Great blue lobelia

Sun

Full sun to partial shade; afternoon shade is helpful in hot climates. Full sun suits consistently wet ground.

Water

Keep the soil consistently moist to wet. Water deeply during dry spells; do not let the root zone dry out.

Soil

Humus-rich, moisture-retentive soil, including clay, provided it stays moist. Tolerates periodically saturated ground and a range of soil pH.

Height

2-3 ft (0.6-0.9 m)

Spread

1-1.5 ft (0.3-0.45 m)

Flowers

Late summer to early autumn; blue to violet-blue, two-lipped flowers with pale throats.

Foliage

Green, lance-shaped leaves with toothed edges, arranged along upright stems; dies back for winter.

Growth

Moderate

Care needed

Medium

• Pollinators
• Deer resistant
• Cold hardy
• Heat tolerant
• Low allergen

Northern hemisphere months. In Australia, shift them by six.

Difficulty

Moderate

Toughness

Fairly tough

Pick your zone, country, light and whether you have pets.

1. Planting day

   1 ft (0.3 m)

2. After 1 year

   1.5 ft (0.45 m)

3. After 2 years

   2 ft (0.6 m)

4. After 3 years

   2.5 ft (0.75 m)

   full size

About 3 years to full size.

From a typical nursery size, at an average rate for this kind of plant. A rule of thumb, not a forecast.

Cats and dogs

Toxic

Native to

the US and Canada

Growth rate

Moderate

• Plant in spring or early autumn where the soil will remain moist through summer.
• Water during dry spells, especially while plants are establishing.
• Apply a thin layer of compost in spring; rich soil rarely needs additional fertiliser.
• Mulch around the plants to conserve moisture, keeping mulch off the crowns.
• Divide crowded clumps in spring every few years, and remove unwanted seedlings while small.

Pruning. Remove spent flower spikes if you want to limit self-seeding, or leave some to ripen for seed. Cut dead stems to just above the crown in late winter or early spring before new growth lengthens.

Late winter

Cut back remaining dead stems without damaging new basal growth.

Spring

Plant or divide clumps, water thoroughly, and add compost and mulch.

Early summer

Check soil moisture regularly and water before the root zone dries.

Late summer

Enjoy the flowers and remove faded spikes if seedlings are not wanted.

Autumn

Collect ripe seed from dry capsules, or leave some stems to self-seed.

• Plant in groups beside a pond, above the permanently submerged zone, for upright blue flower spikes.
• Use in the moist part of a rain garden with cardinal flower and swamp milkweed.
• Combine with ferns and astilbes in a damp, partly shaded border for contrasting foliage and late colour.
• Weave through a moist native planting with turtlehead and Joe-Pye weed to extend flowering into early autumn.

Garden styles it suits: Cottage garden, Woodland, Wildlife garden, Pollinator garden, Bog garden and New perennial (prairie).

Plant it with: Cardinal flower, White turtlehead, Joe-Pye weed, Royal fern, Swamp milkweed and Astilbe.

Wildlife: The flowers are particularly valuable to bumblebees and also attract other bees. In its native range, it supports wet-ground pollinator plantings late in the growing season.

• Wilting and scorched leaves in dry soil: water deeply, mulch, and move plants to a damper position if drying is recurrent.
• Slugs and snails eating young shoots: inspect new growth, remove pests, and use wildlife-conscious slug controls if needed.
• Rust or leaf spots: remove badly affected foliage, improve spacing, and water the soil rather than the leaves.
• Unwanted seedlings in moist bare soil: remove spent flower spikes before seed ripens and pull seedlings while small.

All parts contain toxic alkaloids and should not be eaten by people, cats, dogs or horses. Ingestion can cause vomiting, diarrhoea, weakness and, in serious cases, breathing or heart problems. Seek medical or veterinary advice promptly after suspected ingestion.

• Cardinal flower (Lobelia cardinalis)

  The usual species has bright red rather than blue flowers, with narrower, more prominently projecting flower parts; both grow upright in damp ground.

• Edging lobelia (Lobelia erinus)

  This familiar bedding plant forms low mounds or trailing stems covered in small flowers, rather than tall, leafy stems bearing upright flower spikes.

Lobelia honours the Flemish botanist Matthias de l’Obel, while siphilitica refers to the plant’s historical use as a supposed treatment for syphilis, not an effective or safe remedy.

Propagation. Divide clumps in spring. Sow the fine seed on the surface of moist seed compost without burying it; light aids germination. A period of cold, moist conditions can improve germination, so outdoor autumn sowing is useful where winters are cold. Plants may also self-seed in suitable damp ground.

Accepted botanical name

Lobelia siphilitica

In the US and Canada

Native in Canada and the lower 48 states.

Native range

Alabama, Arkansas, Colorado, Connecticut, Delaware, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky and Louisiana.

Measured

Average height 80 cm.

Sources: GBIF (CC BY 4.0), USDA PLANTS and Trefle (CC BY 4.0). The care and design notes are LandscapeGPT's own.