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

SYNONYMS:

• hydrilla: waterthyme, Elodea verticillata (L.f.) F. Muell., Serpicula verticillata L.f., Hydrilla lithuanica (Andrz. ex Besser) Dandy
• Brazilian elodea: Brazilian or dense waterweed, leafy elodea, egeria, Philotria densa (Planch.) Small & St. John, Elodea densa (Planch.) Caspary, Anacharis densa (Planch.) Victorin
• common elodea: Canadian waterweed or pondweed, broad waterweed, American elodea, Philotria canadensis (Michx.) Britt., Philotria linearis Rydb., Elodea brandegae St. John, Elodea ioensis Wylie, Elodea linearis (Rydb.) St. John, Elodea planchonii Caspary, Anacharis canadensis (Michx.) Planch., Anacharis canadensis var. planchonii (Caspary) Victorin

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.

• hydrilla: Noxious. Many biotypes exist, including monoecious and dioecious types (male and female flowers on the same plant; male and female flowers on separate plants, respectively). At publication time monoecious and female dioecious populations occur in California. Hydrilla occurs in tropical to temperate regions on all continents except Antarctica. Besides stem fragments, plants form 2 types of specialized perennating buds enclosed by tough leaf scales called turions. Introduced from Eurasia.
• Brazilian elodea: Dioecious. At publication time, populations in the U.S. consist only of male plants. Brazilian elodea is more common than hydrilla in California, but less threatening from a management perspective. Commonly sold as aquarium décor, plants can naturalize in warm temperate to cool sub-tropical regions when unwanted aquarium contents are released into lakes, ponds, or waterways. Introduced from eastern South America.
• common elodea: Dioecious. Common elodea is native to North America, where it is an important component of natural aquatic ecosystems. Populations are rarely troublesome in natural habitats, but plants can become dominant in altered or created aquatic systems, especially when bicarbonate, reduced iron, and phosphorus are plentiful. It is widely distributed throughout the world and considered a serious weed in Europe and Australia.

back to top of page

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.

• hydrilla: Leaves (3)4-8(12)-whorled. Middle and upper leaves often 5-8-whorled, 6-20 mm long, 1-4 mm wide. Leaves below growing tips straight or curved. Toothed margins usually visible without magnification. Lower surface midveins smooth or minutely sharp-toothed. Leaf axils have pairs of minute scales (squamulae intravaginales) up to 0.5 mm long, fringed with finger-like orange-brown hairs, usually visible with magnification on some mature leaves and branch points. Dioecious plants sometimes appear more robust than monoecious plants. Monoecious plants often branch freely near the substrate, producing many stolons and a forest of vertical shoots. Dioecious plants often branch freely near the water surface, forming large submerged mats.
• Brazilian elodea: Plants typically look larger and leafier than hydrilla or common elodea. Leaves 3-6(8)-whorled. Middle and upper leaves 15-40 mm long, 2-5 mm wide. Leaves below growing tips often curved downwards. Lower surface midvein smooth or minutely toothed.
• common elodea: Leaves dark green, usually peppered with large dark cells (visible with low magnification). Middle and upper leaves typically 3-whorled, 6-15(20) mm long, 1-5 mm wide. Leaves below growing tips often slightly curved downwards. Lower surface midvein smooth.

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.

• hydrilla: Plants often produce a mat of creeping above and below ground stolons that develop subterranean turions at the tips (referred to as tubers). Subterranean turions tough, whitish to brown-black, plump, ovoid, 4-15 mm long, typically to 15 cm deep in the substrate, remain attached until parent stolon decomposes. Dioecious subterranean turions are usually larger than monoecious turions.
• Brazilian elodea: Adventitious roots and lateral branches only grow from double nodes (specialized nodes separated by a shortened internode) typically spaced along stems at 6-12 node intervals. Only fragments with a double node develop into new plants.
• common elodea: Roots in substrate are more wiry than those of hydrilla or Brazilian elodea. Nodes with axillary buds usually occur at ~ 10 cm intervals.

back to top of page

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).

• hydrilla: Sepals and petals translucent, white to reddish, 3-5 mm long, staminate petals linear. Staminate and pistillate flowers 1 per spathe. Staminate flowers deciduous, free-floating at maturity, ephemeral. Stamens 3, explosively release pollen at maturity. Staminodes minute (< 1 mm long), clear or lacking. Wind-pollinated.
• Brazilian elodea: Often flowers freely. Flowers extend up to 3 cm above water surface. Nectaries shiny green, 3-lobed, minute. Petals glossy white, broadly ovate, wrinkled. Sepals green, ~ ½ petal length. Staminate flowers 2-5 per spathe, persistent. Staminate petals mostly 7-12 mm long. Stamens 9. Pistillate flowers 1 per spathe. Pistillate petals 5-8 mm long. Staminodes 1-3 mm long, yellow to red-orange, papillose. Insect-pollinated.
• common elodea: Flowers infrequently. Pistillate and staminate plants seldom grow in the same area. Some flowers bisexual. Staminate and pistillate flowers 1 per spathe, persistent. Sepals green, often purple-striated, ~ equal to petals. Petals fragile, white, narrower than sepals. Staminate petals 3-6 mm long. Stamens (7)9(18). Pistillate petals 2-4 mm long. Sterile stamens 1-2 mm long, greenish-white to pale violet. Water-pollinated.

