Your Guide to Growing Beautiful Lobelia Seedlings

Lobelia, with its dazzling cascades of blue, purple, red, or white blooms, brings unparalleled charm to gardens and containers. Starting lobelia from seed is a rewarding way to fill your space with these vibrant flowers. Whether you’re aiming for overflowing hanging baskets or dramatic perennial borders, mastering the art of growing lobelia seedlings is the first exciting step.

Here’s what you need to know:

Lobelia thrives in sun or partial shade and consistently moist soil.
Annual lobelias (Lobelia erinus) are perfect for containers and hanging baskets, while perennial types like Lobelia cardinalis add drama to beds.
Starting lobelia seedlings indoors early in the season gives you a head start on vibrant summer color.
Caring for young seedlings involves providing warmth, light, and careful watering.
Perennial lobelias require minimal care once established but appreciate winter protection in colder climates.

Ready to embark on this colorful journey? Let’s dig in and discover how to nurture tiny lobelia seedlings into stunning displays.

Getting Started with Lobelia Varieties

Before you sow, it’s helpful to know the two main types you’ll likely encounter: annual and perennial lobelias.

Annual lobelias, typically varieties of Lobelia erinus, are the familiar trailing or mounding plants seen spilling from pots and baskets. They flower profusely all summer until the first frost and are generally easy to grow from seed, making them a prime candidate for starting as lobelia seedlings.

Perennial lobelias, including species like Lobelia cardinalis (Cardinal Flower), Lobelia siphilitica (Blue Cardinal Flower), and hybrids like Lobelia x speciosa, offer striking upright flower spikes and often dramatic foliage. While some perennial types can be started from seed, division or cuttings are also common propagation methods. They tend to be short-lived but return year after year in suitable conditions, often moist to wet soil, thriving in sun or partial shade. Some perennial types, like Lobelia cardinalis, prefer consistently wet conditions, even boggy soil, in sun or partial shade (typically hardy in USDA Zones 3-9), while others like Lobelia tupa (‘Devil’s Tobacco’) need full sun and well-drained soil and may require winter protection (hardy in Zones 8-10). Handle perennial lobelias with gloves as their milky sap can irritate skin.

Sowing the Seeds: Your Guide to Lobelia Seedlings

Starting lobelia seedlings indoors is one of the most rewarding parts of the gardening year. You get to witness the miracle of germination and watch tiny specks transform into robust plants ready for your garden.

When to Sow

For summer blooms, start annual lobelia seedlings indoors early, typically 10-12 weeks before your last expected frost. This usually falls around late February or March. Perennial lobelia seeds can also be sown in early spring.

What You’ll Need

Gather your supplies:

High-quality lobelia seeds (they are very tiny!).
Seed starting mix or multi-purpose compost mixed with a little sand for drainage.
Seed trays or small pots.
Clear plastic bags, a propagator lid, or a heated propagator.
Spray bottle for gentle watering.
Optional: Silver sand to mix with seeds, slow-release fertilizer.

Step-by-Step Sowing Guide

Fill your seed trays or pots with moist seed starting mix. Gently firm the surface.
Lobelia seeds are minute. To help distribute them evenly, you can mix the tiny seeds with a pinch of silver sand.
Scatter the seed/sand mixture thinly over the surface of the soil. Do not cover the seeds with soil, as they need light to germinate.
Mist the surface gently with a spray bottle to settle the seeds without burying them.
Cover the trays or pots with a clear plastic lid or place them inside a clear plastic bag to create a humid environment.
Provide warmth. Lobelia seeds germinate best with soil temperatures between 18-24°C (65-75°F). A heated propagator is ideal, or place trays in a warm spot like on top of a refrigerator or near a heater (but not directly on it) until germination. Ensure they receive light during this period.

Caring for Your Lobelia Seedlings

Germination can take anywhere from 1 to 3 weeks. Once tiny green shoots appear:

Remove the plastic cover immediately to allow airflow and prevent damping off (a fungal disease).
Move the seedlings to a bright location with plenty of light. A south-facing windowsill is good, but supplemental light from grow lights will prevent them from becoming leggy. Keep lights just a few inches above the seedlings for 12-16 hours a day.
Keep the soil consistently moist, but not waterlogged. Watering from the bottom by placing trays in a shallow tray of water for a few minutes is an excellent way to ensure roots get moisture without disturbing the delicate lobelia seedlings.

