Hydrilla verticillata

Non-Native to Florida

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HYDRILLA VERTICILLATA (L. f.) Royle
hydrilla, water thyme
Hydrocharitaceae/Frog's-bit Family

pronounced: hid-ril-lah ver-ti-ki-lah-ta (long/short marks)
from: hydr (G.): water
verticillus (L.): the whorl of a spindle
"water plant with whorls of leaves"

Synonymy:
none known

Hydrilla is a submersed plant. It can grow to the surface and form dense mats. It may be found in all types of water bodies.

Hydrilla stems are slender, branched and up to 25 feet long. Hydrilla's small leaves are strap-like and pointed. They grow in whorls of four to eight around the stem. The leaf margins are distinctly saw-toothed. Hydrilla often has one or more sharp teeth along the length of the leaf mid-rib. Hydrilla produces tiny white flowers on long stalks. It also produces 1/4 inch turions at the leaf axils and potato-like tubers attached to the roots in the mud.

 

Habit:

• Hydrilla is a herbaceous perennial.
• Submersed, "obligate" (requiring a wet habitat).
• Forming dense stands of very long stems (25 ft.) in the water.
• Reproduces mainly by regrowth of stem fragments; also reproduces by growth of axillary buds(turions) and subterranean tubers; tubers can remain viable for more than 4 years (Van & Steward 1990).
• A single tuber can grow to produce more than 6,000 new tubers per m² (Sutton et al. 1992).

 

Habitat:

• Hydrilla can grow in almost any freshwater: springs, lakes, marshes, ditches, rivers, tidal zones.
• Can grow in only a few inches of water, or in water more than 20 feet deep.
• Can grow in oligotrophic (low nutrient) to eutrophic (high nutrient) conditions.
• Can grow in 7% salinity of seawater (Haller 1974).
• Temperature tolerance: hydrilla is somewhat winter-hardy; its optimum growth temperature, 20-27^o C (68-81^o F); its maximum temperature, 30^o C (86^o F) (Kasselmann 1995).
• U.S. southern populations overwinter as perennials; northern populations overwinter and regrow from tubers.
• Can grow in only 1% of full sunlight.
• Low light compensation and saturation points and low CO₂ compensation point make it a competitive plant because it can start growing in low light before other plants do (Van et al. 1976; Bowes 1977).

 

Hydrilla verticillata (L. f.) Royle
Original description: Linnaeus 1782; Royle 1839

• Monocot, forb, submersed.
• Rooted in the hydro-soil, adventitious roots are white.
• The plant is submersed, except when branches have reached and grown across the water surface; sometimes found as detached floating mats.
• Appearance can vary substantially, depending on growth conditions (Pieterse et al. 1985).
• Stems submersed, slender (about 1/32 in. thick) and sinewy, long (to 25 feet).
• Profuse branching occurs near the water surface.
• Leaves small (5/8 in. long), strap-like, pointed tips, conspicuous midrib; arranged in whorls of 4 to 8, joined directly to the stem, whorl internodes 1/8 to 2 in. long.
• Leaf margins distinctly visible saw-toothed; often with one or more sharp teeth along the underside midrib.
• Leaf color green; clean leaves are transluscent; topped out leaves, bleached by the sun and attacked by fungus and bacteria, may appear yellowish to brownish-green.
• Flowers, male and female.
• Female flowers solitary, tiny, white, floating on the surface; reaching to the surface on "long" (to 4 in.) threadlike stalks, stalks attached at leaf axils near the stem tips; at surface, flower opens to form a wide funnel into the water; petal rims hold flower to surface and prevent water from getting into flower; female petals 6, to 1/16 in. long; stamens 3, styles 3, stigmas 3.
• Male flowers tiny, greenish, closely attached to leaf axils toward stem tips, until they break loose and rise to the surface where they free-float, sometimes in large numbers, where they fertilize the female flowers by randomly bumping into them; male flowers at surface like "inverted bells."
• Other reproductive parts
• Turions ("buds" in some of the leaf axils), are dark green, cylindrical, to 1/4 in. round, occasional.
• Subterranean turions ("tubers"), are yellowish, potato-like, attached to the root tips in the hydrosoil, to 1/2 in. long, 1/2 inch broad.

 

Hydrilla verticillata might be confused with at least two other submersed, long-stemmed plants having small narrow leaves.

Here is a comparison:

• Native elodea (Elodea canadensis):
--elodea leaves occur in whorls of 3 around the stem, Hydrilla verticillata leaves are in whorls of 4 to 8 around the stem.
• Non-native egeria (Egeria densa):
--egeria leaves are in whorls of 4-5; Hydrilla leaves are in whorls of 4 to 8.

 

Origin:

• There is only one species of Hydrilla in the world.
• Hydrilla verticillata's dioecious type (plants having female flowers only) originates from southern India. Hydrilla's monoecious type (plants having male and female flowers on the same plant) is probably from Korea. (Madeira et al. 1997).
• Occurs in Europe, Asia, Australia and the Pacific, Africa, South America and North America.

 

Distribution in the U.S.:

• Hydrilla verticillata is present from Florida to Connecticut and west to California and Washington. Its dioecious form is mainly in the southern U.S.; north of South Carolina hydrilla is mainly monoecious.

The best way to track the spread of invasive aquatic plants may be to identify the drainage basins (watersheds) they have been discovered in. Drainage maps give useful information to eco-managers because drainage maps show precisely where the plants are, making it easier for managers to infer where the plants might go next, and thus where to take preventive measures.

 

How it got here:

• Hydrilla verticillata was probably brought to the Tampa and Miami, Florida areas as an aquarium plant in the late 1950s; by the 1970s, it was established throughout Florida. The monoecious type was introduced separately much later in the Potomac Basin (Enrionmental Laboratory 1985).

Hydrilla verticillata continues to be sold through aquarium supply dealers and over the Internet, even though the plant is on the U.S. Federal Noxious Weed List.

 

Potential to spread elsewhere in U.S.:

• Hydrilla spreads to new waters mainly as fragments on boats and trailers.
• In Russia, hydrilla grows to 50^o N latitude--equivalent to the US/Canadian border.

 

Problems/Effects:

• Hydrilla verticillata is an invisible menace, invisible that is until it fills the lake or river that it infests, "topping out" at the surface. Hydrilla can grow an inch a day. When hydrilla invades, ecologically-important native submersed plants such as pondweeds (Potamogeton spp.), tapegrass (Vallisneria americana) and coontail (Ceratophyllum demersum) are shaded out by hydrilla's thick mats, or are simply outcompeted, and eliminated (van Dijk 1985).
  
• Millions of dollars are spent each year on herbicides and mechanical harvesters in Florida alone in an effort to place hydrilla under "maintenance control"
• Hydrilla verticillata greatly slows water flow and clogs irrigation and flood-control canals; in Florida, large mats of fragments collect at culverts and clog essential water control pumping stations.
• Hydrilla seriously interferes with boating, both recreational and commercial, and prevents swimming and fishing; major infestations limit sportfish weight and size (Colle & Shireman 1980).
• Dense hydrilla infestations can alter water chemistry and oxygen levels (Pesacreta 1988).

 

Control:

Due to decades of university, state and federal research and experience with Hydrilla verticillata in the U.S., several methods have been developed to help in its management:

In some cases, lake drawdowns may help manage hydrilla by letting the exposed plants die and decompose.

The action of mechanical harvestors and chopping machines remove hydrilla from the water and transport it to disposal on shore; chopping machines, unfortunately, fragment the hydrilla plants and may actually increase the plant's distribution.

Biocontrol fish and insects have been introduced to control hydrilla. The herbivorous (plant-eating) biological control fish, the Chinese grass carp, has a good preference for hydrilla (Cassani). Learn much more about the Chinese grass carp on this page of this web site. Other biological control work has been done for this species, including tuber-feeding weevils and leaf-eating flies.

Registered aquatic herbicides do provide temporary control of hydrilla;

From the University of Florida Aquatic Weed Management Guide, Vandiver 1999:

According to this Guide, ... copper, diquat, endothall and fluridone; As always, comply with federal law by following the herbicide label instructions, permissible sites and application rates.

 

What can you do?

First, clean your boat before you leave the ramp! Transporting plant fragments on boats, trailers, and in livewells is the main introduction route to new lakes and rivers.
But, there's plenty more you can do to help.

 

Laws and lists:

Hydrilla verticillata
• Is "state-listed" in Arizona, Arkansas, California, Florida, New Hampshire, North Carolina, Oregon, Puerto Rico, South Carolina, Arkansas, California and Washington.
• Is on the Florida Prohibited Plants list, Florida Department of Environmental Protection:
• Is on the Federal List of Noxious Weeds (USDA/APHIS, 2000).
• Is on the Florida Exotic Pest Plant Council list:
• Category I - "plants invading and disrupting native plant communities in Florida."

 

Want to know more?

The information contained on this wep page was extracted from published scientific literature and agency reports. It is important to know that plant research, like most areas of scientific research, is still relatively young and incomplete--much may have been published about the physiology of one plant but not about its management; much may have been published about how to culture and grow another plant but not about its natural ecology. Thousands of research articles may have been published about one invasive plant, but perhaps only a dozen about another.

If you want to read the research yourself, perhaps to clarify or expand an area of information contained here, or to help determine your own line of research, you are welcome to query the world's largest collection of international scientific literature about aquatic, wetland and invasive plants, the APIRS bibliographic database, which contains more than 54,000 citations and their content keywords. Or you might want to ask us to do it for you and mail or e-mail the search results to you.

 

This is the literature about Hydrilla verticillata that was used to develop this web page. More research items about this plant may be found at APIRS:

• Bsiouny FM, Haller WT, Garrard LA. 1978. Survival of hydrilla(Hydrilla verticillata) plants and propagules after removal from the aquatic habitat. Weed Sci. 26:502-504.
• Blackburn RD, Weldon LW, Yeo RR, Taylor TM. 1969. Identification and distribution of certain similar-appearing submersed aquatic weeds in Florida. Hyacinth Control J. 8:17-23.
• Bowes G, Holaday AS, Van TK, Haller WT. 1977. Photosynthetic and photorespiratory carbon metabolism in aquatic plants. In Proceedings 4th Int. Congress of Photosynthesis, Reading (UK). pp. 289-298.
• Cassani JR. 1996. Managing aquatic vegetation with grass garp. A guide for water resource managers. American Fisheries Soc., Introduced Fish Sect., Bethesda, MD., 196 pp.
• Colle DE, Shireman JV. 1980. Coefficients of condition for largemouth bass, bluegill and redear sunfish in hydrilla-infested lakes. Transactions of the American Fisheries Society 109:521-531.
• Cook CDK, Lund RL. 1982. A revision of the genus Hydrilla (Hydrocharitaceae). Aquat. Bot. 13:485-504.
• Environmental Laboratory, Army Corps of Engineers. 1985. Monoecious Hydrilla in the Potomac River. Miscellaneous Paper MP A-85-5. US Army Engineer Waterways Experiment Station, Vicksburg, MS.
• Haller WT, Miller JL, Garrard LA. 1976. Seasonal production and germination of hydrilla vegetative propagules. J. Aquat. Plant Manage. 14:26-29.
• Haller WT, Sutton DL, Barlowe WC. 1974. Effects of salinity on growth of several aquatic macrophytes. Ecology 55($):891-894.
• Kasselmann C. 1995. Aquarienpflanzen. Egen Ulmer GMBH & Co., Stuttgart. 472 pp. (In German)
• Madeira P, Van T, Steward K, Schnell R. 1997. Random amplified polymorphic DNA analysis of the phenetic relationships among world-wide accessions of Hydrilla verticillata. Aquatic Botany 59:217-236.
• McCann JA et al. 1996. Nonindigenous aquatic and selected terrestrial species of Florida-Status, pathway, and time of introduction, present distribution, and significant ecological and economic effects. Southeastern Biological Science Center, Gainesville, 256 pp.
• Pesacreta G. 1988. Water Chemistry from North Carolina Piedmont Impoundments with Hydrilla (Hydrilla verticillata (L.f.) Royle). Ph.D. dissertation, North Carolina State University, Raleigh.
• Pieterse AH. 1981. Hydrilla verticillata - a review. Abstr. Trop. Agric. 7:9-34.
• Salvucci ME, Bowes G. 1983. Two photosynthetic mechanisms mediating the low potorespiratory state in submersed aquatic angiosperms. Plant Physiol. 73:488-496.
• Spencer DF, Anderson LWJ. 1986. Photoperiod responses in monoecious and dioecious Hydrilla verticillata. Weed Sci. 34:551-557.
• Steward KK, Van TK. 1987. Comparative studies of monoecious and dioecious hydrilla (Hydrilla verticillata) biotypes. Weed Sci. 35:204-210.
• Stocker RK. 1996. Control technologies for use against the submersed aquatic weeds hydrilla and hygrophila. Semi-Annual Rept., USDA/ARS-IFAS, Gainesville, FL, 68 pp.
• Van TK, Haller WT, Bowes G. 1976. Comparison of the photosynthetic characteristics of three submersed aquatic plants. Plant Physiol. 58:761-768.
• Van TK, Steward KK. 1990. Longevity of monoecious hydrilla propagules. J. Aquat. Plant Manage. 28:74-76
• Van TK, Steward KK. 1982. Evaluation of chemicals for aquatic plant control. Annual Report 1981, Ft. Lauderdale, FL, 66 pp.
• van Dijk G. 1985. Vallisneria and its interactions with other species. Aquatics 7(3):6-10.
• Vandiver VV. 1999. Florida aquatic weed management guide. Univ. of FL, IFAS, Cooperative Extension Service, Publ. SP-55, 130 pp.
• Verkleij JAC, Pieterse AH, Horneman GJT, Torenbeek M. 1983. A comparative study of the morphology and isoenzyme patterns of Hydrilla verticillata (L.f.) Royle. Aquat. Bot. 17:43-59.

 

Other web sites that treat Hydrilla verticillata:

Visit the Osceola County Hydrilla & Hygrophila Demonstration Project web site for current research and information.

View the booklet: Hydrilla verticillata: The Perfect Aquatic Weed.

Download the Recognition Card of Hydrilla verticillata (PDF 1.3 MB).

See more images and information about hydrilla, as contained in the book, Identification & Biology of Non-Native Plants in Florida's Natural Areas.

Here is the PDF file of Hydrilla Management in Florida: A Summary and Discussion of Issues Identified by Professionals with Future Management Recommendations Final Document: 2005.

See the UF/IFAS Assessment, which lists plants according to their invasive status in Florida.

View the herbarium specimen image of the University of Florida Herbarium Digital Imaging Projects.

This web page was authored in June, 2001, by Victor Ramey (Center for Aquatic and Invasive Plants, University of Florida), with significant contribution from Barbara Peichel (Sea Grant, University of Minnesota). The information contained herein is based on the literature found in the APIRS database.