Illustration
Northern Blue Flag Iris
Iris versicolor
Also called northern blue flag iris versicolor, northern blue flag, blue flag iris, blue flag, harlequin blue flag and larger blue flag
Northern Blue Flag Iris blooms from May through June.
Months here are Northern Hemisphere months; in the Southern Hemisphere, shift them by six months.
Growing Northern Blue Flag Iris
Plant it in full sun. It prefers moderate water. It is hardy in USDA zones 3–9. Expect it to reach about 2.5 ft tall and 2.5 ft wide at maturity, in roughly 2 years (a house reading).
What was checked
Mature height and spread, hardiness zones, sun, water, bloom months and color and the trait tags were checked against named botanical sources for this plant, with what the sources could not settle written down; years to mature size rests on a recorded house rule where no publisher states one, and is marked so; its common name and alternate names have not been checked yet. 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 Northern Blue Flag Iris
Plant Northern Blue Flag Iris 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 3 to 9, spring means March to May and fall means August or September. 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 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 Northern Blue Flag Iris get?
2.5 fttall5 ft 7 inan adult2.5 ftwideSee Northern Blue Flag Iris 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 Northern Blue Flag Iris
Derived from the mature spread above by a fixed rule, not checked separately: spacing is the spread, center to center.
Space Northern Blue Flag Iris about 2.5 ft apart, center to center, so each plant has room to reach its full width.
Add Northern Blue Flag Iris 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
Northern Blue Flag Iris' height, spread, hardiness zones, sun, water, bloom months and color, trait tags, soil, diseases, pests and toxicity were checked against 10 published sources on September 26, 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, trait tags, soil, diseases, pests and toxicity
- NC State ExtensionGardener Plant Toolboxheight, spread, hardiness zones, sun, water, bloom months and color, trait tags, soil, diseases, pests and toxicity
- Plant Addictsthe queue's store; brand line American Beauties Native Plantsheight, spread, hardiness zones, sun, water, bloom months and color, trait tags, soil, diseases and pests
- The Morton Arboretumheight, spread, hardiness zones, sun, water, bloom months and color, trait tags, soil, diseases and pests
- Penn State ExtensionRain Garden Plants: Blue Flag Irisheight, hardiness zones, sun, water, bloom months and color, trait tags, soil, pests and toxicity
- Chicago Botanic GardenPlant Finderheight, hardiness zones, sun, water, bloom months and color, trait tags and pests
- Lady Bird Johnson Wildflower CenterNative Plants Databaseheight, sun, water, bloom months and color, trait tags, soil and toxicity
- Prairie Moon NurseryWinona, Minnesotaheight, hardiness zones, sun, water, bloom months and color, soil and toxicity
- University of Illinois ExtensionHort Answersheight, hardiness zones, sun, water, bloom months and color and diseases
- prairienursery.comPrairie Nursery (Westfield, Wisconsin)height, hardiness zones, bloom months and color and trait tags
How the checking works, and what each standard covers, is on the verified plants page.
Good to know about Northern Blue Flag Iris
Curated from university extensions, botanic gardens and other primary sources, with the source named on every note.
Safety
Toxic to dogs, cats and horses; the rhizome is the most toxic part. Eating it causes drooling, vomiting, diarrhea and lethargy. Sap can irritate skin. per ASPCA and NC State
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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.