When the seedlings have their first set of true leaves and are large enough to handle (this can take a few weeks as they grow slowly at first):

Pricking Out: Carefully transplant clusters of seedlings (they are often grown in small groups) into individual small pots or cell packs filled with a slightly richer potting mix. Handle them by a leaf, not the stem, which is easily damaged.
Potting On: As they grow larger and roots begin to fill their small pots, move them into larger pots (around 7-9cm) to give them more space to develop before planting out.

Transplanting Lobelia Seedlings Outdoors

Lobelia is tender and cannot tolerate frost. Wait until all risk of frost has passed in your area, usually in late May, before planting your lobelia seedlings outdoors.

Harden Off: Before planting, gradually acclimate your seedlings to outdoor conditions over a week or two. Start by placing them outside in a sheltered spot for a few hours each day, increasing the time and exposure to sun and wind daily.
Prepare the planting area by incorporating compost or other organic matter.
When planting annual lobelias in containers or hanging baskets, mix garden compost into the potting mix to help retain moisture. You can also add a slow-release fertilizer at this stage.
Plant seedlings (or small groups) about 10-15cm (4-6 inches) apart for annual types, depending on their mature size and desired look. Perennial types may need different spacing; check the specific variety’s requirements.
Water thoroughly after planting.

Trailing annual lobelia spilling from a hanging basket, perfect for partial shade

Choosing the Right Spot: Where Lobelia Thrives

Selecting the ideal location is key to happy, free-flowering lobelia plants, whether they started as lobelia seedlings or were purchased as plugs.

Light and Soil Requirements

Most lobelias perform best in a location that receives full sun to partial shade. In hotter climates, partial shade, especially during the hottest part of the day, can help prolong flowering and prevent the plants from drying out too quickly.

Critically, lobelias need reliably moist soil. They do not tolerate drying out. This is particularly true for many perennial types like Lobelia cardinalis, which naturally grow in damp meadows or along stream banks and need consistently wet, fertile soil. Even annual types in containers will struggle if their soil dries out.

Annual vs. Perennial Needs

Annual Lobelias (Lobelia erinus): Perfect for containers, window boxes, and hanging baskets where their trailing or mounding habit shines. Partial shade is often beneficial in hot summers to keep them looking fresh and flowering longer.
Perennial Lobelias: Needs vary slightly.
- Lobelia cardinalis requires very moist, fertile soil and thrives in sun or partial shade.
- Lobelia tupa prefers full sun in a sheltered spot with fertile, well-drained soil.
- Lobelia x speciosa varieties also do well in moist soil in sun or partial shade.

Adding slow-release fertilizer granules to potting mix when planting annual lobelia in a container

Planting Out Your Lobelia

Whether you’ve nurtured them from lobelia seedlings or bought young plants, proper planting gives them the best start.

For annual lobelias going into pots or baskets, use a good quality multi-purpose compost amended with garden compost or other organic matter to boost moisture retention. Mixing in a slow-release fertilizer at planting time provides steady nutrients for continuous flowering. Plant after the last frost, spacing trailing types about 10-15cm apart.

For perennial lobelias, spring is the ideal planting time. Prepare the bed by digging over the soil and incorporating plenty of organic matter like compost or well-rotted manure, especially if you’re aiming for moisture-loving types like Lobelia cardinalis. Dig a hole the size of the rootball, place the plant so the top of the rootball is level with the soil surface, and backfill. Water in well and mulch around the base with garden compost to help keep the soil moist.

Essential Care for Flourishing Lobelia

Consistent care will keep your lobelia blooming beautifully from summer through fall.

Watering Needs

This is perhaps the most critical aspect of lobelia care. Lobelias, especially those in containers and hanging baskets or moisture-loving perennial varieties, need consistently moist soil. Never let the soil dry out completely, as this severely impacts flowering and can cause the plant to decline rapidly. Water regularly, especially during dry periods and hot weather.

Watering container-grown lobelia to maintain consistently moist soil, essential for healthy plants

Feeding Your Plants

If you added slow-release fertilizer when planting lobelia seedlings or young plants, they should be well-fed for several weeks. For annual lobelias in containers, supplementing with a liquid feed is beneficial. Start with a balanced liquid feed every two weeks in spring to encourage growth, switching to a high-potash feed in summer to promote abundant flowering. Perennial lobelias benefit from a balanced all-purpose feed in spring and early summer, followed by a high-potash feed from midsummer.

