Key takeaways
- Lobelia erinus is the trailing basket annual. Lobelia cardinalis is a tall red perennial
- Sow basket lobelia indoors in January or February
- Don't cover the seed, because it needs light to sprout
- Pinch out the tips at 5cm for bushier plants
- Cut plants back by half in mid-July for more flowers
- Cardinal flower needs soil that stays wet and plenty of sun
“Lobelia” means two very different plants in the UK. There’s the little trailing annual, Lobelia erinus, that spills blue from hanging baskets all summer. And there’s Lobelia cardinalis, a tall red perennial for wet soil and pond edges that looks nothing like its small cousin. Most people only know the basket one. Here’s how to grow both.
When should I sow lobelia seed in the UK?
Sow indoors between mid-January and late February for baskets in flower by early summer. Lobelia is slow, taking around three months from sowing to flowering, and the seed can take up to three weeks to sprout. Start late and your baskets won’t flower until July.
Use a heated propagator at 18-20C. Sow on the surface of damp seed compost and don’t cover it, because lobelia needs light to sprout. Put a clear lid or bag over the tray to keep it humid, and never let the surface dry out. Our guide to sowing seeds indoors has more on propagators and heat mats.
Sow on the surface and don’t cover the seed, as it needs light to sprout. Keep the lid on so the compost never dries out.
Why is lobelia seed so hard to handle?
Lobelia seed is as fine as dust, so it’s very hard to sow evenly. Shake it straight from the packet and you get thick clumps with bare patches in between.
Pelleted seed solves this. Each seed has a little coating, so you can see it and place it. If you only have loose seed, mix it with about ten times as much silver sand and sprinkle the mix thinly. It’s not as neat as pellets, but it works.
What are the best annual lobelia varieties?
Crystal Palace has the deepest blue of any lobelia, almost navy, with bronze leaves. It grows upright, so it’s best for bedding and pot edges. Cambridge Blue is a lighter sky blue. Sapphire is the classic trailing basket lobelia, with a white eye.
Crystal Palace (Lobelia erinus)
A neat, upright plant about 12-15cm tall. The dark bronze leaves make the deep blue flowers really stand out. Lovely for edging and the front of a border. It keeps its shape without pinching and flowers from June to October with a trim.
Sapphire
A trailing lobelia with a white eye in each blue flower. It trails 30-40cm by midsummer. This is the traditional basket lobelia and looks great with white bacopa and pink petunias.
Cambridge Blue
Soft sky-blue flowers on a compact, upright plant about 15cm tall. Use it in pots and bedding when you want a lighter blue than Crystal Palace. It has an airy, open look.
Cascade series
Trailing lobelias bred for baskets, in Blue, Red, White and Mixed. They trail about 40cm. Pick these when you want more colour choice than Sapphire gives you.
Variety comparison at a glance
Here’s how the main varieties compare.
| Variety | Shape | Colour | Size | Good for |
|---|---|---|---|---|
| Crystal Palace | Upright | Deep navy blue | 12-15cm tall | Edging, pot fronts |
| Cambridge Blue | Upright | Sky blue | 15cm tall | Softer colour in pots |
| Sapphire Cascade | Trailing | Blue with white eye | 30-40cm trail | Classic baskets |
| Blue Cascade | Trailing | Mid-blue | 40cm trail | Hanging baskets |
| Red Cascade | Trailing | Carmine red | 40cm trail | A change from blue |
| Cascade Mixed | Trailing | Blues, reds, whites | 40cm trail | Mixed baskets |
| Riviera Blue | Compact upright | Bright blue | 15cm tall | Early bedding |
| White Lady | Trailing | Pure white | 30cm trail | White schemes |
How do I pinch lobelia seedlings for bushier plants?
Pinch out the tip when seedlings are about 5cm tall. Use your fingers or small scissors to take off the top centimetre. The plant then sends out new shoots from lower down, and every shoot flowers, so you get a fuller plant.
Pinch again at 10cm if they’re slow to bush up. Stop once flower buds appear, or you’ll delay the flowers.
Bacopa likes the same treatment, so do them all at once. For more on raising bedding, see our guide to growing annuals from seed.
How do I plant up a lobelia hanging basket?
Line a 35cm basket, fill it with peat-free compost and plant six to eight lobelias around the edge. Put trailing fuchsia, petunias, bacopa or verbena in the middle. Water well and hang it up once frosts are over, usually late May.
Six to eight lobelias round the edge soon trail over the sides. Hang the basket out once frosts are over, usually in late May.
