American white waterlily (Nymphaea odorata)

Synonyms: Castalia odorata, Nymphaea tuberosa

Common Names: Fragrant water lily, white water lily, sweet-scented water lily

Description:

American white waterlily (Nymphaea odorata) is an aquatic plant with round floating leaves and large, fragrant white flowers [1]. Unlike most species MISIN tracks, it is native across eastern and central North America, including the Great Lakes region, where it is a normal part of the wetland flora [1][3]. It becomes a problem mainly where it has been planted and has escaped outside its native range ? such as the Pacific Northwest and parts of the western United States ? and where colonies grow into dense mats that crowd out other plants and block the water surface [1][2]. MISIN tracks it to map where it occurs and spreads.

Habit:

American white waterlily roots in the muck at the bottom of still water and sends round, flat leaves up to the surface on long, flexible stalks [1]. The leaf pads are green on top, often reddish or purple underneath, and have one narrow slit cut to the center ? like a pie with a single slice removed [1]. Its flowers are the easiest clue: showy, sweet-smelling, and white (sometimes pink), 2.4 to 7.5 inches across, floating right on the water and opening in the morning then closing by afternoon [1]. It is easiest to spot in summer, from roughly June into September, when both the pads and the floating white flowers are out [1].

At-a-glance cues:

1. Round floating leaf pads with a single narrow slit to the center [1]
2. Large, fragrant, white (sometimes pink) flowers floating flat on the water, 2.4 to 7.5 inches across [1]
3. Flowers open in the morning and close by afternoon [1]
4. Rooted in bottom muck by thick rhizomes; leaf undersides often reddish-purple [1]

Leaves:

1. Float flat on the water surface on long, flexible stalks from the rootstock [1]
2. Round to nearly circular, with one narrow slit (notch) cut to the center and smooth edges [1]
3. Green and waxy on top; underside often reddish, purplish, or green [1]
4. Typically several inches to about a foot across [1]

Stems:

1. Herbaceous aquatic perennial with no upright above-water stem [1]
2. Leaves and flowers rise on long, flexible underwater stalks from thick, fleshy underground stems (rhizomes) rooted in the bottom [1]
3. Spreads outward along branching runners (stolons) that can reach 3 feet or more [1]

Flowers:

1. Showy and sweet-scented, with many white (occasionally pink) petals [1]
2. 2.4 to 7.5 inches across, floating singly on the water surface [1]
3. Open in the morning and close by afternoon; bloom through the summer [1]
4. Each flower is perfect (has both male and female parts) [1]

Fruit and seeds:

1. Fruit is a leathery, berry-like capsule that ripens underwater as its stalk coils and pulls it below the surface [1]
2. Each capsule holds many small seeds [1]
3. Seeds are spread by water and waterfowl [1]
4. The plant also spreads readily from broken pieces of rhizome [1]

Habitat:

American white waterlily grows in quiet, still, or slow-moving fresh water ? ponds, the calm margins of lakes, marshes, and slow streams [1]. It roots in mucky or silty bottoms in water up to about 6 to 7 feet deep [1]. It is native to the eastern and central United States and southern Canada, where it occurs naturally across the Great Lakes region [1][3]. It has been planted widely as a water-garden ornamental, and beyond its native range ? especially in the Pacific Northwest and parts of the western United States ? it has escaped and become invasive in lakes and ponds [1][2].

Reproduction:

American white waterlily spreads in two main ways, and both are easy to picture [1]. Underground, its thick rhizomes creep outward and send up new pads, so a single plant slowly widens into a clump and then a mat [1]. Broken pieces of rhizome can drift and take root somewhere new, which is how the plant moves between waters on boats, trailers, and equipment, or when people dump trimmings or unwanted pond plants into a lake [1][2]. It also makes seeds: its capsules ripen underwater and release many small seeds that float off or are carried by waterfowl [1].

Once established, colonies expand year after year from the rhizomes [1]. For control where it is unwanted, timing matters ? repeatedly removing the floating pads through the growing season starves the rootstock, and digging out the rhizomes keeps the colony from coming back [1][2].

Impact and Damage:

American white waterlily is not poisonous to people or pets, and in a garden pond it is grown on purpose for its flowers [1]. In its native eastern and central range it is a natural part of wetlands and provides food and cover for fish, insects, and waterfowl [1]. The trouble comes when colonies grow unchecked ? especially where the plant has been introduced outside its native range [1][2]. There, dense stands can spread across hundreds of acres of water surface, shading out and crowding the plants beneath them, lowering the oxygen in the water under the mat, and turning open water into a solid carpet of pads [1][2]. Those mats also block swimming, boating, and fishing and can cut off access to the shoreline [1][2].

Similar species:

Several other floating-leaved water plants are mistaken for American white waterlily. The round pad with a single slit, and the fragrant white floating flower, are the best clues.

Yellow pond-lily (Nuphar variegata), also called spatterdock or bullhead lily, shares round floating pads, but its leaves are more oval with a deeper notch and its flowers are yellow, cup-shaped, and held just above the water rather than white and floating flat [1].

Watershield (Brasenia schreberi) has much smaller, oval floating leaves with no slit and a stalk attached at the center of the pad. Its leaves and stems have a slimy, jelly-like coating, and its small flowers are dull purple, not showy and white [1].

American lotus (Nelumbo lutea) has very large round leaves, many of them held high above the water on stiff stalks rather than floating, and pale yellow flowers. Its leaves have no slit to the center [1].

Hardy garden water lilies (other Nymphaea species and hybrids) are sold for ponds in many flower colors and look much like this species. Where such planted lilies escape into natural waters they raise the same concerns [1].

Monitoring and rapid response:

Signs to watch for:

1. Round floating leaf pads with a single slit to the center, in still or slow water [1]
2. Large, fragrant, white floating flowers in summer that open in the morning and close by afternoon [1]
3. Expanding mats of pads covering more and more of a pond or lake surface [1][2]
4. New colonies appearing where pond or aquarium plants may have been dumped [1][2]

What you can do:

1. In a pond you own where it is unwanted, repeatedly rake, pull, or cut the floating pads through the growing season to weaken the rootstock [1][2]
2. Dig out the thick underground rhizomes where you can reach them, since the colony regrows from them [1][2]
3. Bag and dispose of pads and rhizome pieces on land ? never toss fragments back into the water, where they re-root and spread [1][2]
4. For small patches, a bottom barrier or light-blocking fabric anchored over the plants can smother them [1]
5. Never dump water-garden or aquarium plants into lakes, ponds, or streams [1][2]
6. Lake associations and natural area crews can cut or harvest larger stands over several seasons and should target small, newly established, or out-of-range colonies first [1][2]

When to call a professional:

1. Large, well-established mats covering open water, where lake-wide management is needed [1][2]
2. Work in public waters or situations that need specialized equipment or permits [1][2]
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Washington State Noxious Weed Control Board. Fragrant waterlily (Nymphaea odorata). nwcb.wa.gov
2. King County, Washington, Noxious Weeds Program. Fragrant water lily identification and control. kingcounty.gov
3. USDA NRCS PLANTS Database. Nymphaea odorata Aiton (NYOD). plants.usda.gov

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