Purple lobelia
Lobelia x speciosa 'Vedrariensis' · Campanulaceae
Spikes of rich violet-purple flowers on dark stems, over green leaves flushed with red, from midsummer to early autumn.
- Size
- 3½ ft tall, 14 in wide
- Zones
- 5 to 8
- Light
- Full sun to part sun
- Flowers
- Mid July to mid September
- Pets
- Toxic (moderate)
- Job in a bed
- Accent
A bed built around purple lobelia
The planner designed this bed around purple lobelia when this page was made: a 12 × 6 ft (3.7 × 1.8 m) cottage garden in zone 6, full sun. It chose the other plants from the 966 that suit those conditions, and says below why purple lobelia stands where it does.
The plants
- 1 Purple lobelia × 3, purple
- 2 Dianthus 'Kahori' × 7, pink
- 3 Winter heath × 8, white
- 4 Italian bugloss 'Dropmore' × 3, blue
- 5 Silver sage × 1, silver leaves
- 6 Gray's sedge × 3, green
- 7 Schubert's allium × 4, pink
- 8 Red spider lily × 8, red
Why this plant
- The only spires. The others are open bowls, bells, airy sprays and others. Shape reads at a distance, where colours blur.
- The only purple.
Why here
- Anchors a back corner. At 3½ ft it is among the tallest plants here, and the tallest close the ends of the bed, where they stand in front of nothing.
- A step in the back row. Next to Gray's sedge (2½ ft): no two neighbours there are the same height, so the back steps up and down like a skyline.
- 3 plants in one clump. A single spire is too slight to hold a place alone, so here the accent is a clump.
Why now
- In flower for 9 weeks. From mid July to mid September.
- In company. With dianthus 'Kahori' and Gray's sedge in mid July: 3 kinds at once.
Planned as a cottage garden: 25 of the 25 plants (100%), bulbs aside, stand alone or in groups of no more than five, small enough for the kinds to mingle. The most in flower at one time is 4 kinds, from late May. Dianthus 'Kahori' flowers longest, for 17 weeks.
The planner's own design check: 12 Excellent, 3 Good, 2 Satisfactory. It flags one thing to watch: Nothing on this plan is highly poisonous or an aggressive spreader.
Open this bed in the planner Change its size, the sun or the style, move plants about, or swap one for another.
Where purple lobelia goes in a border
At 3½ ft it belongs in the middle of a bed. In the planner's ten height classes it is moderately tall (3 to 4 ft).
Its job: accent. An upright plant used singly, like a punctuation mark, and repeated at intervals to give the bed rhythm.
A hybrid between the cardinal flower and the great blue lobelia, hardier and longer-lived than the red. It tolerates ordinary border soil if watered in dry spells.
- Purple spires for late summer
- Moist borders
- Hummingbirds and bees
What to plant with purple lobelia
Plants recorded as good partners for it, and what each pairing does:
- Sneezeweed: purple against yellow, opposite each other on the colour wheel, if you choose those colours, in flower together for six weeks; happy in the same moist soil.
- Japanese anemone: in flower together for five weeks, from mid August; happy in the same moist soil.
- Tufted hair grass: in flower together for six weeks, from mid July; a low mound at the foot of its spires; happy in the same moist soil.
Questions about purple lobelia
Does purple lobelia need sun or shade?
Purple lobelia grows in full sun (6 or more hours of direct sun a day) and part sun (4 to 6 hours of sun, including some afternoon sun).
It wants soil that stays evenly moist, rich in organic matter, and plenty of water.
What zones does purple lobelia grow in?
USDA zones 5 to 8. It is hardy to zone 5, which means it survives winter nights down to about −10 °F (−23 °C).
Above zone 8 it struggles: the zones measure winter cold, but a plant's top zone usually means the summers are too hot or the winters too mild for it to rest.
Gardening in Canada? Use the USDA zone for your town: Canada's own zone map uses different numbers.
