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Purple lobelia flowers in a hanging basket
With masses of tiny vibrant flowers seeming to float above the leaves, lobelia provides a trailing carpet of vivid colour. Check out these simple tips to learn how to keep them in prime condition in your garden.

What you need to know about lobelia

Name: lobelia, Lobelia erinus, edging lobelia, Lobelia cardinalis, cardinal flower.

Height: annuals usually 10–20cm tall; perennials and shrubs usually from 60cm to 1.5m tall.

Foliage: small oval or lance-shaped green leaves.

Climate: can be grown in both cool and warm temperate climates.

Soil: requires a moist, but well-drained, rich organic soil.

Position: most lobelias will grow in full sun or partial shade.

Flowering: tiny colourful two-lipped flowers that repeatedly bloom, from spring until the first frosts. 

Feeding: use a controlled-release fertiliser when planting. Containerised plants may require a water-soluble fertiliser every month.

Watering: requires constantly moist soil and does not like to dry out. Ensure that you water frequently during the hot summer months.

Appearance and characteristics of lobelia

The lobelia that most people are familiar with is the annual low-growing mounding or trailing type, with masses of small blue, purple or white flowers. These are usually seen edging borders or spilling over the edge of pots, containers or hanging baskets. However, there are also upright or clump-forming tall perennials and shrubs suitable for growing in damp borders or by the waterside.

Lobelia originates mainly from the temperate regions of the Americas and Africa, with some perennial species indigenous to Australia. They usually form low mounds covered in flowers, or clumps or rosettes of foliage with upright spikes of two-lipped flowers, in shades of blue, purple, red, pink and white. Most species can be easily grown throughout the temperate areas of Australia and New Zealand. They should be positioned in full sun or partial shade, with protection from the hot afternoon sun in warmer areas. Annual lobelia varieties will survive light frosts, but will be killed by the first hard frost. Some of the perennial species will grow in the tropical regions, and others in the cold mountainous regions of Australia.

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How to plant and grow lobelia

If growing lobelia seedlings indoors, sow in late winter. If growing outside directly, sow seeds from early spring onwards.

Lobelia prefers a well-drained, humus-rich organic soil that also retains moisture. The annual varieties resent growing in heavy or waterlogged soils, particularly in winter, which can be fatal. Many of the perennial types require constantly moist soil, as they grow naturally in waterside locations such as streams and ponds. Lobelia grows best in a soil pH between 6 and 7, so test your soil first with a pH kit.

Mulch perennial types in spring with compost or organic matter to improve the soil and retain moisture.
In hanging baskets, pots or containers, always use a premium standard potting mix, and keep the plant well-watered throughout summer. Remember to give your lobelia a thorough soaking whenever it dries out.

Caring for lobelia

Apply controlled-release organic fertiliser around the plants during the beginning of spring and summer. Regular liquid feeding after deadheading a flush of flowers will also encourage quick regrowth and repeat flowering.

How and when to prune lobelia

  • Always pinch out the young growing tips of annual lobelia seedlings when they are around 2–3cm high to induce bushiness and to produce more flowers. They will take a little longer to flower, but it will be worth it, resulting in increased numbers and a more compact overall shape. 
  • Prune after the first flush of flowers to encourage fresh regrowth and flowering. 
  • For most herbaceous varieties, deadhead the main flowering spikes to encourage further flowers, and cut stems down to the base in winter.

Diseases and pests affecting lobelia

Lobelia is usually pest and disease free, but can occasionally be troubled by botrytis and root rot when grown in wet or waterlogged conditions. Remove the affected foliage, spray with a fungicide and improve the growing conditions to remedy the problem. Occasionally thrips and mites may also cause damage. These can be controlled with an insecticide

How to propagate lobelia

Growing lobelia from seed

Annual species of lobelia are usually grown from very fine seed. This should be sown on the surface of seed trays in late winter, if grown inside, or from early spring onwards, if sown outside. 

Prick out the seedlings in small clumps to increase their size, and always plant the seedlings out in spring, after danger of frost has passed. 

Dividing lobelia clumps

Perennial species of lobelia are usually propagated by dividing the clumps during autumn or spring. Basal cuttings may also be used for some varieties. 

If you like this then try

Alyssum: annual ground cover with white, pink and purple flowers, perfect for edging flower beds and planting in hanging baskets and containers.

Pansy (viola): brightly coloured perennial with dark blotches on the flowers, usually grown as annuals and flowering from late winter through to spring, and sometimes summer in cooler climates.

Marigold: golden yellow and orange-flowered bedding or edging plant providing sunny shades of colour in the garden.

Start growing today

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