Start designing

Edging lobelia

Lobelia erinus. Family Campanulaceae. Annual.

Also called Annual lobelia, Garden lobelia, Trailing lobelia.

Edging lobelia is a tender, short-lived perennial usually grown as an annual for its masses of small blue, purple, white or pink flowers. Compact forms make neat border edges, while trailing forms spill over pots, window boxes and hanging baskets. It flowers best in mild weather and may pause or decline during hot summers.

Hardiness

USDA zones 2a to 11b

UK hardiness: H2 (tender, 1 to 5°C)

  1. 1
  2. 2
  3. 3
  4. 4
  5. 5
  6. 6
  7. 7
  8. 8
  9. 9
  10. 10
  11. 11
  12. 12
  13. 13

Grows outdoors in the shaded zones. Select a zone to see what else grows there.

At a glance

Sun
Full sun to partial shade; give afternoon shade where summers are hot.
Water
Keep evenly moist but not waterlogged. Small pots and hanging baskets may need daily watering in warm, windy weather.
Soil
Fertile, moisture-retentive, well-drained soil; use a good-quality container potting mix in pots.
Height
0.25-0.75 ft (0.08-0.23 m)
Spread
0.5-1.5 ft (0.15-0.45 m), depending on variety
Flowers
Spring to autumn in mild conditions, often slowing in summer heat; blue, violet, purple, white or pink flowers, frequently with a white eye.
Foliage
Evergreen. Small green to bronze-green leaves on fine, branching stems; foliage can persist in frost-free conditions.
Growth
Fast; fills out and flowers within one growing season.
Care needed
Medium

Care

  • Plant out after frost risk has passed, spacing plants about 0.3-0.5 ft (0.1-0.15 m) apart.
  • Water before the root ball dries out completely, directing water onto the soil rather than repeatedly soaking the foliage.
  • Feed container plants every two weeks with a balanced liquid fertiliser at the label rate.
  • Provide afternoon shade and check basket moisture frequently during hot spells.
  • Keep drainage holes clear and do not leave pots standing in water.

Pruning. After the first flush of flowers, trim straggly stems back by about one-third, then water and feed to encourage fresh growth. Routine deadheading is unnecessary.

Through the year

Late winter
Surface-sow seed indoors in bright light at about 18-24°C; do not cover the tiny seeds.
Spring
Prick out seedlings in small clumps and gradually harden them off before planting outdoors.
Late spring
Plant out after the last frost and water thoroughly; in hot, frost-free climates, use it primarily as a cool-season bedding plant.
Early summer
Begin regular feeding of container plants and check their water needs frequently.
Midsummer
Trim tired growth after flowering and move portable containers out of strong afternoon sun.
Autumn
Remove plants killed by frost; in frost-free gardens, replace exhausted plants when flowering and growth deteriorate.

In the garden

  • Edge paths and small flower beds with compact varieties for a low ribbon of colour.
  • Use trailing varieties around hanging baskets and window boxes to soften their edges.
  • Plant blue forms beneath white pelargoniums or pale petunias for a cool-coloured container display.
  • Fill gaps at the front of seasonal borders without hiding taller plants.

Garden styles it suits: Cottage garden, Traditional, Victorian, Blues and silvers, Pastel border and Small-space.

Plant it with: Petunias, Pelargoniums, Sweet alyssum, Busy Lizzies, Pansies and Bacopa.

Wildlife: Flowers attract small bees and other flower-visiting insects. Avoid spraying open flowers with insecticides.

Problems to watch for

  • Wilting and loss of flowers in heat or drought: keep roots evenly moist, provide afternoon shade and trim back tired growth; badly scorched plants may not recover.
  • Root or stem rot in saturated soil: improve drainage, empty saucers and remove rotting plants.
  • Aphids on soft shoots: wash off small colonies with water or use an insecticidal soap labelled for ornamental plants.
  • Slugs and snails damaging young plants: inspect after dark, remove them and use iron-phosphate bait where needed, following the label.

Safety

Treat all parts as poisonous if eaten by people, cats, dogs or horses. Lobelia contains toxic alkaloids; ingestion can cause vomiting, diarrhoea, drooling and weakness, with potentially serious heart or breathing effects after larger amounts. Seek medical or veterinary advice after ingestion.

Easily confused with

  • Bacopa (Chaenostoma cordatum)

    Another common basket trailer, but its flowers have five nearly equal, rounded lobes and its leaves are broader. Lobelia flowers have two small upper lobes and three larger lower lobes.

  • Blue star creeper (Isotoma fluviatilis)

    Forms a creeping, ground-hugging mat with pale blue, evenly star-shaped flowers. Edging lobelia is bushier or trailing and has distinctly two-lipped flowers.

The name

Lobelia honours the Flemish botanist Matthias de l'Obel, while edging lobelia refers to its traditional use along the fronts of flower beds.

Propagation. Surface-sow seed indoors about 8-12 weeks before the last expected frost; light aids germination. Handle seedlings in small clumps rather than individually. Named varieties may not come true from seed; some are propagated from softwood cuttings.

Checked against open data

Accepted botanical name
Lobelia erinus
In the US and Canada
Introduced in Hawaii and the lower 48 states.
Native range
Angola, Botswana, Cape Provinces, Caprivi Strip, Chad, Ethiopia, Free State, KwaZulu-Natal, Lesotho, Malawi, Mozambique and Namibia.
Measured
Average height 18 cm.

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

See it in your own yard.

Upload a photo of your yard and see it redesigned with plants that suit your climate, each checked against your zone.

Redesign your yard
Identify a plant