applying a protective layer of mulch after the ground freezes helps insulate the crown from freeze-thaw cycles. Annual lobelias require no such care; they are simply removed from the garden after being killed by frost, as their life cycle is complete.
