To understand the behavior of lobelia grown indoors, one must first examine its inherent biological programming. The genus Lobelia encompasses a wide variety of species, but the common flowering types popular in cultivation, such as Lobelia erinus, are technically tender perennials. This means that in their native habitats or in climates that remain consistently warm and frost-free year-round, they possess the genetic capacity to live for more than two years, regrowing from their root systems. However, their sensitivity to cold temperatures often leads them to be cultivated and sold as annuals in outdoor gardening, where they complete their entire life cycle—from seed, to flower, to seed production—within a single growing season before being killed by frost.
When grown inside, the plant is removed from the primary external factor that dictates its annual behavior: winter frost. The controlled conditions of an indoor environment fundamentally alter the equation. Without a killing frost, the lobelia plant does not have its life cycle forcibly ended. Provided with adequate warmth, light, and moisture, an indoor lobelia can bypass the seasonal die-back it would experience outdoors. Therefore, from the plant's physiological perspective, being moved indoors allows it to express its true, perennial nature. It can continue to grow and, with proper care, survive for several years.
For a lobelia plant to thrive as a perennial indoors, several of its core needs must be met continuously. Light is the most critical factor. As a plant adapted to full sun or partial shade outdoors, it requires very bright, direct light indoors to perform sufficient photosynthesis to sustain itself long-term. A south-facing window is often ideal. Without intense light, the plant will become leggy, fail to produce abundant flowers, and gradually weaken. Temperature is another crucial element; lobelias prefer moderate conditions between 65-75°F (18-24°C) and must be protected from both cold drafts and excessive dry heat from vents. Consistent moisture is required, but the plant is highly susceptible to root rot, so well-draining soil is a non-negotiable necessity to maintain perennial health.
Even under ideal indoor conditions, a lobelia plant will likely exhibit periods of vigorous growth and flowering followed by periods of rest. After a major blooming period, the plant may appear tired or stop producing flowers. This is not the plant dying but rather entering a semi-dormant state to conserve energy. From the plant's viewpoint, this is a natural survival strategy. To encourage renewed growth and flowering, a gardener can prune the plant back significantly, which reduces the energy required to sustain foliage and stimulates the production of new, bushier growth. This cycle of bloom, rest, and pruning can be repeated, reinforcing its perennial habit indoors.
Biologically, the common lobelia used as a bedding plant is a tender perennial. When grown indoors where the threat of frost is eliminated and its core needs are consistently met, it can and will behave as a perennial. It will not complete its life cycle in one season but will persist, going through repeated growth and bloom cycles for multiple years. Its survival is entirely dependent on the simulated environment mimicking its preferred conditions of bright light, moderate temperatures, and moist but well-drained soil, allowing it to express its inherent perennial nature.
