- Scientific name: Iris prismatica
- Species of Greatest Conservation Need (MA State Wildlife Action Plan)
Description
Close up of a slender blue flag flower.
Iris prismatica is a wild species of Iris with large, bright blue flowers to 8 cm (3.15 in) in overall width, that resembles familiar garden varieties of Irises but occurs in wetlands and wet meadows. It closely resembles the common marsh blue flag Iris (Iris versicolor) though with visibly narrower leaves, up to 7 mm (0.28 in) in width. In addition, the fruit capsules, up to 4 cm (0.16 in) in length, are sharply triangular in this species versus the rounded capsule of the more common species. And though one would likely have to dig one up to be sure, the rhizomes on slender blue flag are like a narrow rope or boot lace that extends from plant to plant making a rhizomatous clone, rather than the thick rhizomes of the mash blue flag that grows in a tight clump. The rhizomes of slender blue flag are very close to the surface. The only other tall, violet flowered Iris with narrow leaves is the introduced garden Siberian iris (Iris sibirica). It differs from Iris prismatica by having hollow stems, fruits with two rows of seeds per locule, not found in natural wetlands, and typically only growing near places that are or were landscaped at one time.
Life cycle and behavior
Slender blue flag is an herbaceous perennial that is a wetland obligate species.
Population status
Slender blue flag occurs currently only in Essex, Bristol, Barnstable, Dukes and Nantucket Counties (iNaturalist 2025). Previous it was also common in Middlesex, Suffolk, and Plymouth County. Populations have sharply diminished over time. Massachusetts herbarium records show 137 specimen records with a legible date. Of those, only 27 occurred in the last 90 years. The median year is 1911 (Consortium of Northeastern Herbaria 2025). iNaturalist has 32 research grade records in the state, the vast majority on Nantucket and Martha’s Vineyard. These should be considered a relatively reliable reflection of the current populations as the flower is large and brightly colored, and when flowering, the flower is at the same level of the top of the surrounding vegetation making the species quite visible from afar. It occurs only in eastern Massachusetts where most of the human population is and where most iNaturalist observations occur (over 1.16 million vascular plant species observations statewide as of 1 April 2025).
Distribution and abundance
Slender blue flag is known historically from all US states that touch on the Atlantic Ocean, plus Alabama and two inland states—West Virginia and Tennessee. It is rare to uncommon throughout that range, and considered critically imperiled in New Hampshire, North Carolina, Pennsylvania and Alabama (NatureServe 2025). iNaturalist (2025) shows 222 research-grade observations in the U.S. with no current populations in Maine, New Hampshire and Georgia, but occurrences throughout the rest of its historic range.
In New York there are 15 remaining populations of slender blue flag, which is listed as state-threatened. (New York Natural Heritage Program 2025).
Distribution in Massachusetts
1999-2024
Based on records in the Natural Heritage Database
Habitat
In Massachusetts, the habitat is in wet hay meadows harvested once a year for forage. These sites are often strongly influenced by introduced cool-season European grasses such as sweet vernal grass (Anthoxanthum odoratum) and so are somewhat degraded. Other habitats include edges of older cranberry bogs where sand disturbance has occurred, and the edges of salt marshes with freshwater influences.
In New York, slender blue flag is known from a variety of open, wet, coastal habitats including sea level fens, the edges of salt and brackish meadows, as well as from diverse disturbed habitats such as burned or scraped old fields, ditches, and roadside swales. (New York Natural Heritage Program 2025).
Healthy habitats are vital for supporting native wildlife and plants. Explore habitats and learn about conservation and restoration in Massachusetts.
Nantucket wet meadow with slender blue flag. This is one of Massachusetts best populations.
Threats
Blue slender flag
Changes in hydrology and overgrowth by invasive species are the two main threats to this species. Ironically, probably the best site for this species is on Nantucket at a disused cranberry bog that is undergoing full scale wetland restoration. It is too soon at this date (April 2025) to know how slender blue flag will be affected by these changes, and though restoration plans did account for keeping as much of this as possible, hydrologic changes are expected to occur.
Conservation
Much more survey work to find more populations. One obvious approach is to systematically review old specimen data for locations and resurvey those sites to see if the plants might still be there. Surveys of known populations are needed to establish numbers and gather data on associated species, soil conditions and former or current management practices on all populations.
Management should consist of annual mowing where possible to control shrub and invasive species. Prescribed fire should be considered in dry years to maintain open habitat.
Acknowledgement
MassWildlife acknowledges the expertise of Alexey Zinovjev and Irina Kadis, whose blog post (2013) contributed substantially to the development of this fact sheet and the conservation of this species.
References
Consortium of Northeastern Herbaria. 2025. Herbarium records.
https://portal.neherbaria.org/portal/collections/list.php. Accessed 1 April 2025
Cullina M, Connolly B, Sorrie B, Somers P (2011) The vascular plants of Massachusetts: a county checklist, 1st revision. Massachusetts Natural Heritage & Endangered Species Massachusetts Division of Fisheries and Wildlife, Westborough, MA
Gleason, Henry A., and Arthur Cronquist. Manual of Vascular Plants of Northeastern United States and Adjacent Canada, Second Edition. Bronx, NY: The New York Botanical Garden, 1991.
Haines A (2011) New England Wild Flower Society’s Flora Novae Angliae: a manual for the identification of native and naturalized higher vascular plants of New England. Yale University Press. 1008 pp.
iNaturalist 2025. Available from https://www.inaturalist.org. Accessed 1 April 2025
NatureServe. 2025. NatureServe Network Biodiversity Location Data accessed through NatureServe Explorer [web application]. NatureServe, Arlington, Virginia. Available https://explorer.natureserve.org/. Accessed: 3/31/2025
New York Natural Heritage Program. 2025. Online Conservation Guide for Iris prismatica. Available from: https://guides.nynhp.org/slender-blue-flag/. Accessed March 31, 2025.
Zinovjev, A., and I. Kadis. 2013. Slender blueflag (Iris prismatica Pursh) in Massachusetts. Website http://salicicola.com/projects/flora/iris/prismatica/ [accessed 1 April 2025].
Contact
| Date published: | April 25, 2025 |
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