Edging lobelia (Lobelia erinus) is a low, trailing flower with small blue, violet, white or rose blooms, long used in borders, hanging baskets and window boxes. Arkansas Extension gives it about 6 inches tall with clumps spreading 12 to 16 inches; NC State Extension lists 6 to 9 inches tall and 6 to 12 inches wide. It is hardy only in zones 10 to 11, so most gardeners treat it as an annual. All parts are poisonous (NC State).
Lobelia (Edging Lobelia) at a glance
| Type | Flower |
|---|---|
| Season | Cool-season |
| Sun | Full sun to part shade |
| Plant spacing | 10 in |
| Row spacing | 10 in |
| Per square foot | 1 |
| Seed depth | 0.0625 in |
| Days to maturity | Start seed ~8 weeks before planting out; blooms spring to early summer |
| How to start | Start edging lobelia seed about 8 weeks before planting out after frost, give it afternoon shade in hot summers, keep the soil evenly moist and cut it back after each flush of bloom |
| Water | 1 in per week |
These are the same numbers our plant spacing, square foot and harvest date calculators use. See every crop in the plant database.
When to plant lobelia
Arkansas Extension says to start seed about 8 weeks before the expected planting date; plants will not tolerate late spring frosts, so set them out after your last frost. Lobelia performs best in cool springs and early summer rather than hot or humid weather (NC State), so in warm regions plant it for spring. See the planting calendar. NC State notes that seed is recommended but difficult, and purchased plants are more successful.
How to plant lobelia
- Sowing: the sources do not give a seed depth; follow your seed packet.
- Light: full sun (6+ hours) to partial shade (NC State). In hot-summer areas it is best located where it gets some shade (Arkansas).
- Soil: organically rich, moist and well-drained.
- Spacing: neither source gives a planting distance. Mature clumps spread 6 to 16 inches, so check your plant tag. See the plant spacing calculator.
Caring for lobelia
Keep the soil consistently moist; lobelia does not tolerate drought but is easy to overwater, and in southern summers high moisture and heat often cause root rot and plant death (Arkansas). See the garden watering calculator and the overwatering checker.
Feed every two weeks during the growing season (NC State). Cut back plants that have stopped blooming: in northern areas this keeps them flowering, and in hotter areas it prepares them to bloom again when fall conditions improve. Under wet conditions watch for damping-off, stem rot and root rot. It is listed as deer resistant. See the pest identifier.
Companion plants
| Grow nearby | Containers, mixed hanging baskets, window boxes and rock gardens; butterflies and pollinators visit the flowers |
|---|---|
| Keep apart | Hot, humid summers, drought and soggy soil |
Arkansas Extension notes it was a classic Victorian bedding plant. Check other pairings with the companion plant finder.
Common problems
| Problem | What you see | What to do |
|---|---|---|
| Plants fade in summer heat | Foliage and flowers decline | Lobelia prefers cool temperatures; give afternoon shade and cut back, and it may resume blooming in fall (Arkansas, NC State). |
| Root rot or damping-off | Seedlings collapse, or roots and stems turn soft | Under wet conditions these are the main risks; keep soil moist but not soggy. See the overwatering checker. |
| Dry, crispy plants | Foliage wilts and browns | It has little or no drought tolerance; keep the soil consistently moist, especially in containers. |
Not sure what you are looking at? The plant problem identifier ranks likely causes from the symptoms you see.
When and how to harvest
This is an ornamental edging and container plant rather than a harvest crop. Safety: NC State rates toxicity as medium; all parts contain alkaloids (lobelamine and lobeline), and large amounts can cause nausea, vomiting, diarrhea and convulsions, so keep it away from children and pets and do not eat it.
Plan your lobelia
- Plant spacing calculator: how many plants fit your bed
- Planting calendar: sowing and transplant dates for your frost dates
- Frost dates by ZIP code: your last spring and first fall frost
- Harvest date calculator: when your crop will be ready
Frequently asked questions
How far apart should I plant lobelias?
Space lobelias 10 inches apart in rows 10 inches apart. In a square foot garden, plant 1 per square foot.
How deep do I plant lobelia seed?
The sources do not give a depth; follow your seed packet.
How long do lobelias take to grow?
Start seed about 8 weeks before the expected planting date.
How much water do lobelias need?
About 1 inch of water a week, from rain and watering combined. Keep consistently moist; no drought tolerance, but easy to overwater.
When do I start lobelia seed?
About 8 weeks before the expected planting date, with plants set out after the last spring frost.
Does lobelia like heat?
No, it performs best in cool springs and early summer and declines in hot, humid summers.
Is lobelia poisonous?
Yes, NC State lists all parts as poisonous (medium severity).