Illustration
Southern Blue Flag
Iris virginica
Also called virginia iris and virginia blue flag
Also comes in purple and white.
Southern Blue Flag blooms from May through June.
Months here are Northern Hemisphere months; in the Southern Hemisphere, shift them by six months.
Growing Southern Blue Flag
Plant it in full sun. It prefers high water. It is hardy in USDA zones 5–9. Expect it to reach about 2.5 ft tall and 2 ft wide at maturity, in roughly 2 years (a house reading).
Is Southern Blue Flag toxic to dogs or cats?
- Listed as toxic to dogs and cats by NC State Extension and Pet Poison Helpline.
- Listed as poisonous to people by NC State Extension.
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 Southern Blue Flag
Plant Southern Blue Flag 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 7, the middle of its hardy zones 5 to 9, spring means March to May and fall means September or October. Once settled, it blooms from May to June.
Month-by-month timing for your zone is on the planting calendar; if you garden near zone 7, the middle of this plant's hardy range, start with zone 7 in March. Derived from the plant's category, zones and bloom months plus typical zone frost dates, not checked separately.
How big does Southern Blue Flag get?
2.5 fttall5 ft 7 inan adult2 ftwideSee Southern Blue Flag 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 Southern Blue Flag
Derived from the mature spread above by a fixed rule, not checked separately: spacing is the spread, center to center.
Space Southern Blue Flag about 2 ft apart, center to center, so each plant has room to reach its full width.
Add Southern Blue Flag 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
Southern Blue Flag's height, spread, hardiness zones, sun, water, bloom months and color and trait tags were checked against 9 published sources on October 24, 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 GardenPlant Finderheight, spread, hardiness zones, sun, water, bloom months and color and trait tags
- NC State ExtensionGardener Plant Toolboxheight, spread, hardiness zones, sun, water, bloom months and color and trait tags
- illinoispollinators.orgUniversity of Illinois Extension, Illinois Pollinatorsheight, spread, hardiness zones, sun, water and bloom months and color
- hoffmannursery.comHoffman Nurseryheight, hardiness zones, sun, water and bloom months and color
- Missouri Department of Conservationheight, sun, water, bloom months and color and trait tags
- tnyards.utk.eduUniversity of Tennessee Institute of Agriculture, Tennessee Smart Yardsheight, sun, water, bloom months and color and trait tags
- University of Illinois ExtensionHort Answersheight, hardiness zones, sun, water and bloom months and color
- earthsourceinc.netEarth Source / Heartland Restoration Services, native seed and plant n...height, spread, bloom months and color and trait tags
- Chicago Botanic GardenPlant Finderhardiness zones, sun 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 28 states and 2 provinces
United States: Alabama, Arkansas, District of Columbia, Florida, Georgia, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maryland, Michigan, Minnesota, Mississippi, Missouri, Nebraska, New York, North Carolina, Ohio, Oklahoma, Pennsylvania, South Carolina, Tennessee, Texas, Virginia, West Virginia, Wisconsin and 16 more
Recorded growing wild in 670 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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Design a garden with this
Drop this plant onto a 3D plan, see how it fills in, and watch your whole garden bloom across the year.