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Great Blue Lobelia
Lobelia siphilitica
Also called blue cardinal flower, great lobelia, big blue lobelia and blue lobelia
Great Blue Lobelia blooms from July through October.
Months here are Northern Hemisphere months; in the Southern Hemisphere, shift them by six months.
Growing Great Blue Lobelia
Plant it in part shade. It prefers high water. It is hardy in USDA zones 4–9. Expect it to reach about 2.5 ft tall and 1.5 ft wide at maturity, in roughly 1 year (a house reading).
Is Great Blue Lobelia toxic to dogs or cats?
- Listed as toxic to dogs and cats by Royal Horticultural Society.
- Listed as poisonous to people by NC State Extension and Royal Horticultural Society.
Lists differ and none is complete. If a pet eats a plant and seems unwell, call a veterinarian or a pet poison helpline.
What was checked
Nine facts were checked against named botanical sources, each recorded beside the sources that carried it: mature height and spread, hardiness zones, sun, water, bloom months and color, the trait tags, and years to mature size, which rests on a recorded house rule where no publisher states one and is marked so. Soil, diseases, pests, toxicity and the plant's own names have not yet been read for this plant, and its trait tags are the ones a feature uses rather than every word a page prints. Each fact above carries its own mark. The planting, spacing and size sections are derived from these checked facts by a fixed rule, not checked separately, and the plant type is the library's own classification.
When to plant Great Blue Lobelia
Plant Great Blue Lobelia in early spring as soon as the soil is workable, or in early fall at least six weeks before your first frost, so roots establish in cool weather. In zone 6, the middle of its hardy zones 4 to 9, spring means March to May and fall means August or September. Once settled, it blooms from July to October.
Month-by-month timing for your zone is on the planting calendar; if you garden near zone 6, the middle of this plant's hardy range, start with zone 6 in March. Derived from the plant's category, zones and bloom months plus typical zone frost dates, not checked separately.
How big does Great Blue Lobelia get?
2.5 fttall5 ft 7 inan adult1.5 ftwideSee Great Blue Lobelia in a garden plan: the planner shows it in 3D beside everything else you add, growing toward this size over the years.
How far apart to plant Great Blue Lobelia
Derived from the mature spread above by a fixed rule, not checked separately: spacing is the spread, center to center.
Space Great Blue Lobelia about 1.5 ft apart, center to center, so each plant has room to reach its full width.
Add Great Blue Lobelia to a garden plan and the planner draws it at its full width, so you see the spacing before you buy.
Where these facts come from
Great Blue Lobelia's height, spread, hardiness zones, sun, water, bloom months and color and trait tags were checked against 8 published sources on October 21, 2026. Each one is linked below with every fact it carried, which can include a fact this page still marks as not yet checked, because a source carrying a value is not the same as the value being held to the standard. Facts not in that list, and the planting, spacing and size sections, are not covered by these sources.
- Missouri Botanical Gardenheight, spread, hardiness zones, sun, water, bloom months and color and trait tags
- NC State ExtensionPlant Toolboxheight, spread, hardiness zones, sun, water, bloom months and color and trait tags
- University of Illinois Extensionheight, hardiness zones, sun, water, bloom months and color and trait tags
- Mt. Cuba Centerheight, hardiness zones, sun, water and bloom months and color
- Penn State Extensionheight, hardiness zones, water, bloom months and color and trait tags
- Prairie Moon Nurseryheight, spread, hardiness zones, water and bloom months and color
- Chicago Botanic Gardenheight, hardiness zones, water and bloom months and color
- RHSRoyal Horticultural Societyheight, spread, water and bloom months and color
How the checking works, and what each standard covers, is on the verified plants page.
Where it grows wild
Native to 37 states and 2 provinces
United States: Alabama, Arkansas, Colorado, Connecticut, Delaware, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Nebraska, New Hampshire, New Jersey, New York, North Carolina, North Dakota, Ohio, Oklahoma, Pennsylvania, South Carolina, South Dakota, Tennessee, Texas, Vermont, Virginia, West Virginia, Wisconsin, Wyoming and 25 more
Recorded growing wild in 1,113 counties.
Range data from the World Checklist of Vascular Plants, Royal Botanic Gardens, Kew (CC BY 4.0, adapted) and USDA PLANTS. How the native range is credited.
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Drop this plant onto a 3D plan, see how it fills in, and watch your whole garden bloom across the year.