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

• hydrilla: Fruits loment-like, narrowly cylindrical, 5-15 mm long, smooth or with few to several short to long irregular spines, do not open to release seeds. Seeds 1-5, ellipsoid, 2-3 mm long, smooth, brown.
• Brazilian elodea: Fruits and seeds typically not produced in California. Capsules ellipsoid, 7-15 mm long, 3-6 mm in diameter at widest part, thin-walled, translucent, open irregularly to release seeds. Seeds ellipsoid, 5-8 mm long, 2 mm wide, with a beak 3-4 mm long, covered with elongated papillae.
• common elodea: Capsules ellipsoid to ovoid, ~5-7 mm long, 2-3 mm wide, with a beak ~ 5-6 mm long, translucent, thin-walled, open irregularly to release seeds. Seeds ellipsoid, 4-6 mm long, with a short beak less than 1 mm long. Base of beak collared by wart-like cells.

OTHER VEGETATIVE REPRODUCTIVE STRUCTURES:

• hydrilla: Stem turions axillary, sometimes terminal on monoeicous plants or under certain conditions, tough, green, conical to ovoid, 3-12 mm long, deciduous.
• Brazilian elodea: Does not develop turions.
• common elodea: Stem turions terminal, appear as dense, dark green, leafy growing tips with nodes very close together, usually remain attached to parent stems.

back to top of page

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.

• hydrilla: Inhabits water 0.5-3(12) m deep. Grows at 10-35º C. Tolerates low light and variable water quality, including brackish, turbid, and polluted water. Turions survive near freezing temperatures.
• Brazilian elodea: Inhabits acidic to alkaline waters to 1-2(7) m deep. Highly susceptible to iron deficiency. Grows best under low light (optimal ~ 100 lux) at 10-25º C. Tolerates turbidity and short periods under ice. Does not survive prolonged periods of near freezing temperatures.
• common elodea: Sometimes inhabits sheltered areas of fast-flowing streams. Grows best under high light in high-calcium to slightly brackish water, 10-25º C, to 5 m deep. Tolerates some shade, little turbidity, and near freezing temperatures.

DISTRIBUTION:

• hydrilla: Eastern North Coast Ranges (Clear Lake area, Lake Co.), Cascade Range (Shasta Lake area, Shasta Co.), north and central Sierra Nevada foothills (nw Yuba, c & nw Calaveras, sw Amador, ne San Joaquin, c & nw Tulare cos.), Sonoran Desert (Imperial Valley, Imperial Co.); to Southern U.S., Central America. At publication time, previous infestations now eradicated occurred in the Sacramento Valley (n Sutter Co.), southern North Coast Ranges (ce Sonoma Co.), San Francisco Bay region (n San Mateo Co.), Central Coast (nw Monterey Co.), South Coast (se Santa Barbara, s Los Angeles, w San Diego cos.), and Desert regions (wc San Bernardino, c Riverside cos.). To 200 m (650 ft).
• Brazilian elodea: Northern and southern Sierra Nevada foothills, San Joaquin Valley, San Francisco Bay region, southeastern Great Basin, San Jacinto Mountains; throughout most of U.S., except some northern and mid-western states. To 2200 m (7200 ft). Expected to expand range in California.
• common elodea: Western North Coast Ranges, high elevations of the Sierra Nevada and Cascade Range, Central Valley, San Francisco Bay region, middle and eastern Transverse Ranges (San Gabriel & San Bernardino Mtns.), Great Basin; throughout most of U.S. To 2600 m (8500 ft).

back to top of page

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.

• hydrilla: Reproduces by stolons, stem fragments, stem turions, and subterranean turions (tubers). Turion and tuber initiation, production, dormancy and germination are variable, depending on the biotype and environmental conditions. A period of low temperatures often stimulates turion and tuber germination, especially in the monoecious biotype. Stem turions and tubers typically germinate in spring when conditions become favorable or remain dormant. Stem turions mostly develop late summer through fall as day length shortens, but dioecious plants may initiate some in spring. Stem turions separate from the parent stem in late fall or at maturity. Free-floating plants typically produce more stem turions than rooted plants, and monoecious plants often produce more than dioecious plants. Dense colonies usually develop fewer stem turions. Monoecious turions usually germinate readily, survive ~ 1 year under field conditions, and remain viable for up to ~ 8 hours under dry conditions. Tubers typically develop mid-summer through winter, but monoecious plants may also form them in spring. Monoecious plants often produce 50% or more tubers in a shorter period than dioecious plants, but dioecious tubers are often larger, more likely to remain dormant, and survive for longer periods. Under field conditions monoecious tubers survive for up to 5 years. Dioecious tubers may survive for up to 10 years. Dioecious tubers can survive up to 2-3 days under dry conditions. Tubers can survive ingestion and regurgitation by waterfowl and are resistant to many herbicides.
• Brazilian elodea: Reproduces by stolons and stem fragments.
• common elodea: Reproduces by stolons, stem fragments, and terminal turions. Turions typically develop in fall and appear to lack a true dormancy period.

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.

CONTROL METHODS:

Hydrilla control information can be accessed through the Western Aquatic Plant Management Society's hydrilla page at the following web address: http://www.wapms.org/plants/hydrilla.html

back to top of page