First, you must understand that we are not all the same. In the horticultural world, we are broadly categorized. The vibrant, low-growing Lobelia erinus (edging or trailing lobelia) you see in containers and garden borders is typically treated as a tender annual in most of the USA. We cannot survive frost and complete our entire life cycle in one growing season. However, our cousins like Lobelia cardinalis (Cardinal Flower) and Lobelia siphilitica (Great Blue Lobelia) are hardy herbaceous perennials. We die back to the ground in winter and return from our roots each spring in USDA zones 4-9, bringing tall, stunning spikes of color to your garden.
To help us thrive, you must replicate our preferred natural habitat. We Lobelia plants crave consistently moist, well-draining soil that is rich in organic matter. Think of the damp meadows and stream banks many of us call home. While we love moisture, we despise having our roots sit in waterlogged soil, which leads to rot. For light, most of us prefer full sun to partial shade. In hotter regions of the USA, providing us with afternoon shade is crucial to prevent us from wilting and shutting down flower production during the peak summer heat.
For annual varieties, you will often start us from seed indoors 10-12 weeks before the last expected spring frost. Our seeds are tiny and need light to germinate, so press them gently into the soil surface without covering them. Keep the seed tray consistently moist and warm. For perennial types, you can divide our clumps in early spring every 2-3 years. This rejuvenates us and gives you new plants. Whether planting seedlings or divisions, space us according to our mature size and water us in thoroughly to settle the soil around our roots.
Our most critical demand is consistent moisture. Please check the soil regularly, especially during dry spells and for those of us in containers. A layer of organic mulch around our base helps immeasurably by conserving water, cooling our roots, and suppressing weeds. To encourage a long season of prolific blooming, feed us with a balanced, water-soluble fertilizer every 4-6 weeks. For annual lobelias, a mid-summer trim—cutting us back by about half—can revitalize us and promote a fresh, dense flush of flowers and foliage as the weather cools.
While generally resilient, we can face a few challenges. Our love for moist soil can sometimes attract unwanted attention. Slugs and snails find our tender leaves quite delicious, especially on young plants. Diatomaceous earth or beer traps can deter them. In crowded or humid conditions, we can be susceptible to foliar diseases like powdery mildew or botrytis. Ensuring good air circulation around us by proper spacing and avoiding overhead watering, especially in the evening, is the best defense to keep our foliage dry and healthy.
