Great Blue Lobelia (Lobelia siphilitica) is a North American native perennial known for its striking spikes of vibrant blue flowers. It offers a welcome splash of color from late summer into mid-fall. It is a favored choice for gardeners aiming to attract pollinators, as its blooms are a magnet for hummingbirds, bumblebees, and butterflies. Its upright growth adds visual interest to garden spaces.
Ideal Growing Conditions
Great Blue Lobelia thrives in consistently moist, rich, and well-draining soil. It naturally inhabits wet prairies, marshland borders, pond edges, and stream banks, indicating its preference for significant moisture. While adaptable to various soil types, it performs best when the soil is rich in organic matter. A soil pH between 6.5 and 7.5, which is slightly acidic to neutral, is optimal for nutrient absorption.
This plant can grow in a range of light conditions, from full sun to partial shade, and can even tolerate some heavy shade. In hotter climates or full sun exposures, it requires consistent moisture to prevent stress. If the soil dries out, especially during summer dry spells, supplemental watering becomes necessary. It is not drought tolerant and relies on steady moisture for healthy growth.
Planting Your Great Blue Lobelia
The best times to plant Great Blue Lobelia from nursery stock are in spring or fall. Before planting, prepare the site by enriching the soil with organic compost, particularly if your soil is sandy or lacks nutrients. This helps improve both drainage and moisture retention, crucial for the plant’s establishment.
When planting multiple specimens, space them 12 to 18 inches apart to allow for mature growth and spread. For starting from seed, tiny seeds require light to germinate, so they should be surface-sown and not buried deeply. Seeds can be sown outdoors in late fall for natural overwintering and spring germination, or indoors with cold, moist stratification before planting. Seedlings can be transplanted once they are a few inches tall, maintaining consistent moisture.
Ongoing Care and Maintenance
Consistent watering is important for Great Blue Lobelia, especially during dry periods, as it prefers moist conditions. Aim for about 1 inch of water per week, whether from rainfall or supplemental irrigation, allowing the top two to three inches of soil to dry slightly between waterings. Mulching helps retain soil moisture and keeps the root zone cool.
Fertilization is minimal if the soil is rich in organic matter. If plants show signs of nutrient deficiency, a balanced liquid fertilizer (such as 10-10-10) can be applied at half strength every 4-6 weeks during the active growing season, or a slow-release granular fertilizer in early spring. Over-fertilization should be avoided, as it can lead to excessive foliage at the expense of blooms.
Deadheading spent flower spikes can encourage a neater appearance and may promote reblooming, though it also prevents the plant from self-seeding. Great Blue Lobelia is a short-lived perennial, often lasting only a few years, but it readily self-seeds, ensuring its return without becoming invasive. In late fall, avoid cutting back the stalks; instead, mulch lightly to protect the plant’s roots and any newly formed rosettes for winter. Dividing clumps in spring can also help maintain plant vigor every 3-5 years. It is resistant to most pests and diseases.
Troubleshooting Common Problems
Great Blue Lobelia is robust, but issues often stem from improper moisture levels. Wilting can indicate too little water, especially during dry spells or in full sun, requiring prompt and thorough irrigation. Conversely, waterlogging can lead to root rot, as the plant prefers moist but not soggy conditions. Ensuring good drainage in the soil is important to prevent water from accumulating around the roots.
Poor or overly dry soil can also cause the plant to struggle, manifesting as weak growth or reduced flowering. Amending the soil with organic matter before planting and maintaining consistent moisture levels can mitigate these problems. While pest and disease-resistant, slugs may occasionally feed on the leaves. Deer and rabbits are deterred, making it a good choice for areas where these animals are a concern.