Non-native Invasive Freshwater Plants

Fragrant Water Lily (Nymphaea odorata)

Technical Information

Description

Fragrant water lilies  are one of the most easily recognized of all the aquatic plants. Beautiful large white or occasionally pink many-petaled flowers float on the water's surface surrounded by large, round green leaves. Mature Nymphaea odorata leaves are often circular, cleft at the base, smooth to 25 cm across, often purple on the lower surface, with most of the leaves floating. The leaves are attached to flexible underwater stalks rising from thick fleshy rhizomes. The horizontal creeping and branching rhizomes (2-3 cm in diameter) are attached by adventitious roots arising in groups below the leaf bases. The petioles leave crescent-shaped scars on the rhizome when shed. Showy flowers rise on long solitary stalks and are borne at the surface of the water or elevated slightly above it. The white (sometimes pink) fragrant flowers measure up to 25 cm across and have yellow centers surrounded by 25 or more petals. After the flower has finished, the stalk forms a spiral and draws the fruit below the water. The fruit is a capsule 1-2 cm in diameter containing many seeds (2-3 mm long).

Economic Importance

The fragrant water lily, along with other Nymphaea species, is a premier plant for cultivation in containers and ornamental ponds. Most Nymphaea varieties are not difficult to grow. Many hybrids have been developed in hues such as rose, saffron, yellow, purple, and vermilion. The fragrant water lily and its hybrids is an extremely popular water garden plant and can be readily obtained at nurseries and through mail order catalogs. Because of its great beauty, northwest property owners have introduced this non-native plant into many Washington lakes; often by transplanting plants from one lake into another. Of 15 lakes surveyed in 1994 in King County, Nymphaea odorata appeared on the species list of all 15 lakes. Shallow lakes are particularly vulnerable to being totally covered by fragrant water lilies  as is 110 acre Giffin Lake in the Sunnyside area of eastern Washington. An aerial photograph of this lake taken in April 1974 shows open water and data indicate that 11-25 percent of the lake was covered by emersed plants (unknown species). Twenty years later, 100 percent of the lake's surface was covered by water lilies.

The photograph shows Chambers Lake in Lacey Washington. This lake has a large population of water lilies. Left unmanaged, water lilies will restrict lake-front access and eliminate swimming opportunities. Requests for water lily control represent a high percentage of the herbicide permit requests received by Ecology. In several of the integrated aquatic plant management plans funded by Ecology, the fragrant water lily was considered the second nuisance plant after Eurasian watermilfoil and was targeted for control.

Geographic Distribution

Nymphaea odorata is native to the eastern half of North America, including southern Canada. It has been introduced as an ornamental in many parts of the world and is now found throughout North America and Canada. Although found throughout Washington, the fragrant water lily is especially prevalent in western Washington lakes where it has been intentionally planted by property owners who admired the showy flowers.

Habitat

The fragrant water lily is a floating-leaved aquatic perennial herb that grows rooted in mucky or silty sediments in water up to six to seven feet deep. It prefers quiet waters such as ponds, lake margins, and slow streams and will grow in acid or alkaline waters. When unmanaged, it tends to form dense monospecific stands that can cover hundreds of acres and can persist until senescence in the fall. Beaver, moose, muskrat, porcupine, and deer eat water lily leaves and roots and waterfowl eat the seeds. Water lilies also provide excellent cover for largemouth bass, sunfish, and frogs. However, when allowed to grow in dense stands, the floating leaves prevent wind mixing and extensive areas of low oxygen can develop under water lily beds during the summer. When managed to form a patchy distribution interspersed with open water, water lilies can provide excellent habitat.

History

It is believed that the fragrant water lily was introduced into Washington during the Alaska Pacific Yukon Exposition held in Seattle in the late 1800s. water lily culture in the western hemisphere began in 1786 when Nymphaea sp. was first introduced into the large estates and botanic gardens of England.

Growth and Development

Each spring (April) new shoots appear from the rhizomes and grow up through the water until they reach the surface. The flowers appear from June to September. Each blossom opens in the morning and closes in the early afternoon for two to five consecutive days. Pollination is performed mainly by beetles, but bees have also been observed visiting the flowers. After the flowers have closed for the final time, the flower stalk "corkscrews" and draws the developing fruit below the water. The plant senesces in the fall and over winters as the rhizome.

Reproduction

The fragrant water lily reproduces through both seeds and rhizome spread. Nymphaea rhizomes can be cut into 10 cm or larger pieces for propagation. A planted rhizome will cover about a 15-foot-diameter circle in five years.

Response to Herbicides

Westerdahl and Getsinger report excellent control of the fragrant water lily with glyphosate. Good control was obtained with endothall dipotassium salt and fluridone. Generally glyphosate is the recommended herbicide for water lily control because it can be directly applied to the floating leaves, unlike fluridone or endothall which must be applied to the water. The application of glyphosate allows specific plants or areas of plants to be targeted for removal. Generally two applications of glyphosate are needed. The second application controls the plants that were missed during the first herbicide application. A drawback of using herbicides, is the "uplifting" of mats of decomposing water lily roots that can form large floating islands in the waterbody after the herbicides have killed the plants. Harvesting water lilies  before treating the water with a systemic herbicide such as fluridone has been shown to stress the plants and provide greater impact of the herbicide to the plants (as was demonstrated during the 1991 fluridone application to Long Lake, Thurston County).

Response to Cultural Methods

Localized control (in swimming areas and around docks) can be achieved by covering the sediment with a opaque fabric which blocks light from the plants (bottom screening). However, it is sometimes very difficult to place and secure the fabric to densely packed, tough, fleshy water lily rhizomes. Several lake residents have reported success in eliminating water lilies from waterfront lots by the process of carbohydrate depletion. During each growing season, residents faithfully removed all emerging leaves. They reported that it took about two to three seasons to kill the plants. Managers of reservoirs and some lake systems may have the ability to lower the water level as a method of managing aquatic plants. Response of the fragrant water lily to water level drawdown has been variable.

Response to Mechanical Methods

Mechanical controls such as cutting and harvesting are popular methods of controlling water lilies. Cutting is less efficient than harvesting because cut plants must then be removed from the water. Harvesters both cut and collect the plants. Both methods create open areas of water. However, because water lilies grow in shallow water and grow rapidly, they must be cut several times a year. Harvesting has been used extensively on Long Lake, Thurston County to control water lilies. Underwater rototilling (called rotovation) was successfully used to remove water lilies  from a small Seattle area lake where the drowning of two people was attributed to the presence of dense plant beds. Rotovation dislodges the large, fleshy water lily rhizomes which can then be removed from the water. Although rotovation is a much more expensive process than harvesting or cutting, it results in the permanent removal of water lily rhizomes. Thurston County has experimented with using a barge-mounted backhoe to excavate water lily rhizomes from the sediment. Like rotovation, excavating the rhizomes results in permanent removal of the plant. Both rotovation and excavation requires that the project proponent obtain a number of environmental permits before proceeding.

Biocontrol Potentials

General release of biocontrol agents for the fragrant water lily would not be popular because water lilies are so widely propagated for ornamental ponds. However, a number of insects, such as aphids, have been observed feeding on and impacting water lilies. The University of Washington experimented with using triploid grass carp to remove fragrant and other species of water lilies from Chambers Lake, Thurston County by stocking very high rates of fish. However little or no impact of the fish on water lilies was observed in that lake. It is generally accepted that grass carp can not be effectively used for fragrant water lily management in Washington. Therefore, there are no effective biological control agents available at this time for water lily control. Nor are there likely to be any.

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Problems with this page, contact Kathy Hamel at kham461@ecy.wa.gov