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Thread-like Naiad (Najas gracillima)

Life history

Species overview

Thread-like Naiad (Najas gracillima), a Wisconsin Special Concern plant, is found in oligotrophic lakes with soft water, often in calm, somewhat shallow water. Blooming occurs July; fruiting occurs August through early September. The optimal identification period for this species is July through August.



  • Distinguishing characteristics: A submerged plant with flexible stems up to 5 dm long and 0.2 to 0.7 mm thick. Of the other Najas species found in Wisconsin, Najas minor has stiffer, recurved leaves; Najas marina is dioecious and has prickly internodes and midvein; Najas guadalupensis has leaves with an abruptly acute tip; Najas flexilis has slender leaves that taper to an apex.
  • Flower characteristics: Flowers are tiny and axillary, with a perianth-like spathe. Male flowers have a single anther; female flowers have a style that is offset from the fruit's apex.
  • Fruit characteristics: Light brown and 2 to 3 mm long and 0.4 to 0.7 mm wide, with 20-45 rows of pits.
  • Leaf characteristics: Internodes are .01 to 3.2 cm long and are withouit prickles. Leaves are 0.5 to 3 cm long and 0.1 to .05 mm wide, with an expanded base that is fringed across the summit. The leaf margins are minutely serrulate with 13 to 17 teeth on each side that are difficult to see even with a hand lens.


  • Blooming phenology: July
  • Fruiting phenology: August through early September
  • Optimum time to identify: The optimal identification period for this species is July through August


  • Growth form: Aquatic forb-herb
  • Vegetative reproduction:
  • Life cycle: Annual
  • Comments: Associated species: Nymphaea odorata, Brassenia schreberi, Dulichium arundinacea, Potamogeton epihydrus, Nuphar variegata, Sagittaria latifolia, Utricularia vulgaris, Potamogeton pusillus, and Potamogeton amplifolius

State status

Status and Natural Heritage Inventory documented occurrences in Wisconsin

The table below provides information about the protected status - state and federal - and the rank (S and G Ranks) for Thread-like Naiad (Najas gracillima). See the Working List Key for more information about abbreviations. Counties shaded blue have documented occurrences for this species in the Wisconsin Natural Heritage Inventory database. The map is provided as a general reference of where this species has been found to date and is not meant as a range map.

Documented locations of Najas gracillima in the Natural Heritage Inventory Database as of July 2015.

Summary Information
State StatusSC
Federal Status in Wisconsinnone
State RankS3S4
Global RankG5?
Tracked by NHIY

Habitats and landscapes

The Natural Heritage Inventory has developed scores indicating the degree to which each of Wisconsin's rare plant species is associated with a particular natural community or ecological landscape. This information is similar to that found in the Wildlife Action Plan for animals. As this is a work in progress, we welcome your suggestions and feedback.

General habitat information

  • Habitat description: Found in oligotrophic lakes with soft water, often in calm, somewhat shallow water.
  • Soils: Sandy or silty soils.

Natural communities

This table lists the natural communities that are associated with Thread-like Naiad. Scores for natural community associations are: "significant" association (score=3), "moderate association" (score=2) or the species can be present but is only weakly associated with the community (score=1).

Ecological landscapes

This table lists the ecological landscape association scores for Thread-like Naiad. The scores (3=High, 2=Moderate, 1=Low, 0=None) also correspond to the map.

Map of the Ecological Landscapes of Wisconsin.

Species guidance

The Endangered Resources Program has developed avoidance measures and management guidelines for plants on the Natural Heritage Working List. These are a work in progress, and we welcome your suggestions and feedback. Sources used in developing this information can be found here.

Avoidance measures

These are specific actions designed to avoid "take" (mortality) of this species.

  • No avoidance measures have been developed for this species.

Management guidance

Management guidelines are additional considerations that may help maintain or enhance habitat for this species

  • No guidance has been developed for this species.


Thread-like Naiad Photo.

Photo © Paul Skawinski.

Support for Wisconsin's rare plant information has been provided by the Division of Forestry, the Endangered Resources Fund and the Wisconsin Rare Plant Preservation Fund. To donate, visit the Natural Resources Foundation of Wisconsin [exit DNR].

Last revised: Wednesday, May 05, 2021
Southwest Savanna Southern Lake Michigan Coastal Western Coulees and Ridges Southeast Glacial Plains Central Sand Hills Central Lake Michigan Coastal Central Sand Plains Northern Lake Michigan Coastal Northern Lake Michigan Coastal Northeast Sands Western Prairie North Central Forest Northern Highlands Northwest Lowlands Northwest Sands Northwest Lowlands Superior Coastal Plains Forest Transition