Ornamental · Campanulaceae
Lobelia
Lobelia erinus
Give it cool weather, part shade, and steady moisture, and forgiving lobelia rewards you with clouds of blue from spring into fall.
Partial sun
Every 2–4 days
36–81°F · 2–27°C
70–85 days
Overview
Lobelia is that soft, frothy little edger you see lining paths and spilling out of baskets in a haze of blue, and it is a kind plant to learn on. It belongs to the bellflower family and grows as a tender annual in most gardens, keeping to a modest 4 to 9 inches (10 to 23 cm) tall while it fills out into a gentle mat or a trailing curtain of bloom.
The one thing worth understanding is its timing, and it is easy. Lobelia is a cool-season plant that shines in the mild stretches of spring and fall, flowering from April into early summer while the weather stays gentle. If a hot, sticky summer makes it sulk, that is normal and completely recoverable, which is why so many gardeners simply cut it back and let cooler weather bring on a fresh flush.
Once it settles in, lobelia asks for very little from you: rich, moist soil, a spot sheltered from the fiercest afternoon sun, and a drink before it ever dries out. Get those small things right and one planting will carry weeks of color for you. The single point to keep in mind is that the whole plant is toxic, so tuck it somewhere pets and grazing animals cannot reach.
Care
The short version: give lobelia part shade where summers are hot, rich soil kept evenly moist, a balanced feed every couple of weeks, and a confident midsummer cutback to bring the flowers back in fall.
Light
Lobelia is happiest in part sun, and it will take full sun for you in cool climates. Where summers turn hot, give it afternoon shade, since it is really heat and glare, not a lack of light, that push it out of bloom too soon.
Water
Aim to keep the soil evenly moist at all times, because lobelia has no patience for drought and fades quickly if it dries out. Water at the base every 2 to 4 days, a little more in heat, and just avoid leaving it soggy so you do not invite rot.
Soil
Set it into rich, well-drained ground full of organic matter, whether that is loam, sand, or clay. It is relaxed about pH, content anywhere from 6.0 to 7.5, as long as the soil holds moisture without turning waterlogged.
Temperature
This is a cool-weather plant that copes with a range of about 36°F to 81°F (2°C to 27°C). It does its best work in the mild ends of the season and starts to struggle once summer heat and humidity settle in.
Humidity
Ordinary outdoor air suits lobelia just fine, so there is no number to chase. Muggy summer heat is the true problem, not dry air, since damp, still air around the plant is what invites 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 happily instead of running out of steam partway through.
Pruning
When midsummer heat takes the shine off, go ahead and shear the whole plant back by about a third. It looks a bit bare for a week, then pushes fresh growth that reblooms once cooler fall weather arrives, so trust the cut.
Size & spacing
Lobelia stays low for you, 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 just what makes it so easy to line beds with and to fill the front of containers.
When to plant
Lobelia runs on cool weather, so the calendar is your friend here. Start the slow seed indoors well ahead of your last frost, move plants out once spring has truly settled, and then enjoy the flowers from April into summer, longer still 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 all lobelia trouble comes down to two easy patterns: too much water early on and too much heat later. Spot the signs below while they are still small and you can almost always coax the plant back. Match what you are seeing, then act gently before it spreads.
Do not panic if seedlings tip over at the base
If your little seedlings pinch in at the soil line and topple, that is damping-off, a fungus that moves in when a cold seed tray stays too wet. It is disheartening, but it is very preventable. Next time reach for a clean, well-draining mix, water with a gentler hand, open up the airflow around the tray, and quietly clear away any seedlings that have already given out.
Fix:
- Use sterile, well-draining seed mix
- Improve air circulation and reduce watering
- Remove and discard affected seedlings
You can head off wilting and soft, browning roots
When soil stays wet and drains poorly, stem and root rot can set in, and the giveaway is a plant that keeps wilting even though the ground is plenty damp, with roots that have gone dark and soft. The good news is the fix is simple: help the water drain away and water less often. If a plant is already too far gone, gently lift it out so the rest can carry on.
Fix:
- Improve drainage and reduce watering frequency
- Remove severely affected plants
It is fine if the blooms fade when summer heats up
Take heart, because this one is not your fault: lobelia loves cool weather, so heat and humidity stress naturally wear it down as the season warms. When it looks tired and stops flowering, simply trim it back by about a third, keep the soil evenly moist through the hot spell, and offer afternoon shade if your summers run warm. Many plants happily rebloom once cooler fall weather comes 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
Think of lobelia as a look-but-don't-eat plant, and you will be fine. Every part 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. Just plant it where animals and small children cannot get at it.
Toxic to
Cats · Dogs · Horses · Rabbits · Livestock · Birds · People
Cats Toxic
Because lobeline runs through every part of the plant, a curious cat that samples it may start drooling, lose interest in food, and grow quiet and listless. Vomiting and diarrhea tend to come next, and a large mouthful can unsettle the heartbeat and breathing.
Dogs Toxic
A dog that mouths the leaves or flowers will likely drool a lot and vomit, with belly pain and loose stools not far behind. Should it eat a real quantity, the alkaloids begin to reach the heart and breathing.
Horses Toxic
Horses meet the same alkaloids with drooling, an upset stomach, and a dull, low mood. A big dose can disturb the heart rhythm, so it is wise to keep lobelia well away from paddocks and hay.
Rabbits Toxic
Since the alkaloids reach through the whole plant, there is no part a rabbit can safely graze. Drooling and gut upset are the usual result, and a bigger feed brings worse.
Livestock Toxic
Cattle, sheep, and goats that graze the foliage can be poisoned, showing salivation, vomiting, and weakness. A heavy feed risks heart and breathing trouble, so fence lobelia away from pasture.
Birds Toxic
The lobeline held in 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 reach the heart.
People Caution
No part of lobelia is meant to be eaten, and swallowing it brings nausea, vomiting, and drooling. A large amount can affect the heart and breathing, so keep curious little ones 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 lovely shades of blue. These are the names you are most likely to meet.
Crystal Palace
You will love this compact, upright edger for its deep blue flowers glowing over bronze-tinged leaves.
Cambridge Blue
This tidy little mounding type is an easy charmer, offering soft sky-blue flowers.
Sapphire
If you want something that spills from a hanging basket, this trailing kind carries blue flowers each with a friendly white eye.
Laguna series
These trailing cultivars were bred to shrug off heat, so they keep going in summer containers when others tire.
How to propagate
Lobelia is grown from seed, and it rewards a patient hand more than a hurried one.
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 is absolutely doable, it just asks for a little patience. The seed is dust-fine and slow to wake up, so sow it indoors 8 to 10 weeks before your last frost, scatter it on the surface, and keep it evenly moist and warm. Give it about 10 to 12 weeks to reach a plant ready for the garden, and if that feels like a lot, know that picking up nursery transplants is a perfectly good shortcut.
Myths
Edging lobelia is the same plant as the medicinal cardinal flower.
It is easy to assume they are one and the same, but annual edging lobelia (Lobelia erinus) is its own species, separate from cardinal flower (Lobelia cardinalis). The important thing to hold onto is the shared risk: every Lobelia carries 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, and it will thank you for part shade wherever summers turn hot. Mid-summer heat and humidity usually make it decline, which is exactly why it looks its happiest in spring and fall rather than at the height of the season.