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Hydrilla

Hydrilla Leaf credit: Robert Vidéki, Doronicum Kft., Bugwood.orgHydrilla credit: Robert Vidéki, Doronicum Kft., Bugwood.orgHydrilla root & bulb credit: G.F. Hrusa, California Dept of Food & AgricultureHydrilla credit: Photo by Richard Old, www.xidservices.com

(Hydrilla verticillata)

Common Names

water thyme, Indian star-vine, Wasserquirl

Description

Regulated Plant, Not a Montana Listed Noxious Weed

Hydrilla is not an easy aquatic to identify due to its similarity to other species and its appearance, which can vary a great deal depending on conditions it grows in. Another complication in identifying Hydrilla is that this plant can either be monoecious (both male and female flowers on the same plant and is believed to have come from Korea) or dioecious (male and female flowers on different plants and is believed to have come from India) because each has some unique growth characteristics. In Montana the monoecious plant is the most likely to be found. Stems are very thin (approximately 1/32 inches in diameter) and can reach up to 30 feet in length. Leaves are small with pointed tips and have visible toothed edges. This plant branches out near the water’s surface and submerged fresh leaves are translucent and green, but near the water’s surface the leaves may bleach out and become yellow or brownish-green.  The female plant produces a single, white, tiny flower that floats on the surface of the water, but is attached to a long, thread-like stalk.  The male plants produce tiny green, white, red or brown flowers that break off of from the plant and float to the surface. Hydrilla is very prolific because it can reproduce four different ways: through fragmentation, tubers, turions, and by seed. Turions are small (1/4 inch in length) dark green or brown, spine-like structures that are produced along the stems and break free from the plant to sink to the bottom where they may remain dormant for several years. Tubers also form at the end of the root and are not generally visible. They are white or yellow in color, appear potato-like and also occur in the fall. Both turions and tubers can withstand extreme conditions including snow and ice cover, herbicides and even being eaten and spit out by birds! Hydrilla is commonly confused with Brazilian elodea (Egeria densa) and American Elodea (Elodea canadensis). Where it has invaded, it forms a monospecies that can cover hundreds of acres and clog the waterways.

Key Features

Four to eight leaves of Hydrilla occur in whorls around the stem (generally five leaves per whorl). The leaves have serrated edges or small spines along the leaf edge and the middle vein of the leaf is often a red color when it has not been exposed to sunlight or air.

Habitat

Hydrilla can grow in almost any freshwater; however, it does not prefer fast moving waters. It can grow in just a few inches of water or at depths of 20 feet deep, and it can grow in either high or low nutrient soils. It survives in harsh climates, easily surviving Montana winters. In northern climates, this plant over winters and re-grows from tubers. It can thrive in extremely shady conditions which gives it an advantage over many other species that require more sunlight. 

Currently found in the following counties:

Hydrilla has not been found in Montana

IWM

Interesting Facts

Hydrilla can grow in almost any freshwater; however, it does not prefer fast moving waters. It can grow in just a few inches of water or at depths of 20 feet deep, and it can grow in either high or low nutrient soils. It survives in harsh climates, easily surviving Montana winters. In northern climates, this plant over winters and re-grows from tubers. It can thrive in extremely shady conditions which gives it an advantage over many other species that require more sunlight. 

Commonly Confused Plants

Brazilian elodea, American Elodea

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Photo Credits (top to bottom): Robert Vidéki, Doronicum Kft., Bugwood.org; Robert Vidéki, Doronicum Kft., Bugwood.org;G.F. Hrusa, California Dept of Food & Agriculture; Photo by Richard Old, www.xidservices.com

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