Ornamental · Campanulaceae
Lobelia
Lobelia erinus
Give it cool weather, part shade, and steady moisture, and lobelia covers itself in a fine haze of small blue flowers from spring into fall.
Partial sun
Every 2–4 days
36–81°F · 2–27°C
70–85 days
Overview
Lobelia is a low plant of fine texture, its slender stems dense with small leaves and, above them, a scatter of tiny blue flowers so thick it reads as a soft haze. It belongs to the bellflower family and grows as a tender annual in most gardens, holding to just 4 to 9 inches (10 to 23 cm) tall while the stems either build a low mat or spill over an edge in a trailing curtain of bloom.
Up close the color is the thing. As a cool-season plant it carries its densest, freshest blue through the mild weeks of spring and fall, flowering from April into early summer where the weather stays gentle, each little bloom set among narrow green leaves. In hot, muggy summer the small leaves tire and the flowers thin, so gardeners cut the plant back and wait for cooler weather to fill it in again.
The plant itself is undemanding once its fine roots take hold: rich, moist soil, a place out of the harshest afternoon sun, and water before the stems ever dry. Meet that and the low mat stays full of color for weeks. One fact runs through all of it, leaf, stem, and flower, since the whole plant is toxic, so set it where pets and grazing animals cannot reach.
Care
The short version: part shade where summers are hot, rich soil kept evenly moist, a balanced feed every couple of weeks, and a hard midsummer cutback that fills the low mat with fresh blue flowers again in fall.
Light
Lobelia holds its densest color in part sun and will take full sun in cool climates. Where summers turn hot, give it afternoon shade, since heat and glare, not too little light, are what thin the small flowers and end the bloom early.
Water
Keep the soil evenly moist at all times, because lobelia has no tolerance for drought and the fine stems wilt fast once they dry out. Water at the base every 2 to 4 days, more in heat, but never leave it soggy or the roots rot.
Soil
Set the fine roots into rich, well-drained ground full of organic matter, whether loam, sand, or clay. It is easy about pH, content anywhere from 6.0 to 7.5, as long as the soil holds moisture without turning waterlogged.
Temperature
This cool-weather plant carries its fresh green leaves and blue flowers across a range of about 36°F to 81°F (2°C to 27°C). It looks its best at the mild ends of that range and tires once summer heat and humidity set in.
Humidity
Ordinary outdoor air suits it, so there is no level to hit. Muggy summer heat is the real problem, not dry air, since damp, stagnant air around the low, leafy growth invites the rots that trouble it.
Feeding
Feed every two weeks through the growing season with a balanced water-soluble fertilizer. That steady, light diet keeps a fast grower dense with small flowers rather than thin and leggy by mid-season.
Pruning
When midsummer heat frays the mat, shear the whole plant back by about a third. The stubble looks bare for a week, then pushes fine new stems and leaves that rebloom once cooler fall weather arrives.
Size & spacing
Lobelia stays low, about 4 to 9 inches (10 to 23 cm) tall, and spreads 6 to 12 inches (15 to 30 cm) wide. That low, dense, mounding-to-trailing habit is what packs so many small flowers into the front of a bed or over the lip of a container.
When to plant
Lobelia runs on cool weather, so the calendar matters. Start the dust-fine seed indoors well before your last frost, move the young plants out once spring has settled, and watch the small blue flowers come from April into summer, longer where the season stays mild.
Northern hemisphere
Southern hemisphere
Typical months for a temperate climate: start earlier in warm zones, later where spring frosts linger.
- Sow: Start seed indoors 8 to 10 weeks before the last spring frost; seed can be slow and difficult to germinate.
- Plant: Transplant after the last frost once cool spring weather is settled.
- Harvest: April to June, continuing into summer in cool climates
Hardiness
USDA hardiness zones: 10–11
Temperature: down to -1°C
Troubleshooting
Most lobelia trouble comes down to two things: too much water early on and too much heat later. Catch the signs in the leaves and stems below while they are still small and you can usually turn the plant around. Match what you see, then act before it spreads.
Thread-thin seedlings pinch and fall at the soil
The thread-fine stems of young seedlings darken and go water-soaked right where they meet the soil, then narrow to a weak pinch and fold over: that is damping-off, a soil fungus that works in cold, wet ground. The little plants tip in a group, limp against the surface. A clean, well-draining mix, less water, more moving air, and the removal of the collapsed seedlings clear it up.
