A family of well-cultivated species in a variety of colors, sizes and seasonal growth patterns, lobelia offers something for every gardener to enjoy. From tall spikes bearing scarlet red blooms to trailing, low-mounding bright blue buds, lobelia also comes in white, pink, and purple shades of flowers. Whether you choose a perennial species like Lobelia cardinalis or L. siphilitica, or annual species L. erinus, growing requirements vary little.
All lobelia, whether annual or perennial, prefer partial shade. When choosing a site, be sure that the area will have at least partial shade for most of the day and protection from the harshest mid-day sunshine for best results.
Most lobelia planting beds need additional organic matter, such as composted leaves or manure, to improve drainage and add nutrients. Till up to 3 inches of organic matter up to 8 inches deep into the soil prior to planting.
Keep soil moist to improve lobelia plant size and health, and to encourage blooming. Annuals require up to 1 1/2 inches of water per week, according to Clemson University Cooperative Extension experts. Perennials should be watered less frequently and deeply to encourage root establishment for subsequent seasons. Allow the topsoil to dry completely between waterings.
A 10-10-10 balanced fertilizer should be applied and thoroughly combined with the soil at a rate of 1 lb. per 100 square feet at the time of bed preparation. Additional fertilizer may be applied, in liquid form, to improve annual growth or bloom as needed.
Provide an organic mulch cover, such as composted pine needles, around perennial plants as fall approaches to protect root systems over the winter. Perennial species of lobelia can be cut back to just above ground level after blooming has ceased.