Lobelia is a diverse genus within the family Campanulaceae, comprising over 400 species that range from annual bedding plants to perennial shrubs. When considering Lobelia for indoor cultivation, the primary focus shifts to species and cultivars that tolerate the lower light levels, stable temperatures, and container life of an indoor environment. The key is selecting types that are compact, have a longer blooming period, and do not require the intense, full-sun conditions that many outdoor Lobelias demand. These plants are typically photoperiodic, meaning their flowering is influenced by day length, which can be managed indoors with artificial lighting.
Lobelia erinus is the most commonly recognized species, famed for its prolific blooms in shades of blue, violet, white, and pink. For indoor growing, compact or trailing cultivars are most suitable. The 'Cascade' or 'Fountain' series, for example, are bred for their vigorous trailing habit, making them ideal for hanging baskets placed near a bright window. These cultivars are technically short-lived perennials but are often grown as annuals. They thrive in cool, bright conditions. A south or east-facing window that provides several hours of indirect light is optimal. The plant's physiology requires consistent moisture but is highly susceptible to root rot, necessitating a well-draining, peat-based potting mix.
While the wild species Lobelia cardinalis is a tall, moisture-loving perennial for bog gardens, the cultivar 'Queen Victoria' has been successfully adapted as a dramatic indoor or conservatory plant. This variety is prized for its striking bronze to burgundy foliage and brilliant scarlet flower spikes. Unlike L. erinus, it is a true herbaceous perennial. Its growth cycle involves a period of winter dormancy. To grow it indoors, it requires a very bright location, such as a south-facing window, and consistently moist soil. It is much larger than other indoor Lobelias, often reaching 2-3 feet in height, making it a substantial specimen plant. Its high humidity requirement can be a challenge in dry indoor air, often necessitating a pebble tray or humidifier.
Similar to L. cardinalis, Lobelia siphilitica is a hardy perennial known for its blue flowers. Compact cultivars can be grown indoors in large containers. It shares similar cultural requirements with L. cardinalis, demanding bright light and constantly moist soil. Its value indoors is as a structural plant with vertical flower spikes. It is less common as a houseplant than L. cardinalis but offers a unique color variation for the dedicated indoor gardener seeking a challenge. Like its relative, it will require a cool, dormant period to thrive over multiple years.
For a truly unconventional indoor specimen, Lobelia tupa offers a dramatic, architectural presence. This perennial species can grow quite large but can be container-controlled. It features tall stems, large, fuzzy, grey-green leaves, and unusual, tubular, reddish-orange flowers. It is less about continuous floral display and more about its bold foliage and form. It requires a very sunny spot and excellent air circulation to prevent fungal issues. Due to its size and specific needs, it is best suited for a bright sunroom or conservatory rather than a typical living room.
Regardless of the type, all Lobelias share certain physiological traits that must be managed indoors. They are generally cool-season plants, preferring temperatures between 60-70°F (15-21°C). High temperatures can cause flowering to cease and may induce dormancy. Most Lobelia species have a shallow, fibrous root system that is efficient at nutrient uptake but dries out quickly. This necessitates frequent watering, but with careful attention to drainage to prevent waterlogging. Furthermore, Lobelias are considered toxic due to the presence of various alkaloids, such as lobeline, which can cause discomfort if ingested by humans or pets, a crucial safety consideration for an indoor plant.
