Ornamental · Campanulaceae
Lobelia
Lobelia erinus
Give it cool weather, part shade, and steady moisture, and lobelia throws out clouds of blue from spring straight into fall.
Partial sun
Every 2–4 days
36–81°F · 2–27°C
70–85 days
Overview
Lobelia is the low, frothy edger that lines paths and pours out of baskets in a haze of blue, and it delivers a lot of color for very little fuss. It belongs to the bellflower family and grows as a tender annual in most gardens, staying compact at 4 to 9 inches (10 to 23 cm) tall while it spreads into a soft mat or a trailing curtain of flowers.
The real hook is the timing. This cool-season plant hits its stride in the mild stretches of spring and fall, flowering from April into early summer where the weather stays gentle. Throw it into a hot, muggy summer and it sulks, so gardeners simply cut it back and let cooler weather spark a whole second flush.
Once it digs in, it barely asks for anything: rich, moist soil, a spot ducking the harshest afternoon sun, and water before it dries out. Nail those and a single planting keeps the color coming for weeks. Just keep one fact front and center, since the whole plant is toxic, and plant it where pets and grazing animals cannot reach it.
Care
The short version: part shade where summers are hot, rich soil kept evenly moist, a balanced feed every couple of weeks, and a bold midsummer cutback that fires the flowers back up in fall.
Light
Lobelia thrives in part sun and will take full sun in cool climates. Where summers turn hot, hand it afternoon shade, because heat and glare, not too little light, are what shove it out of bloom early.
Water
Keep the soil evenly moist at all times, since lobelia has zero tolerance for drought and fades fast the moment it dries out. Water at the base every 2 to 4 days, more in heat, but never let it sit soggy or you hand it rot.
Soil
Drop it into rich, well-drained ground loaded with organic matter, whether loam, sand, or clay. It shrugs off pH concerns, happy anywhere from 6.0 to 7.5, as long as the soil holds moisture without staying waterlogged.
Temperature
This cool-weather plant powers through a range of about 36°F to 81°F (2°C to 27°C). It thrives at the mild ends of the season and starts to flag once summer heat and humidity settle in.
Humidity
Ordinary outdoor air suits it fine, so skip chasing a number. Muggy summer heat is the real culprit, not dry air, since damp, stagnant air around the plant 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 pumping out flowers instead of running out of steam mid-season.
Pruning
When midsummer heat dulls the display, shear the whole plant back by about a third. It looks rough for a week, then explodes into fresh growth that reblooms once cooler fall weather lands.
Size & spacing
Lobelia stays low, around 4 to 9 inches (10 to 23 cm) tall, and spreads 6 to 12 inches (15 to 30 cm) wide. That compact, mounding-to-trailing habit is what makes it a knockout for edging beds and packing the front of containers.
When to plant
Lobelia runs on cool weather, so the calendar sets the pace. Kick off the slow seed indoors well before your last frost, push plants out once spring has settled, and ride the flowers 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
Nearly every lobelia hiccup traces back to two things: too much water early on and too much heat later. Spot the signs below while they are still small and you can usually turn the plant right around. Match what you see, then jump on it before it spreads.
Seedlings keel over and rot at the base
A tray that keels over at the soil line has run into damping-off, a soil fungus that pounces on young seedlings kept cold and wet. Watch for water-soaked stems that pinch in at the base, then a sudden collapse. Reset with a clean, well-draining mix, dial back the watering, get air moving around the tray, and clear out the seedlings that have already gone.
Fix:
- Use sterile, well-draining seed mix
- Improve air circulation and reduce watering
- Remove and discard affected seedlings
Plant droops while roots turn brown and soft
Soil that stays wet and refuses to drain hands you stem and root rot. The dead giveaway is a plant that keeps drooping even in soaked ground, with roots gone dark and mushy underneath. Open up the drainage, stretch the gaps between waterings, and yank any plant that is clearly beyond rescue.
Fix:
- Improve drainage and reduce watering frequency
- Remove severely affected plants
Blooms trail off once summer heats up
Lobelia lives for cool weather, so heat and humidity stress knock the wind out of it as the season warms. The plant goes ragged and downs tools on flowering once the temperatures climb. Chop it back by about a third, keep the soil evenly moist through the heat, and slide it into afternoon shade where summers run hot, and it charges back into bloom when cool fall weather rolls around.
Fix:
- Cut plants back by about one-third to encourage fall rebloom
- Maintain consistent moisture during heat
- Provide afternoon shade in hot climates
Toxicity
Grow lobelia as a look-but-don't-eat plant. Every part holds lobeline and related alkaloids, so eating it makes pets, livestock, and people sick, with nausea, vomiting, and drooling, plus heart and breathing effects from a large amount. Plant it where animals and small children cannot get at it.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Toxic
Lobeline runs through every part of the plant, so a cat that takes a bite can start drooling, turn away from food, and go dull and listless. Vomiting and diarrhea tend to follow fast, and a big mouthful can throw the heartbeat and breathing off.
Dogs Toxic
A dog that chomps the leaves or flowers will likely drool hard and vomit, with stomach pain and loose stools right behind. Let it eat enough and the alkaloids start hitting the heart and breathing.
Horses Toxic
Horses run into the same alkaloids and answer with drooling, a sour stomach, and a flat, depressed mood. A large dose can throw the heart rhythm off, so keep lobelia well clear of paddocks and hay.
Rabbits Toxic
The alkaloids charge through the whole plant, so no part is safe for a rabbit to graze. Count on drooling and gut upset, and worse from a bigger feed.
Livestock Toxic
Cattle, sheep, and goats can be poisoned by grazing the foliage, which sets off salivation, vomiting, and weakness. Heavy grazing risks heart and breathing trouble, so fence it clear of pasture.
Birds Toxic
The lobeline packed into the leaves and flowers makes lobelia an unsafe nibble for pet or backyard birds. Even a small taste can upset the gut, and more can hit the heart.
People Caution
No part of lobelia is food, and eating it sets off nausea, vomiting, and drooling. A large amount can affect the heart and breathing, so keep curious kids 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 rolls out in upright edging types and trailing types for baskets, mostly in shades of blue. These are the names you will run into most.
Crystal Palace
A compact, upright edger that grabs attention with deep blue flowers blazing over bronze-tinged foliage.
Cambridge Blue
A tidy mounding edger that carries soft sky-blue flowers with quiet charm.
Sapphire
A trailing type whose blue, white-eyed flowers tumble beautifully from a hanging basket.
Laguna series
A crew of trailing cultivars bred to shrug off heat and keep powering through summer containers.
How to propagate
Lobelia is grown from seed, and it pays back 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.
Growing lobelia from seed makes a satisfying project, as long as patience comes along for the ride. The seed is dust-fine and slow to wake up, so fire it up indoors 8 to 10 weeks before your last frost, scatter it across the surface, and hold it evenly moist and warm. Give it about 10 to 12 weeks to hit garden-ready size, and if that stretch feels risky, nursery transplants get you there without the wait.
Myths
Edging lobelia is the same plant as the medicinal cardinal flower.
Annual edging lobelia (Lobelia erinus) is its own species, separate from cardinal flower (Lobelia cardinalis), even when gardeners toss them into one bucket. The thing they truly share is the hazard, so lock this in: 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 actually a cool-season annual that begs for part shade where summers turn hot. Mid-summer heat and humidity usually drag it into decline, which is exactly why it lights up in spring and fall rather than at the peak of the season.