Hydrilla [Hydrilla verticillata (L.f.) Royle][HYLLI][CDFA list: A][CalEPPC: Red Alert][Federal Noxious Weed] Photographs Maps of Distribution

Brazilian elodea [Egeria densa Planch.][ELDDE][CalEPPC: A-2] Photographs

Common elodea [Elodea canadensis Rich.][ELDCA] Photographs

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GENERAL DESCRIPTION: Submerged aquatic perennials. Plants are genetically variable and highly plastic depending on environmental conditions. Stems typically grow rooted in the substrate, but fragment easily into free-floating pieces that root at nodes. Fragments may start new colonies when carried elsewhere. Brazilian elodea and hydrilla can aggressively invade new aquatic environments, displace native aquatic vegetation by forming dense stands or large sub-surface mats, and alter the dynamics of aquatic ecosystems. Other detrimental impacts from heavy infestations can include water flow impediment in waterways, increased flooding, clogged pumps and boat propellers, diminished water clarity, reduced use of lakes and waterways for recreational activities, and economic loss.

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SEEDLINGS: Seldom encountered.hydrilla: Stalk below cotyledon (hypocotyl) lacking. Rootlets whorled. Cotyledon sheath 2-5 mm long, glabrous, often whitish-green with purplish dots. Cotyledon blade lanceolate, 6-8 mm long, 1-2 mm wide, +/- pinched in just above the base (attenuate), margins smooth. Taproot and first leaves develop simultaneously. First leaves compound. Leaflets 3-8, linear, 5-8 mm long, 0.5-1 mm wide, +/- sessile, tips acute, margins minutely toothed.

MATURE PLANT: Leaves sessile, whorled, linear to +/- lanceolate, often scale-like and opposite on lower stems. The number of leaves per node is unreliable for species identification. Margins minutely toothed, visible with low magnification.

ROOTS and UNDERGROUND STRUCTURES: Roots slender, unbranched, in substrate grow to ~ 20 cm long. Certain nodes on stems and stolons develop adventitious roots. Adventitious roots of hydrilla and common elodea typically develop only at nodes with dormant axillary buds or branches.

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FLOWERS: June-October. Flowers extend to the water’s surface on a long thread-like flower tube several centimeters long from an axillary spathe (fused bracts). Petals, sepals, styles 3. Ovary inferior. Pistillate (female) flowers often have staminodes (sterile stamens).

FRUITS and SEEDS: Fruit production and seed set is typically low.


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POSTSENESCENCE CHARACTERISTICS: Plants decompose rapidly. Stems of Brazilian elodea and common elodea typically decline when water temperatures climb above 25º C. Hydrilla and Brazilian elodea stems usually die during periods of prolonged near freezing temperatures. Common elodea may turn blackish and die-back during the cold season or survive green and intact under ice in near freezing conditions.

HABITAT: Slow-flowing or still water in ditches, sloughs, canals, rivers, ponds, lakes, reservoirs; often in nutrient-rich substrates.


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PROPAGATION/PHENOLOGY: All reproduce vegetatively. Seed is rarely produced in California. Vegetative parts disperse with flooding, waterfowl, and human activities, such as fishing and boating.

MANAGEMENT FAVORING/DISCOURAGING SURVIVAL: Removing and destroying stem fragments from recreational equipment, such as boat propellers, docking lines, and fishing gear can help prevent the spread of hydrilla and Brazilian elodea. Suddenly removing dense canopies of hydrilla by mechanical harvesting or herbicide treatment may stimulate turion germination. Sterile triploid grass carp consume hydrilla and are useful in aquatic systems where total removal of all submerged vegetation is acceptable.

SIMILAR SPECIES: Grassy naiad [Najas graminea Del.] and southern naiad [Najas guadalupensis (Sprengel) Magnus] are easily distinguished from the above species by having sessile flowers and +/-whorled linear leaves 0.5-1 mm wide with sheathing bases that are mostly crowded on short lateral branches.


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