Fix:
- Use sterile, well-draining seed mix
- Improve air circulation and reduce watering
- Remove and discard affected seedlings
Slack stems droop while roots go brown and soft
The slender stems lose their spring and hang slack even over wet ground, and below the surface the fine roots have turned from pale and firm to dark and mushy: this is stem and root rot from soil that stays wet and drains poorly. The wilt holds on no matter how damp the soil. Better drainage, less frequent water, and the removal of the worst plants bring it back in hand.
Fix:
- Improve drainage and reduce watering frequency
- Remove severely affected plants
The blue mat frays and stops flowering in heat
The fine, once-crowded blue flowers thin out and the small leaves lose their fresh green as heat and humidity stress press on this cool-weather plant. The whole mat looks frayed and tired and the bloom simply stops as temperatures climb. Cut it back by about a third, hold the soil evenly moist through the heat, and give it afternoon shade where summers run hot, and the fresh growth flowers again when cooler fall weather returns.
Fix:
- Cut plants back by about one-third to encourage fall rebloom
- Maintain consistent moisture during heat
- Provide afternoon shade in hot climates
Toxicity
Treat lobelia as a look-but-don't-eat plant. Every part, from the small leaves to the blue flowers, holds lobeline and related alkaloids, so eating it makes pets, livestock, and people sick, with nausea, vomiting, and drooling, and heart and breathing effects from a large amount. Set it where animals and small children cannot reach it.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Toxic
The small leaves and blue flowers alike carry lobeline through and through, so a cat that bites them can drool, turn from its food, and go dull and listless. Vomiting and diarrhea follow, and a large mouthful can throw off the heartbeat and breathing.
Dogs Toxic
A dog that crushes the fine stems and flowers between its teeth tends to drool heavily and vomit, with stomach pain and loose stools close behind. Enough of it lets the alkaloids reach the heart and breathing.
Horses Toxic
Horses meet the same alkaloids in the leaves and stems with drooling, a sour stomach, and a flat, depressed mood. A large dose can disturb the heart rhythm, so keep lobelia well clear of paddocks and hay.
Rabbits Toxic
The alkaloids run through every soft leaf and stem, so no part is safe for a rabbit to graze. Drooling and gut upset are the usual result, and worse from a larger feed.
Livestock Toxic
Cattle, sheep, and goats can be poisoned by grazing the low, leafy growth, which brings on salivation, vomiting, and weakness. Heavy grazing risks heart and breathing trouble, so fence it away from pasture.
Birds Toxic
The lobeline held in the little leaves and flowers makes lobelia an unsafe nibble for pet or backyard birds. Even a small taste can upset the gut, and more can affect the heart.
People Caution
No part of lobelia is food, not the flowers and not the leaves, and eating it brings on nausea, vomiting, and drooling. A large amount can affect the heart and breathing, so keep curious children away from the plants.
Keep away from grazing animals and pets.
Helpline numbers
United States
ASPCA Animal Poison Control: (888) 426-4435 (fee applies)
Poison Control: (800) 222-1222
Canada
Poison Centres: 1-844-764-7669
Varieties
Lobelia comes in upright edging types and trailing types for baskets, its small flowers mostly in shades of blue. These are the names you will meet most often.
Crystal Palace
A compact, upright edger whose deep blue flowers sit against foliage tinged a dark bronze.
Cambridge Blue
A tidy mounding edger dressed in flowers of a soft, pale sky-blue.
Sapphire
A trailing type whose blue flowers each hold a small white eye at the center as they spill from a hanging basket.
Laguna series
A group of trailing cultivars bred to hold their color through heat and keep flowering in summer containers.
How to propagate
Lobelia grows from seed so fine it looks like dust, and it rewards patience more than speed.
SeedModerate
Start indoors 8 to 10 weeks before the last frost. · Roughly 10 to 12 weeks from sowing to transplant-ready size.
Lobelia grows from seed so fine it looks like dust, and that seed is slow to break. Scatter it across the surface indoors 8 to 10 weeks before your last frost, leaving it uncovered, and keep it evenly moist and warm as the tiny green threads appear. Count on about 10 to 12 weeks from sowing to a plant of transplant size, and where that wait does not suit, nursery transplants fill in.
Myths
Edging lobelia is the same plant as the medicinal cardinal flower.
Annual edging lobelia (Lobelia erinus) is a distinct species from cardinal flower (Lobelia cardinalis), even where the two are grouped together. Look past the leaves and flowers to the one thing they share: every Lobelia holds toxic lobeline alkaloids, and none is safe to eat.
Lobelia is a true sun-lover that thrives in summer heat.
It is really a cool-season annual that wants part shade where summers turn hot. Mid-summer heat and humidity usually make it decline, the small leaves fading and the blue flowers thinning, which is why it looks its freshest in spring and fall rather than at the peak of the season.