Lobelia: Trailing Blue Classic for UK Gardens

The name lobelia covers two very different UK garden plants. There is the tiny trailing annual Lobelia erinus that drips blue from every hanging basket across Britain every summer. And then there is Lobelia cardinalis, a tall red perennial for wet clay and pondsides that looks nothing like its small cousin. Most gardeners only know the trailing one. This guide covers both properly.

My own Staffordshire trials have focused on squeezing the longest possible flowering season out of the annual forms. The answer turns out to be early sowing, pelleted seed, and a hard midsummer trim. For more on summer planting schemes, see our guide to the best annual bedding plants for UK gardens.

When should I sow lobelia seed in the UK?

Sow lobelia seed indoors between mid-January and late February for flowering baskets by May. Lobelia needs 12-14 weeks from sowing to first flower. Germination alone takes 3 weeks which is slow compared to almost any other annual. Start too late and your baskets will not flower until July.

Use a heated propagator set to 18-20C. Sow on the surface of damp seed compost. Do not cover the seed. Lobelia needs light to germinate. Cover the tray with a clear lid or bag to keep humidity high. Never let the surface dry out or germination stops permanently. See our guide to sowing seeds indoors for the full method with heat mats and propagators.

Why is lobelia seed so hard to handle?

Raw lobelia seed is almost as fine as dust and impossible to sow evenly. A single packet contains several thousand seeds. Sprinkle directly from the packet and you get dense clumps with bald patches between. The result is patchy trays and wasted seed.

Pelleted seed solves this completely. Each seed sits inside a clay coating that breaks down when damp. You can pick up individual pelleted seeds with damp tweezers. Sow 2-3 pellets per cell or every 2cm in a tray. The extra cost (roughly £1 more per packet) is worth every penny. Pelleted seed has transformed my sowing success rate.

If you only have raw seed, mix it with silver sand at 1 part seed to 10 parts sand. Sprinkle the mix thinly. It is not as accurate as pelleted but workable.

What are the best annual lobelia varieties?

Crystal Palace has the deepest blue flower of any lobelia variety. Almost navy with bronze foliage. It is upright rather than trailing so works best in bedding schemes and pot edges. Cambridge Blue is lighter sky blue with green leaves. Sapphire is a classic trailing type with a white eye.

Crystal Palace (Lobelia erinus)

Upright compact habit reaching 12-15cm. Dark bronze foliage makes the deep blue flowers pop. Excellent for edging and front-of-border bedding. Flowers from June to October with trimming. Hold its shape well without pinching.

Sapphire

Trailing variety with a distinctive white eye in each blue flower. Trail length 30-40cm by midsummer. The traditional hanging basket lobelia. Works brilliantly with white bacopa and pink petunias.

Cambridge Blue

Soft sky-blue flowers on a compact upright plant. Grows 15cm tall. Useful in pots and bedding where you want a lighter colour than Crystal Palace. Open habit makes it airy rather than dense.

Cascade series

Trailing forms bred for hanging baskets. Available in Blue Cascade, Red Cascade, White Cascade, and Cascade Mixed. Trail length reaches 40cm+. Colour choice is the main reason to pick this series over Sapphire.

Variety comparison at a glance

Here is how the main UK lobelia varieties compare for habit, colour, and best use.

How do I pinch lobelia seedlings for bushier plants?

Pinch out the growing tip when seedlings reach 5cm tall to force side shoots. Use fingers or small scissors to remove the top 1cm of growth. The plant responds by producing 2-3 new shoots from the leaf joints below. Each new shoot flowers, giving you denser coverage.

Pinch once at 5cm and again at 10cm if the plants are slow to bush up. Do not pinch after the first flower buds form or you delay flowering for 2-3 weeks.

Bacopa and lobelia respond to the same technique so pinch everything in the same session. For more tips on boosting seedling performance, see our guide to growing annuals from seed.

How do I plant up a lobelia hanging basket?

Line a 35cm basket with moss or coir liner, fill with peat-free compost, and plant 6-8 lobelia around the edge. Combine with trailing fuchsia, petunias, bacopa, or verbena in the centre for a mixed effect. Water deeply at planting and hang up once the frosts are over in late May.

Recipe for a classic blue lobelia basket

• 6 x Lobelia Sapphire Cascade (edge)
• 3 x white Bacopa (midway)
• 1 x blue trailing petunia (centre)
• 2 x silver Helichrysum petiolare for foliage

For a full basket layout guide see our hanging baskets how-to. Pair lobelia with petunias for a classic blue and pink scheme that UK gardeners have planted for generations.

How do I keep my lobelia flowering into autumn?

Trim the whole basket back by half in mid-July. Use shears, do not be delicate. The plants look bare for a week then burst back into flower within 2-3 weeks. Water heavily and liquid feed with a high-potash fertiliser (tomato feed works) once a week.

Without the trim, flowering slows in August and the plants look tired by September. With the trim, baskets stay in full colour until the first frost — usually late October in the West Midlands.

Keep watering daily through hot weather. Hanging baskets dry out faster than anything else in the garden. Lobelia wilts dramatically when dry but usually recovers if watered quickly. Chronic drought kills them permanently.

