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Montana Field Guide

Montana Field Guides

Hydrilla - Hydrilla verticillata
Other Names:  Waterthyme, Water-thyme, Water-thyme

Regulated Weed: Priority 3
Non-native Species
Not Documented

Global Rank: GNR
State Rank: SNA
(see State Rank Reason below)

Agency Status


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State Rank Reason (see State Rank above)
Hydrilla verticillata is not known to occur in Montana. The Montana Department of Agriculture (MDA) classifies it as a Priority 3-Regulated Plant, which means it may not be intentionally spread or sold other than as a contaminant in agricultural products (MDA 2017). It is not a Montana listed noxious weed (MDA 2017). The purpose of this profile is to provide awareness and education.

It remains uncertain as to where Hydrilla is truly native, and is reported to have entered the U.S. in Miami, Florida in 1958 or 1959 (FNA 2000; DiTomaso and Healy 2003). It was introduced as an aquarium plant (FNA 2000).
General Description
PLANTS: Aquatic perennials that grow rooted in the substrate, but easily fragment. Plants produce a matt of stolons that develop tubers and stems that produce turions [see Reproduction section].

LEAVES: Mature leaves are sessile, whorled in groups of (3)4-8(12) linear to lanceolate, 6-20 mm long, and 1-4 mm wide. On lower stems, leaves may be scale-like and opposite. Leaf margins are toothed, which can be observed without magnification. The lower surface of each leaf vein may be smooth or minutely and sharply toothed. At the leaf axils are pairs of small scales (squamulae intravaginales), up to 0.5 mm long, fringed with orange-brown hairs (use hand-lens and examine mature leaves).

INFLORESCENCE: A single flower grows fused bracts (spathe) at a leaf node (DiTomaso and Healy 2003). The flower’s hypanthium forms a long, thread-like tube to reach the water’s surface (DiTomaso and Healy 2003). Flowers detach and float to the surface. Flowers have translucent whitish to reddish petals and sepals of 3-5 mm long. Male flowers have linear petals and 3 stamens that upon maturity dump pollen onto the water’s surface. Flowers are wind-pollinated.

Flowering occurs from June to October (DiTomaso and Healy 2003).

Diagnostic Characteristics
The native Elodea canadensis has dark green leaves that are often speckled with large dark cells (DiTomaso and Healy 2003). Middle and upper leaves are opposite or 3-whorled and those below the growing tips often curve slightly downward (DiTomaso and Healy 2003). Leaves are often relatively longer and narrower in comparison to exotic species. The mid-vein of each leaf is smooth on its underside (DiTomaso and Healy 2003).

The exotic Egeria densa also has whorled leaves, but in groups of 3-8 that are 15-40 mm long and 2-5 mm wide. The mid-vein of each leaf is smooth or minutely toothed on its underside (DiTomaso and Healy 2003). Egeria densa plants often are larger and leafier than Hydrilla verticillata or Elodea canadensis.

Montana’s native Najas flexilis and Najas guadalupensis [Montana Species of Concern] can appear similar, but have sessile flowers and whorled leaves that are more narrow, have dilated bases, and are sheathing.

Range Comments
For maps and other distributional information on non-native species see:
Nonindigenous Aquatic Species Database from the U.S. Geological Survey
Invasive Species Habitat Tool (INHABIT) from the U.S. Geological Survey
Invasive Species Compendium from the Centre for Agriculture and Bioscience International (CABI)
EDDMapS Species Information EDDMapS Species Information

Shallow (0.5 – 3 (12) meters deep) waters of acidic to alkaline lakes and streams (FNA 2000; DiTomaso and Healy 2003).
Predicted Suitable Habitat Model

This species has a Predicted Suitable Habitat Model available.

To learn how these Models were created see

Plants decompose quickly and periods of pro-longed temperatures near freezing can kill plants (DiTomaso and Healy 2003). Plants grow at water temperatures from 10 to 35 degrees Celsius. Turions survive near freezing temperatures. Plants tolerate low light and brackish, turbid, or polluted water quality.

Tubers can survive ingestion and regurgitation by waterfowl and are resistant to herbicides (DiTomaso and Healy 2003).

