Growth Pattern and Biology
Elodea is a perennial submerged aquatic plant that grows in dense stands in still to slow-moving freshwater bodies. In its native North American range, elodea grows in lakes, ponds, rivers, streams, irrigation canals, and drainage ditches across a wide geographic and ecological range. It is most common in clear to moderately turbid water at depths of 0.5–3 meters, where it receives sufficient light for photosynthesis, and is most abundant in mesotrophic (moderately productive) lakes and rivers with moderate nutrient levels. In nutrient-poor (oligotrophic) water, elodea growth is slow and populations are sparse. In nutrient-rich (eutrophic) water, elodea can grow rapidly and reach nuisance density, though it rarely dominates to the same extent as hydrilla.
Vegetative Growth
Elodea grows from a perennial root system anchored in the sediment. Stems elongate upward through the water column during the growing season (spring through fall), reaching lengths of 0.5–3 meters depending on water depth. Leaves are arranged in whorls of exactly 3 around the stem. Growth is most active at temperatures of 15–25°C (59–77°F); growth slows at higher temperatures and ceases below about 5°C (41°F). In northern water bodies, elodea typically goes dormant in winter, with surviving root crowns producing new stem growth in spring. In mild-winter areas, some growth may continue through winter at reduced rates.
Branching occurs from axillary buds at nodes, creating a bushy, multi-stemmed growth form in established stands. Dense stands form underwater canopies that provide the habitat structure important for fish, waterfowl, and invertebrates. The stem growth pattern — elongating toward the light from sediment-anchored roots — is typical of rooted submerged plants and contrasts with the free-floating growth of coontail.
Reproduction
Elodea reproduces primarily vegetatively in North America, through stem fragmentation and the formation of axillary buds. Stem fragments detached by wave action, current, or disturbance establish new plants when they encounter suitable sediment — the same fragmentation mechanism that makes invasive submerged plants like hydrilla and milfoil so difficult to control. However, because elodea is native and generally not at densities that require management in most North American waters, this fragmentation biology is ecologically beneficial (facilitating natural spread within water bodies) rather than problematic.
Flowers are produced seasonally (summer) and are tiny, white, and floating — held at the water surface on long, delicate peduncles (stalks). Elodea canadensis in North America is predominantly dioecious and produces mostly female plants across its introduced range (introduced populations in Europe, for example, have been nearly all female). Sexual reproduction through seeds does occur but is not the primary reproductive mechanism. Seeds germinate in sediment and produce slow-growing seedlings.
Light and Nutrient Requirements
Elodea requires moderate light for growth and is not as shade-tolerant as hydrilla — it cannot photosynthesize at the extremely low light levels (<1% of surface irradiance) that hydrilla tolerates. This difference in light tolerance is one of the key competitive advantages that allows hydrilla to outcompete and eventually displace elodea in water bodies where both are present. In clear lakes, elodea grows at depths of 1–4 meters; in turbid water, its depth is more restricted. Elodea acquires most of its nutrients through the water column (from surrounding water) rather than primarily from sediment via roots, though root uptake also occurs. In nutrient-rich water, elodea grows more rapidly but may be eventually outcompeted by algae that shade the plant community.
Ecological Interactions
Elodea supports diverse invertebrate communities on and among its stems and leaves. Aquatic insects (chironomids, ephemeroptera), microcrustaceans, snails, and other macroinvertebrates colonize elodea beds at higher densities than bare sediment areas. These invertebrate communities serve as food for fish and waterfowl. Canada geese and dabbling ducks graze on elodea directly. Several fish species use elodea beds as spawning habitat and juvenile shelter. The ecological functions are similar to coontail but with the distinction that elodea is a rooted plant — its sediment-anchored root system provides sediment stabilization that coontail's rootless biology does not.
In Alaska, where elodea is non-native and invasive, these same ecological processes work against native species. Dense elodea beds displace native aquatic plants, alter habitat structure for native fish species, and alter the nutrient cycling of native lake systems. For information on elodea management in Alaska and when management of elodea is warranted elsewhere, see elodea control methods.
References
- Cook, C.D.K. & Urmi-König, K. (1985). Revision of the genus Elodea. Aquatic Botany 21:111–156.
- Catling, P.M. & Wojtas, W. (1986). The waterweed flora of Canada. Canadian Journal of Botany 64:1257–1270.
- Alaska Department of Fish and Game. Elodea in Alaska. adfg.alaska.gov