Ornamental · Campanulaceae
Lobelia
Lobelia erinus
Give it cool weather, part shade, and steady moisture, and lobelia spills out clouds of blue from spring right into fall.
Partial sun
Every 2–4 days
36–81°F · 2–27°C
70–85 days
Overview
Lobelia is that low, frothy edger you've seen lining paths and tumbling out of baskets in a haze of blue. It's part of the bellflower family, and in most gardens it's grown as a tender annual. It stays pretty small at 4 to 9 inches (10 to 23 cm) tall, and it'll either fill out into a soft mat or trail down in a curtain of flowers.
So what makes it tick? Timing, mostly. This is a cool-season plant that does its best in the mild stretches of spring and fall, flowering from April into early summer wherever the weather stays gentle. Push it into a hot, muggy summer and it sulks, which is exactly why so many gardeners cut it back and wait for cooler weather to draw out a second flush.
Once it's settled in, it doesn't ask for much: rich, moist soil, a spot out of the harshest afternoon sun, and a drink before it ever dries out. Get those right and one planting will hand you weeks of color. The one thing to keep in mind is that the whole plant is toxic, so put it somewhere pets and grazing animals can't get to 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 good hard trim in midsummer to bring the flowers back in fall.
Light
Lobelia likes part sun best, and it'll take full sun in cool climates. Where summers get hot, give it afternoon shade, because it's the heat and glare, not too little light, that push it out of bloom early.
Water
Keep the soil evenly moist at all times, since lobelia has zero tolerance for drought and fades fast if it dries out. Water at the base every 2 to 4 days, more when it's hot, but don't let it sit soggy or you're asking for rot.
Soil
Give it rich, well-drained ground packed with organic matter, whether that's loam, sand, or clay. It's easygoing about pH too, happy anywhere from 6.0 to 7.5, as long as the soil holds moisture without staying waterlogged.
Temperature
It's a cool-weather plant that handles a range of about 36°F to 81°F (2°C to 27°C). It does great at the mild ends of the season and starts to struggle once summer heat and humidity settle in.
Humidity
Regular outdoor air suits it fine, so there's no number to hit. Muggy summer heat is the real problem, not dry air, because damp, stagnant air around the plant is what brings on the rots that trouble it.
Feeding
Feed it every two weeks through the growing season with a balanced water-soluble fertilizer. That steady, light diet keeps a fast grower flowering instead of running out of gas partway through.
Pruning
When midsummer heat takes the shine off, just shear the whole plant back by about a third. It looks a bit rough for a week, then pushes out fresh growth that reblooms once cooler fall weather shows up.
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 so handy for edging beds and filling the front of containers.
When to plant
Lobelia runs on cool weather, so timing really is everything here. Start the slow seed indoors well before your last frost, move plants out once spring has settled, and then enjoy 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
Almost all lobelia trouble comes down to two things: too much water early on, and too much heat later. Catch the signs below while they're still small and you can usually turn the plant around. Just match what you see, then act before it spreads.
Seedlings tip over and rot right at the base
So the seedlings pinched in at the soil line and just fell over? That's damping-off, a soil fungus that loves a cold, soggy seed tray. You'll spot water-soaked stems that go narrow at the base, and then the whole thing collapses at once. Move to a clean, well-draining mix, ease up on the water, get more air moving around the tray, and clear out any seedlings that are already gone.
Fix:
- Use sterile, well-draining seed mix
- Improve air circulation and reduce watering
- Remove and discard affected seedlings
It keeps wilting and the roots go brown and soft
When the soil just won't drain and stays wet, stem and root rot can move in. The tell is a plant that keeps wilting even though the ground is soaked, with roots that have turned dark and mushy. Sort out the drainage, water a good bit less often, and pull anything that's too far gone to save.
Fix:
- Improve drainage and reduce watering frequency
- Remove severely affected plants
Flowers and leaves fade when summer gets hot
Here's the thing about lobelia: it's a cool-weather fan, so heat and humidity stress just wear it down as things warm up. When it goes ragged and stops blooming, that's your cue. Trim it back by about a third, keep the soil evenly moist through the heat, and give it afternoon shade if your summers are toasty, and plenty of plants will bounce back with a rebloom once cooler fall weather rolls in.
Fix:
- Cut plants back by about one-third to encourage fall rebloom
- Maintain consistent moisture during heat
- Provide afternoon shade in hot climates
Toxicity
Think of 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 can't get at it.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Toxic
Since lobeline is in every part of the plant, a cat that takes a nibble might start drooling, lose its appetite, and go quiet and listless. Vomiting and diarrhea usually aren't far behind, and a big mouthful can throw off the heartbeat and breathing.
Dogs Toxic
A dog that chews on the leaves or flowers will probably drool a lot and vomit, with a sore stomach and loose stools right after. And if it eats enough, the alkaloids start to reach the heart and breathing.
Horses Toxic
Horses get the same alkaloids, so expect drooling, an upset stomach, and a dull, low mood. A big dose can mess with the heart rhythm, so it's smart to keep lobelia well away from paddocks and hay.
Rabbits Toxic
The alkaloids reach right through the plant, so there's no part a rabbit can safely graze. You'll see drooling and a queasy gut, and it gets worse with a bigger feed.
Livestock Toxic
Cattle, sheep, and goats can be poisoned from grazing the leaves, which brings on drooling, vomiting, and weakness. Eat a lot and you're looking at heart and breathing trouble, so fence it off from pasture.
Birds Toxic
The lobeline in the leaves and flowers makes lobelia a bad nibble for pet or backyard birds. Even a small taste can upset the gut, and more can reach the heart.
People Caution
None of lobelia is food, and eating it brings on nausea, vomiting, and drooling. A large amount can affect the heart and breathing, so it's best to 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 comes in upright edging types and trailing types for baskets, mostly in shades of blue. These are the names you'll bump into most.
Crystal Palace
This one's a compact, upright pick for edging, and it stands out with deep blue flowers over bronze-tinged foliage.
Cambridge Blue
It's a neat little mounding edger that carries soft sky-blue flowers.
Sapphire
Want something for a hanging basket? This trailing type spills blue flowers with a white eye right over the edge.
Laguna series
These trailing cultivars were bred to handle heat, so they keep going in summer containers when others give up.
How to propagate
Lobelia is grown from seed, though 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.
You can grow lobelia from seed, but it'll test your patience a bit. The seed is dust-fine and slow to sprout, so start it indoors 8 to 10 weeks before your last frost, sprinkle it on the surface, and keep it evenly moist and warm. Figure on about 10 to 12 weeks to get a plant ready for the garden, and honestly, if that feels like a gamble, grabbing nursery transplants is the safer bet.
Myths
Edging lobelia is the same plant as the medicinal cardinal flower.
They sound alike, but annual edging lobelia (Lobelia erinus) is a different species from cardinal flower (Lobelia cardinalis), even if people lump them together. What they really share is the risk, and that's the part to remember. Every Lobelia holds toxic lobeline alkaloids, and none of it is safe to eat.
Lobelia is a true sun-lover that thrives in summer heat.
Not quite. It's really a cool-season annual that wants part shade wherever summers turn hot. The heat and humidity of mid-summer usually make it decline, which is why it looks its best in spring and fall instead of the middle of the season.