Pruning and Deadheading

Annual lobelias generally don’t require deadheading; they are self-cleaning. However, if annual plants start to look tired by midsummer, you can cut them back by about a third. This encourages bushier growth and a new flush of blooms. Deadhead perennial varieties once their flower spikes have faded.

Winter Care

Annual lobelias will die back with the first frost and should be removed. For perennial lobelias, cut back the old foliage in autumn. Applying a generous layer of mulch around the crown in autumn provides crucial protection for perennial varieties during cold winters, helping them survive until spring. Some types, particularly Lobelia tupa, are less cold-hardy and may need extra protection or be treated as annuals in colder zones.

Propagating Beyond Seedlings

While starting lobelia seedlings is fantastic, there are other ways to increase your stock of perennial varieties:

Division: Many perennial lobelias can be divided in spring as new growth emerges. Gently lift the clump and split it into smaller sections, ensuring each has roots and shoots. Replant immediately.
Softwood Cuttings: Lobelia cardinalis can be propagated from softwood cuttings taken in summer.

Common Lobelia Problems and How to Fix Them

Even with the best care, you might encounter a few issues.

Wilting: The most common cause is insufficient water. Lobelias must have consistently moist soil. Water deeply and regularly.
Slugs and Snails: These pests are attracted to the new foliage of perennial lobelias in spring. Use your preferred methods to protect emerging plants.
Crown Rot: This can be a problem for perennial lobelias in cold, wet winter soil. Good drainage is important (except for bog-loving types), and mulching helps protect the crown. In areas with very severe, wet winters, you might consider treating less hardy perennials as annuals and replanting each year.

Selecting Lobelia Varieties

The world of lobelia offers diverse options in color and form. Here are a few popular types:

Lobelia x speciosa ‘Scarlet Fan’: Features dark bronze-green foliage and striking spikes of vibrant red flowers. Often grown as a biennial, reaching about 60cm tall.
Lobelia tupa (Devil’s Tobacco): An unusual perennial with dark red-purple stems and blood red, tubular flowers. Can grow up to 1.2m tall and 90cm wide.
Lobelia erinus ‘Sapphire Cascade’: A classic trailing annual variety with beautiful sapphire-blue flowers accented by a white eye. Ideal for hanging baskets, growing to about 15cm tall and 25cm spread.
Lobelia cardinalis ‘Queen Victoria’: A stunning perennial with deep maroon foliage topped with spikes of dark red flowers from August to October. Grows to around 90cm tall.
Lobelia ‘Compton Pink’: A perennial variety with an upright habit and pale pink flowers, perfect for cottage gardens or herbaceous borders. Reaches about 80cm tall.
Lobelia erinus ‘Cambridge Blue’: A compact, mounding annual variety known for its brilliant, sky-blue flowers. Stays relatively small, around 10cm tall and 15cm spread.
Lobelia siphilitica (Blue Cardinal Flower): A perennial variety with tall spires of brilliant blue flowers. Suitable for moist conditions, including bog gardens or pond edges, growing to about 90cm tall.

Vibrant red spires of Lobelia speciosa 'Scarlet Fan' in bloom

Unique tubular red flowers of Lobelia tupa, or Devil's tobacco

Cascading sapphire-blue flowers of trailing Lobelia erinus 'Sapphire Cascade'

Tips for Buying Lobelia

If you prefer to buy rather than start from lobelia seedlings:

Annual lobelias are widely available as small plug plants in spring at garden centers and online – an economical way to get many plants if you can grow them on indoors or in a greenhouse for a few weeks.
Check the specific needs of perennial lobelias before buying to ensure you can provide the required consistently moist soil and light conditions.
For the best selection of perennial varieties, look for specialist nurseries focusing on bog or pond plants, or explore online retailers.

Conclusion

Growing lobelia seedlings into thriving plants that burst with color is a truly rewarding experience. By providing the right start, consistent moisture, and appropriate care, you can enjoy the beauty of lobelia in your garden all season long. Whether you choose the cheerful cascades of annuals or the dramatic spires of perennials, lobelia is sure to be a highlight.

Have you started lobelia from seed? Share your tips and experiences in the comments below! What’s your favorite lobelia variety? Explore more gardening guides and plant features on Thelittle.garden.