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Spiny naiad

(Najas marina)

Photo of spiny naiad
Photo credit: Paul Skawinski

Spiny naiad is a submerged aquatic plant that typically grows in lakes, ponds, reservoirs or slow-moving rivers and streams. Although native to certain areas of the U.S., it is not a native aquatic plant in Wisconsin and may pose a threat to water bodies.

Overview

Other names for this plant include:
  • Common names: holly-leaved naiad, spiny water-nymph, spiny-leaf naiad, alkaline water-nymph, holly-leaved water nymph, large naias, sawtooth, marine naiad, marine water nymph.
  • Scientific names: Najas gracilis, Najas major, Najas marina var. recurvata, Najas major var. angustifolia
Ecological threat:
  • In suitable habitats, spiny naiad can form dense mats of submerged vegetation that can choke out other aquatic plant species. In high densities, it can be a nuisance for boaters and anglers.
  • Spiny naiad is readily eaten by waterfowl, and the seeds of the plant have been shown to germinate after digestion. This means plants could easily be spread by waterfowl.
  • Seeds are considered to be long-lived in the seed bank, which means short times of drawdown or low water are unlikely to be effective in eradicating this plant.
Overview map of spiny naiad classification in WI
Restricted (orange) counties

Classification in Wisconsin: Restricted

Species Assessment Groups (SAG) were assembled to recommend a legal classification for each species considered for NR 40. The recommendation for spiny naiad was based upon this literature review [PDF] developed by the department.

Identification

Leaves: Opposite or in whorls of three on the stem, 0.5 to 4.0 cm long and 0.4-4.5 mm wide. Leaves have 8-13 triangular teeth along the leaf margins and prickles on the underside midrib.

Flowers: Flowers are submerged and inconspicuous. They are water-pollinated and typically solitary in the leaf axils. Flowering occurs from late summer into autumn.

Fruits & seeds: Large amounts of hard, oval, brown seeds are produced and then dispersed by water.

Stems: Brittle and branched towards the upward section of the plant. Internodes on the stem have conspicuous, brown, prickly teeth.

Similar species: Other native naiad species (N. flexilis, N. gracillima, N. guadalupensis) exist in Wisconsin, but can be easily distinguished as they do not have the prickles or easily visible teeth that spiny naiad has. Another invasive naiad, Najas minor or brittle naiad, has the visible teeth along the leaf edges but lacks the prickles at the internodes.

Distribution

See the reported locations of spiny naiad in Wisconsin.

Do you know of additional populations? Send us a report.

Control

Mechanical: Mechanical harvesting or hand-pulling can reduce the population of spiny naiad, but it can grow from stem fragments, so care must be taken to avoid fragmentation or collect all parts. Due to the robust spines on the plant, physical removal by hand can be difficult.

Chemical: A range of aquatic approved herbicides are available and have been shown to be effective in controlling spiny naiad. Aquatic applications of herbicide will require a permit.

Photos

View spiny naiad pictures in our photo gallery!

Resources

Sources for content:
  • DiTomaso, J.M., G.B. Kyser et.al., Weed Control in Natural Areas in the Western United States. Weed Research and Information Center, University of California, 544pp.
  • USGS NAS Factsheet: factsheet [exit DNR]
Links for more information: