Family: Hydrocharitaceae (Tape-Grass Family)
Duration and Habit: Perennial Submerged aquatic
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Hydrilla is a submersed typically rooted perennial plant that can form monospecific stands with dense mats at the surface. Its heavily branched stems can grow up to 9 m (30 ft) long. Leaves are simple and arranged in whorls of 4-8 leaves around the stem. The leaves are small, 2-4 mm (0.1-0.2 in) wide and 6-20 mm (0.2-0.8 in) long, and pointed. The finely serrated edges and typically 1-4 small bumps or teeth along the underside midrib are diagnostic. The whitish or translucent 3-petaled-&-3-sepaled flowers are tiny, only 1.5-3.0 mm (0.0625-0.125 in) in diameter, and float on threadlike stalks that are up to 10 cm (4 in) long.
Native Lookalikes: It is easy to confuse hydrilla with other members of the watermilfoil family such as native common elodea (Elodea canadensis) and non-native Brazilian waterweed (Egeria densa). However, neither has bumps or sharp teeth along the underside leaf midrib, elodea leaf margins are smooth and those of waterweed have minute teeth, and waterweed leaves are longer, 2-3 cm (0.8-1.2 in) long. No other submersed species produces turions, as hydrilla does.
common elodea (Elodea canadensis)
The leaves of common elodea are sparsely arranged on the stem and are typically arranged in whorls of 3. They are not serrated. The flower (not shown) is tiny.
Brazilian waterweed (Egeria densa)
NON-NATIVE. The leaves of Brazilian waterweed are crowded on the stem and typically in whorls of 4 or 5. They are very minutely serrated -- hard to notice in the hand. The flower is relatively large.
All three species (for comparison)
This diagram nicely illustrates the differences among the three species.
Ecological Threat: Hydrilla forms monospecific stands, often outcompeting and replacing native submersed vegetation. Dense underwater stands of hydrilla raise water pH and temperature, and lower dissolved oxygen. While the number of fish is often increased, large fish may become rarer. Hydrilla infestations can promote mosquito habitat. This invasive plant has the potential to affect power generation and agricultural irrigation by blocking intakes and impedes water flow.
Biology & Spread: In Texas, only the female dioecious form (plants with only single-sex flowers) is found and, consequently, seeds are not produced. Hydrilla produces tubers, which facilitate overwintering, as well as turions that can remain viable in the sediment for up to four years.
Otherwise, hydrilla commonly reproduces from fragmented stems that readily regrow, forming new clones of the original plant. Fleshy buds up to 5 cm (2 in) long that look like pine cones, called turions, are often produced from leaf axils (where petioles attach to the stem). These as well as subterranean tubers can grow into new plants. Hydrilla can grow up to 1 inch in a day and stems can reach 30 feet in length. Stems grow until they reach the water surface, filling the entire water column. Several members of this family are popular in the aquarium and ornamental pond industry because they are hardy, tolerant plants. Consequently, some have been introduced to waters in North America where many of those have become severe pest problems. Once established, hydrilla is easily spread through boating and fishing activities and by waterfowl, which feed on the tubers. Regurgitated tubers have been shown to be viable.
History: Hydrilla was first introduced into North America in the mid to late fifties by the aquarium trade. California officials have also traced hydrilla infestations to shipments of mail order waterlilies. Once introduced and established, hydrilla is easily spread through boating and fishing activities and by waterfowl. Hydrilla tubers are readily consumed and regurgitated tubers have been shown to be viable.
U.S. Habitat: Hydrilla is found in lakes, rivers, reservoirs, ponds, and ditches. It tends to form monospecific stands that can cover hundreds of acres. Hydrilla can be found in depths of greater than 20 feet where water clarity is good. Increased water clarity in lakes with invasive zebra mussels (Dreissena polymorpha) can allow hydrilla to grow to greater depths, exacerbating infestations.
U.S. Nativity: Introduced to U.S.
Native Origin: The dioecious form found in Texas is native to the Indian subcontinent whereas the monoecious form is believed to be native to Korea.
U.S. Present: AL, AZ, AR, CA, CT, DE, DC, FL, GA, GU, ID, IL, IN, IA, KS, KY, LA, MD, MA, MS, MO, NJ, NY, NC, OH, OK, PA, PR, SC, TN, TX, VA, WA, WV, WI
Distribution in Texas: Hydrilla is now found across the eastern US from Texas to Maine and is especially widespread in coastal states. Hydrilla is also found in several western states. The distributions of the monoecious and the dioecious forms overlap in some areas and are separate in others. The USGS Nonindigenous Aquatic Species database point map allows you to see the distributions of both.
List All Observations of Hydrilla verticillata reported by Citizen Scientists
Native Texas alternatives include
Scientific research and 30 years of practical experience by aquatic plant managers using herbicides, biological agents, mechanical removal, and physical habitat manipulation have produced relatively successful management programs in Florida and other states. However, in spite of long-term intensive management efforts, hydrilla is still a major weed problem in the states where it has become well-established. Physical removal of hydrilla can be effective in small areas such as boat slips but treatments with herbicides are necessary to control extensive infestations. Care should be taken during physical removal; fragmentation promotes spread. Water drawdowns can be effective for management. Because hydrilla is prohibited in Texas, a permit may be required for physical removal. A nuisance aquatic vegetation treatment proposal is required for chemical, physical, or biological control of any aquatic plant species on a public water body.USE PESTICIDES WISELY: ALWAYS READ THE ENTIRE PESTICIDE LABEL CAREFULLY, FOLLOW ALL MIXING AND APPLICATION INSTRUCTIONS AND WEAR ALL RECOMMENDED PERSONAL PROTECTIVE GEAR AND CLOTHING. CONTACT YOUR STATE DEPARTMENT OF AGRICULTURE FOR ANY ADDITIONAL PESTICIDE USE REQUIREMENTS, RESTRICTIONS OR RECOMMENDATIONS. MENTION OF PESTICIDE PRODUCTS ON THIS WEB SITE DOES NOT CONSTITUTE ENDORSEMENT OF ANY MATERIAL.
The Quiet Invasion: A Guide to Invasive Plants of the Galveston Bay Area. Lisa Gonzalez and Jeff DallaRosa. Houston Advanced Research Center, 2006.
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