What is Lobelia cardinalis and how do I grow it?

Lobelia cardinalis is a hardy perennial from North American wetlands that reaches 90cm tall with brilliant red flower spikes from July to September. It wants wet soil and full sun. This is nothing like the annual trailing lobelia despite sharing the name.

The key to success is moisture. Cardinal flower needs permanently wet roots. Plant in a bog garden, pond margin, or moisture-retentive clay border. On dry soil it sulks and dies within a year. On wet clay it spreads gently and lives for decades.

Hardy to -15C if the roots stay damp. Mulch with bark in autumn for the first two winters. After that it looks after itself.

Lobelia cardinalis care checklist

• Soil: Permanently moist clay or loam
• Sun: Full sun ideal, light shade tolerated
• Height: 70-90cm
• Flowering: July to September
• Hardiness: -15C with wet roots
• Spacing: 30cm apart in drifts of 5+
• Feed: Not needed in bog gardens

See our guide to creating a bog garden for the full construction method.

Is Lobelia tupa worth growing in the UK?

Yes, Lobelia tupa is the most dramatic lobelia for sheltered UK gardens. A Chilean native reaching 1.8m with smoky dark red flower spikes and grey-green leaves. Often called the devil’s tobacco. Much less common than the other species but stunning where it settles.

Hardiness is borderline. In Staffordshire I mulch 15cm deep in November and it survives normal winters. Cold clay wet kills it. Sharp drainage in summer and winter protection is the formula.

Worth growing if you want something most UK gardens do not have. Expect to pay £12-£18 for a 1L pot from specialist nurseries. Cuttings root easily in June if you want to build up stock.

What pests and diseases attack lobelia?

Slugs are the main pest on young lobelia seedlings in the garden. Hanging baskets usually escape slug damage because they are out of reach, but bedding plants get hammered in the first week after planting out. Use copper tape around pots or try nematodes on the soil before planting.

Aphids attack stressed plants in hot weather. Treat with a strong water spray or insecticidal soap. Healthy plants usually shrug off light aphid attacks.

Powdery mildew affects tired plants in late summer. The July trim-back largely prevents it. Water at the base not the leaves. Avoid evening watering in humid weather.

The one thing I wish I had known

Sow lobelia three weeks earlier than any other annual and you get a much longer display. I wasted two seasons sowing in March alongside petunias and cosmos. The resulting baskets did not peak until July and were over by September.

Starting on 20 January under grow lights transformed my results. By early May the plants are bushy with flower buds and ready to hang out after the last frost. They then flower from mid-May to late October, a full six months. The extra effort of early sowing pays off every single year.

For wider sowing timings across the season, consult the UK seed sowing calendar. The RHS plant guide for Lobelia erinus has additional variety suggestions if you want to branch beyond the Cascade series.

Frequently asked questions

When should I sow lobelia seed in the UK?

Sow lobelia seed indoors from mid-January to late February. Lobelia needs 12-14 weeks from sowing to flowering. Germination takes 3 weeks which is slow for annuals. Use a heated propagator at 18-20C. Sow on the surface and do not cover. Pelleted seed is far easier to handle than raw.

Is lobelia a perennial or annual?

It depends on the species — Lobelia erinus is an annual, Lobelia cardinalis is a perennial. The trailing blue lobelia in hanging baskets is Lobelia erinus, a tender annual. The tall red cardinal flower is Lobelia cardinalis, a hardy bog perennial. Both share the name lobelia but grow completely differently.

How do I get lobelia to flower all summer?

Trim annual lobelia hard in mid-July to force a second flush of flower. Cut back by half with shears. Water well and liquid feed once a week. Fresh flowers reappear within 2-3 weeks and last until the first frost. Without trimming, flowers slow in August and tail off by September.

Why is my lobelia seed not germinating?

Lobelia seed needs light to germinate and will not sprout if covered. Sow on the surface of damp seed compost and press gently. Keep at 18-20C with a propagator lid to maintain humidity. Germination takes 2-3 weeks. Never cover with compost. Never let the surface dry out before germination.

What is the best trailing lobelia for hanging baskets?

The Cascade series is the best trailing lobelia for UK hanging baskets. Sapphire Cascade has deep blue flowers with a white eye. Red Cascade brings strong colour. Cascade Mixed gives a full range. Trail length reaches 40cm by midsummer. Crystal Palace is more upright but has deeper blue flowers than any other variety.

Does Lobelia cardinalis grow well in the UK?

Yes, Lobelia cardinalis thrives in UK bog gardens and moist clay borders. It reaches 90cm tall with brilliant red flower spikes from July to September. Needs full sun and permanently moist soil. Hardy to -15C if roots stay wet. Mulch in autumn. It self-seeds gently in the right conditions.

Can I save lobelia seed from year to year?

Yes, annual lobelia produces dust-fine seed that stores well for 3 years. Let the last flowers run to seed in September. Collect the papery pods before they burst. Store in paper envelopes in a cool dark place. Germination drops each year so use fresh seed for the best results.