Recipe for a classic blue lobelia basket
- 6 x Lobelia Sapphire Cascade (around the edge)
- 3 x white bacopa (halfway in)
- 1 x blue trailing petunia (centre)
- 2 x silver Helichrysum petiolare for foliage
Our hanging baskets how-to has full layouts. Lobelia and petunias together make a classic blue and pink basket that never goes out of style.
How do I keep my lobelia flowering into autumn?
Cut the whole basket back by half in mid-July. Use shears and don’t be shy. It looks bare for a week or so, then bursts back into flower within two or three weeks. Water well and give a tomato feed once a week.
Without the trim, flowering slows in August and plants look tired by September. With it, you’ll usually have colour until the first frost.
Water daily in hot weather, as baskets dry out faster than anything else in the garden. Lobelia flops dramatically when dry but usually perks up if you water quickly. Let it dry out again and again, though, and it won’t recover.
What is Lobelia cardinalis and how do I grow it?
Lobelia cardinalis, the cardinal flower, is a tall perennial with bright red spikes from July to September. It grows to about 90cm and comes from North American wetlands. It wants wet soil and full sun, nothing like the basket lobelia.
Moisture is the secret. Plant it in a bog garden, by a pond or in a clay border that stays damp. On dry soil it sulks and soon fades away.
Mulch the crowns with bark in autumn to help them through winter, especially in the first couple of years.
Cardinal flower needs soil that stays wet, so a pond edge or bog garden suits it. Mulch the crowns in autumn for the first few winters.
Lobelia cardinalis care checklist
- Soil: Clay or loam that stays moist
- Sun: Full sun is best, light shade is fine
- Height: 70-90cm
- Flowers: July to September
- Winter: Mulch the crowns in autumn
- Spacing: 30cm apart, in groups of five or more
- Feed: Not needed in a bog garden
Is Lobelia tupa worth growing in the UK?
Yes, if you have a warm, sheltered spot. This Chilean lobelia grows tall, with smoky dark red flower spikes and soft grey-green leaves. It’s sometimes called devil’s tobacco. You won’t see it in many gardens, which is half the fun.
Look for it at specialist nurseries. Give it sun, shelter and soil that doesn’t sit wet in winter.
What pests and diseases attack lobelia?
Slugs are the main pest, especially on young bedding plants. Baskets usually escape because they’re out of reach, but bedding lobelia can get eaten in the first week after planting. Try copper tape around pots, or use nematodes on the soil before planting.
Aphids can turn up on stressed plants in hot weather. Blast them off with a jet of water or squash them. Healthy plants usually shrug off a few.
Powdery mildew can appear on tired plants in late summer. The July trim mostly prevents it. Water the compost, not the leaves, and avoid watering in the evening when it’s humid.
For more sowing dates, see our UK seed sowing calendar. The RHS Lobelia erinus page has more variety ideas if you want to try something new.
Frequently asked questions
When should I sow lobelia seed in the UK?
Sow indoors from mid-January to late February. Lobelia is slow, taking around three months from sowing to flowering, and the seed can take up to three weeks to sprout. Use a heated propagator at 18-20C. Sow on the surface and don’t cover it. Pelleted seed is much easier to handle than loose seed.
Is lobelia a perennial or annual?
It depends which lobelia you mean. The trailing blue one in hanging baskets is Lobelia erinus, grown as an annual. The tall red cardinal flower is Lobelia cardinalis, a perennial for wet soil. They share a name but grow completely differently.
How do I get lobelia to flower all summer?
Cut plants back by half in mid-July. Use shears, then water well and give a liquid feed once a week. New flowers should appear within two or three weeks and keep going until the first frost. Without a trim, flowering slows in August and fades by September.
Why is my lobelia seed not germinating?
It’s probably been covered, and lobelia needs light to sprout. Sow on the surface of damp seed compost and just press it in. Keep it at 18-20C under a propagator lid. It can take up to three weeks, so be patient. Don’t let the surface dry out.
What is the best trailing lobelia for hanging baskets?
The Cascade series is a great choice for baskets. Sapphire has blue flowers with a white eye. Red Cascade and Cascade Mixed give you other colours. Crystal Palace has the deepest blue of all, but it grows upright rather than trailing.
Does Lobelia cardinalis grow well in the UK?
Yes, if you give it wet soil and sun. It grows to about 90cm with bright red flower spikes from July to September. A bog garden, pond edge or damp clay border suits it. Mulch the crowns in autumn to help it through winter.
Can I save lobelia seed from year to year?
Yes, and it keeps for a few years. Let the last flowers set seed in September and collect the papery pods before they split. Store the seed in a paper envelope somewhere cool and dark. Fresher seed sprouts best, so use it up within a couple of years.
Our team produces practical UK gardening guidance from source research, contributor records and established horticultural practice. Editorial standards are overseen by Matt W.
