How to grow lobelia

There are perennial types of lobelia, too. The best known is Lobelia cardinalis, known as the cardinal flower or bog sage. A native American wildflower that is found growing in damp areas, it has bright red flowers that bring a splash of colour to the garden from late summer into autumn. As its name suggests, it likes very moist soil and is suitable for a reliably moist border, bog garden or pond edge.

Lobelia siphilitica, another American native, has long-lasting bright blue flowers. Lobelia tupa, a native of Chile, has dramatic, downy foliage, similar to that of tobacco (hence its common name, Devil's tobacco), held on dark, purple-red stems. It has blood red, tubular flowers.

Lobelia x speciosa varieties are hybrids, usually of L. cardinalis and L. siphilitca. They have spikes of blue, pink, magenta or white flowers from August to October.

Perennial lobelias can be short-lived plants, and are often treated as annuals or biennials. Their milky sap can irritate skin so make sure you wear gloves when handling.

How to grow lobelia

All lobelias need a spot in full sun or part shade, and reliably moist soil. The annual varieties don't need deadheading but can be cut back in mid summer to maintain bushiness and good flowering. Deadhead perennial varieties once they've finished flowering and cut foliage back in autumn. Mulch to protect the crown over winter.

Try our tutorial to grow an oxalis, begonia and lobelia window box or discover more plants for hanging baskets.

Growing lobelia: jump links

• Planting lobelia
• Caring for lobelia
• Propagating lobelia
• Growing lobelia: problem-solving
• Buying lobelia
• Best lobelia to grow

Where to grow lobelia

All lobelias need sun or partial shade, and reliably moist soil.

Annual lobelias

Annual lobelias look great in summer containers, particularly the trailing types, which look great spilling over the edge of a hanging basket. Growing in partial shade will ensure that the plants flower for longer, especially in a very hot summer.

Perennial lobelias

Lobelia cardinalis is best grown in sun or partial shade. It needs very moist, fertile soil that doesn't dry out. Lobelia tupa is best grown in full sun in a sheltered spot and fertile, well-drained soil. It may need protection in hard winters. Lobelia x speciosa varieties also do best in moist soil, in sun or partial shade.

How to plant lobelia

Annual lobelias

If growing annual varieties in pots or hanging baskets, add plenty of garden compost to your multi-purpose compost when planting, to help keep moisture in the soil. You could also add a slow-release feed to keep plants flowering well. Don't plant out until the last frosts have passed – usually at the end of May. Harden off before planting out. Plant 10-15cm apart.

Perennial lobelias

Perennial lobelias are best planted in spring. Dig over the planting area, incorporating lots of organic matter such as compost or well-rotted manure, especially if you're planting Lobelia cardinalis, which needs very moist soil. Dig a hole that's the same size of the rootball. Place the rootball in the planting hole so top of the rootball is level with the soil surface. Water in well. Mulch around the plant with garden compost to help lock moisture in.

Caring for lobelia

Annual lobelias

Water regularly in summer to keep the soil moist – don't let the soil dry out as this will affect flowering and may cause the plant to die back. If you didn't add a slow-release fertiliser when planting, feed with a liquid plant food every two weeks during spring to promote healthy plants, then a high potash feed in summer to prolong flowering. There's no need to deadhead annual lobelias but if your plants are looking tired by midsummer, you could cut them back to maintain bushy growth and encourage repeat flowering.

Perennial lobelias

Water perennial lobelias, especially Lobelia cardinalis, regularly – the soil needs to be kept moist at all times. Feed with an all-purpose, balanced food in spring and early summer, switching to a high potash feed from midsummer. Cut back old flower stems after flowering and trim back the old foliage in autumn. Perennial lobelias, especially Lobelia tupa, may need protection in hard winters - mulch in autumn to protect the crown.

How to propagate lobelia

Sow lobelia seeds in February or March. The seeds are tiny, so you could mix them with some silver sand to make the job easier. They need a temperature of 18-24C (65-75F) to germinate, so use a heated propagator or cover with a polythene bag and keep somewhere warm. Prick out the seedlings when they're large enough to handle, pot them as needed into larger pots, and harden off before planting out when the risk of frost has passed, in late May.

Perennial lobelia varieties can also be divided in spring.

Propagate Lobelia cardinalis by taking softwood cuttings in summer.

Growing lobelia: problem solving

Wilting

This is caused by lack of water – lobelias need a consistently moist soil.

Slugs and snails

Slugs and snails are attracted to the new foliage of perennial lobelias in spring, so make sure you protect them.

Crown rot

This can occur in cold, wet winters. If it's common in your area, it's best to treat perennial lobelias as annual, replacing them every year.

Best varieties of lobelia to grow

Lobelia x speciosa 'Scarlet Fan'

Also known as 'Fan Scarlet', this has dark bronze-green foliage and spikes of vibrant red flowers. Often grown as a biennial.

Height x Spread: 60cm x 25cm

• Buy Lobelia x speciosa 'Fan Scarlet' from Thompson & Morgan

Lobelia tupa (Devil's tobacco)

Also known as Devil's tobacco, this an unusual and eye-catching plant has dark red-purple stems and blood red, tubular flowers.

H x S: 1.2m x 90cm

• Buy Lobelia tupa from Burncoose

Lobelia erinus 'Sapphire Cascade'

– sapphire-blue flowers with a white eye. A trailing variety.

H x S: 15cm x 25cm

• Buy Lobelia 'Sapphire Cascade' seeds from Thompson & Morgan

Lobelia cardinalis 'Queen Victoria' – maroon foliage and spikes of dark red flowers from August to October.

H x S: 90cm x 30cm

• Buy Lobelia cardinalis 'Queen Victoria' from Van Meuwen

Lobelia 'Compton Pink' – a perennial variety that has petals that flower out. They are pale pink, darker pink at the centre. It has an upright habit and looks good in a cottage garden or herbaceous border.

H x S: 80cm x 50cm

• Buy Lobelia 'Compton Pink' from Crocus

Lobelia erinus 'Cambridge Blue' – a compact, variety with brilliant, sky-blue flowers

H x S: 10cm x 15cm

• Buy Lobelia erinus 'Cambridge Blue' seeds from Thompson & Morgan

Lobelia siphilitica (blue cardinal flower) – perennial variety with brilliant blue, tall spires. Suitable for a bog garden or the edge of a pond.

H x S: 90cm x 30cm

• Buy Lobelia siphilitica from Primrose