Blue Lobelia Growing Instructions
Blue lobelia (Lobelia siphilitica) features tall flower spikes and bushy foliage, often growing to 3 feet tall with proper planting and care. The herbaceous perennial grows in U.S. Department of Agriculture plant hardiness zones 4 through 9. Blue lobelia works well in rain gardens or wildflower beds, where the long summer blooming season makes it a centerpiece in the landscape.
Beds that receive at least six hours of sun work well for blue lobelia. The plants can tolerate some afternoon shade if they receive direct morning sun. In warmer climates, it does better in partial shade. Well-drained but rich soil works best. In moderate soil, work in a 2-inch layer of compost before you plant to improve both drainage and to increase the organic matter and natural nutrients in the site. You can grow blue lobelia from seed sown directly in the prepared garden bed or from nursery transplants. Space new plants about 12 inches apart to prevent crowding.
Water and Nutrients
Blue lobelia grows best in moist soil, so regular watering results in the healthiest plants and attractive flowers. The plant needs watering about twice a week, especially during dry periods, so the top 6 inches of soil remain moist at all times. Covering the soil with a 2-inch layer of mulch further helps retain moisture so the soil stays damp. Blue lobelia grows as a wildflower and doesn't usually require fertilizer, preferring organic nutrients from compost. Compost also helps retain soil moisture. Spreading a fresh 1- to 2-inch layer of compost over the garden bed each spring as growth resumes provides for most nutrient needs.
Regular deadheading encourages more flowers and improves the appearance of blue lobelia. Pinch off the old flowers after they wilt but before they set seeds. Blue lobelia grows as a short-lived perennial, often not surviving for more than two or three years, so allowing the flowers to set seeds in late summer encourages reseeding for replacement plants without compromising the summer-long bloom period. Blue lobelia may become leggy in midsummer. Pinch back the tips of each stem to the first or second bud from the top to encourage branching and more compact growth.
Blue lobelia tolerates frost without damage, especially if you leave the old stems in place to protect the roots during cold weather. Only cut back the dead stems in spring when growth resumes. Mulching over the blue lobelia bed provides further insulation. The plants don't require winter watering and too much soil moisture during the dormant period can cause root rot and plant death. You can divide blue lobelia every three years if they are growing well. Dig up the plant after it goes dormant and tease apart the roots into two sections. Replant each section at the same depth it was growing at previously.
Jenny Harrington has been a freelance writer since 2006. Her published articles have appeared in various print and online publications. Previously, she owned her own business, selling handmade items online, wholesale and at crafts fairs. Harrington's specialties include small business information, crafting, decorating and gardening.