Reproductive Characteristics
Hydrilla verticillata can be dioecious (male and female flowers occur on separate plants) or monoecious (male and female flowers occur on same plant) (DiTomaso and Healy 2003). Dioecious plants may appear more robust, branch near the water’s surface to form submerged mats, and have larger tubers. Monoecious plants branch near the substrate, produce stolons and dense, vertical shoots, and have smaller tubers.

The first leaves (cotyledon) appear with no stalk and rootlets that are whorled. The cotyledon’s sheath if 2-5 mm long, glabrous (no hairs), and usually white-green with purplish dots. The cotyledon blade is lanceolate with entire margins and an attenuate base, and 6-8 mm long by 1-2 mm wide. The seedling develops a taproot and its first set of leaves at same time.

Plants reproduce by seeds or vegetative by stolons, stem fragments, stem turions, subterranean and tubers.

Fruits and Seeds
In general, fruit production is low (DiTomaso and Healy 2003). Fruits are narrowly cylindrical, 5-15 mm long, smooth or with irregular spines, and do not open to release the seeds. Fruits release 1-5 seeds which are elliptical, 2-3 mm long, smooth, and brown.

Stem Turions: At the leaf nodes stems develop turions, which are buds enclosed by tough leaf scales. Turions are green, cone-shaped to ovoid, 3-12 mm long, and deciduous.

Turion germination can be stimulated after a period of low temperatures (DiTomaso and Healy 2003). Stem turions germinate in the spring. Stem turions develop on plants from late summer through fall, and separate upon maturity (usually late fall). Monoecious plants often produce more turions than dioecious plants. Dense stands of plants usually produce fewer turions. Monoecious turions may survive up to 4 years under field conditions.

Tubers: Tubers are subterranean structures that are whitish to brown-black, plump, ovoid, 4-15 mm long, that occur to 15 cm deep in the substrate (DiTomaso and Healy 2003). Tubers stay attached to the stolon until it decomposes.

Tubers can germinate in the spring after a period of low temperatures or remain dormant (DiTomaso and Healy 2003). In dioecious plants, tubers usually develop in mid-summer through winter, but monoecious tubers develop in the spring. Dioecious plants produce larger tubers, more likely to remain dormant, and survive longer. Monoecious plants produce more tubers (by 50%) in a relatively short period, which may survive up to 5 years under field conditions. Under dry conditions tubers survive for only a few days to weeks.

Plants disperse vegetatively with flooding, waterfowl, and recreational activities (boating and fishing) (DiTomaso and Healy 2003).

To prevent the spread of Hydrilla, stem fragments must be removed and destroyed from recreational equipment (boat propellers, docking lines, fishing gear, etc.) (DiTomaso and Healy 2003). Turion germination may be stimulated when mechanically harvesting or applying herbicide to dense canopies (DiTomaso and Healy 2003).

Useful Links:
Montana Invasive Species website
Montana Biological Weed Control Coordination Project
Montana Department of Agriculture - Noxious Weeds
Montana Weed Control Association
Montana Weed Control Association Contacts Webpage.
Montana Fish, Wildlife, and Parks - Noxious Weeds
Montana State University Integrated Pest Management Extension
Weed Publications at Montana State University Extension - MontGuides

  • Additional ReferencesLegend:   View Online Publication
    Do you know of a citation we're missing?
    • DiTomaso, J.M. and E.A. Healy. 2003. Aquatic and riparian weeds of the West. Regents of University of California, Division of Agriculture and Natural Resources, Publication 3421.
    • Flora of North America Editorial Committee (FNA). 2000. Flora of North America north of Mexico. Vol. 22. Magnoliophyta: Alismatidae, Arecidae, Commelinidae (in part), and Zingiberidae. Oxford Univ. Press, New York. xxiii + 352 pp.
    • Montana Department of Agriculture (MDA). 2017. Montana Noxious Weed List. February. Helena, Montana.
  • Web Search Engines for Articles on "Hydrilla"
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Citation for data on this website:
Hydrilla — Hydrilla verticillata.  Montana Field Guide.  .  Retrieved on , from