When does purple lobelia flower in my zone?
In zone 5 it flowers mid July to mid September. As a summer flower it moves about half a week for every zone: earlier where it is warmer.
| Zone 5 | Mid July to mid September |
|---|---|
| Zone 6 | Mid July to mid September |
| Zone 7 | Mid July to mid September |
| Zone 8 | Mid July to mid September |
Worked out from Hopkins' bioclimatic law: spring arrives about four days later for each degree of latitude north, and one zone is roughly two degrees. A warm spring or a cold one moves any garden two weeks either way.
How far apart should purple lobelia be planted?
12 in (30 cm) apart, from the middle of one plant to the middle of the next. That is a little closer than its full width of 14 in (35 cm), so a group knits into one patch.
It grows 3½ ft (100 cm) tall. Give it the full distance from anything taller, or it will lean out towards the light.
How many purple lobelia plants do I need?
At 12 in apart, set out in staggered rows:
- 12 to fill 10 square feet (about 1 m²)
- 11 for a single row 10 ft (3 m) long
- 38 to fill a 4 × 8 ft bed on its own
In a mixed border, plant it as one at a time, repeated along the bed at intervals; a slender one reads better as a clump of three.
In the bed above, the planner used three in one group.
Is purple lobelia poisonous to dogs and cats?
Yes: moderate toxicity to people, dogs, cats and horses. All parts are poisonous if eaten in quantity, causing vomiting, weakness and, in serious cases, convulsions.
Rated from the ASPCA. Where references disagree, the more cautious rating is used.
Is there a pet-safe plant like purple lobelia?
These come closest to doing what purple lobelia does, and no toxicity is recorded for them:
- Blazing star: the same job in a bed, the same size and the same colour; flowers mid July to mid August, fine leaves, not medium.
- Spotted bee balm: the same job in a bed and the same flowering time; 16 in shorter, pink, not purple.
- Peach-leaved bellflower: the same job in a bed; 10 in shorter, flowers mid June to mid July.
Matched the way the planner's Swap button matches plants: the same job in a bed, then size, then flowering time.
What flowers after purple lobelia?
Purple lobelia is over by mid September. These start as it finishes and carry the same spot on, so it never goes flat:
- Garden chrysanthemum: from mid September to late October, 24 in tall, to stand in front of it and cover its fading stems.
- Blue-stemmed goldenrod: from early September to mid October, 2½ ft tall, to stand in front of it and cover its fading stems.
- Blue wood aster: from mid September to mid October, 3 ft tall.
What does purple lobelia look like in winter?
Purple lobelia dies down to the ground and there is nothing to see until spring. The old stems can be cut in autumn, or left until spring as shelter for overwintering insects.
More Lobelia
- Cardinal flower Lobelia cardinalis
- Cardinal flower 'Queen Victoria' Lobelia cardinalis 'Queen Victoria'
- Great blue lobelia Lobelia siphilitica
In these guides
Every fact on record
- Type
- Perennial
- Size
- 3½ ft (100 cm) tall, 14 in (35 cm) wide
- Plant
- 12 in (30 cm) apart
- Height class
- Moderately tall (3 to 4 ft)
- Place in the bed
- Middle
- Flowers
- Mid July to mid September; upright spires; purple
- Leaves
- Medium texture, herbaceous; dark green
- Shape
- Upright, clumping
- Hardy in
- USDA zones 5 to 8
- Light
- Full sun, part sun
- Soil
- Rich, evenly moist
- Water
- High
- Feeding
- Light, in spring
- Family
- Campanulaceae
Checked against ASPCA poison lists. Flowering times are for zone 5: later in colder zones, earlier in warmer ones. Toxicity and spread information is a general gardening guide, not veterinary or medical advice, and invasive status varies by region. If a person or pet eats a plant, contact poison control or a vet. Check your local invasive species list before planting anything with a spread warning.