By Julie Christensen
If you think you know lobelias as the common bedding plants found everywhere, you might be surprised to learn that the lobelia family includes more than 400 species, including annuals, perennials, aquatic plants and even shrubs.
The most popular species of lobelia is Lobelia erinus, or common annual lobelia flower. This species is a perennial in U.S. Department of Agriculture plant hardiness zones 10 and 11; otherwise, treat it as an annual. Lobelia has a low, mounding or trailing form, making it ideal for the front of a border or in containers and hanging baskets. It produces thousands of tiny, delicate flowers in a wide variety of colors, including blue, purple, white and red.
Cardinal flower (Lobelia cardinalis) is a perennial species, hardy in USDA zones 2 through 8. This plant grows 2 to 3 feet tall and produces spikes of tubular red flowers. Cardinal lobelia is found in ponds and wetlands. In your yard, it makes a good choice near a water feature or fountain. It prefers moist, acidic soil.
Similarly, blue cardinal flower (Lobelia siphilitica) is another perennial species that prefers moist conditions. Native to the U.S., it’s hardy in USDA zones 4 through 8. Blue cardinal flower grows 2 to 3 feet tall with spikes of blue flowers.
How to Grow Lobelia
Regardless of the type of lobelia you grow, all species grow best in well-draining, but moist to very moist soil. Lobelias tolerate full sun to partial shade, but prefer some shade in hot climates. Lobelias are slow to establish and most gardeners grow them from nursery transplants. You can start annual lobelias from seed, though, 10 to 12 weeks before the last frost. Spread the seed evenly over a tray filled with a starting mix. Don’t cover them with soil though, because they need light to germinate. Store the seeds at 70 to 75 degrees Fahrenheit. Set the seed tray in a sink filled with an inch of water to provide moisture, rather than top watering. This practice discourages damping off, a common problem in lobelia seeds.
Plant lobelias outdoors in spring after the last expected frost. Lighten the soil in flower beds with compost and manure to improve drainage. Be sure to use a light potting mix, rather than garden soil in hanging baskets and containers. Space the young plants 6 inches apart. Water the soil frequently so it stays slightly moist to the touch, especially in hot weather.
Cut annual lobelias back by one-third after the first flush of blooms. This will encourage a compact form and more profuse blooming later. Annual lobelias bloom from early summer to early fall when properly cared for, although the blooms may slow during the heat of mid-summer.
Fertilize annual lobelias every three weeks with a dilute all-purpose plant fertilizer. Fertilize perennial lobelias in the spring as the first flush of growth appears.
Potential Pests and Problems
Annual lobelias are rarely bothered by pest or disease problems, but they may fade during the summer heat. Additionally, the centers of the plants sometimes die and collapse. Regular watering and fertilizing, as well as pruning can alleviate this problem.
Perennial lobelias may suffer from rust, leaf spot or smut. Space the plants so air circulates freely and avoid using overhead sprinklers to keep the leaves dry. Remove any diseased plant parts promptly. Slugs may also afflict perennial lobelias. Remove mulch if slugs are a problem and install slug baits.
Varieties of Lobelia Flowers
- ‘Fountain’ series is an annual lobelia variety with a trailing habit. Use this type for hanging baskets.
- ‘Rosamond’ produces flowers with mauve petals and white centers.
- ‘Rainbow’ series stand 4 to 6 inches tall, with a mounding form. Ideal for the front of a border.
Want to learn more about growing lobelias
Lobelia Erinus from Ohio State University Extension
Lobelia cardinalis from North Carolina State University Extension