European frog-bit (Hydrocharis morsus-ranae)

Common Names: European frogbit, common frogbit

Description:

European frog-bit (Hydrocharis morsus-ranae L.) is a free-floating aquatic plant native to Europe and northern Asia that has spread throughout the Great Lakes basin and connected waterways after escaping from an Ottawa botanical garden in 1939 [1]. Its small, round leaves and ability to form dense, interlocking mats on the water's surface allow it to shade out native aquatic plants and reduce oxygen levels for fish and other aquatic life [2]. MISIN tracks this species because new populations along Great Lakes shorelines are still being confirmed, and early detection gives land managers the best chance of preventing it from spreading further inland [3].

Habit:

European frog-bit is a small free-floating plant that can be mistaken for a miniature water lily. Its round to heart-shaped leaves are typically the size of a quarter to a large coin ? about half an inch to two and a half inches across ? and they sit flat on the water's surface [1]. The underside of each leaf is distinctly purplish-red and spongy along the midvein, a texture caused by air-filled tissue that keeps the plant afloat [5]. The plant grows as a rosette of leaves connected by slender runners, with unbranched threadlike roots hanging freely in the water beneath; it does not anchor to the bottom except in very shallow water [1].

The best time to spot European frog-bit is from late spring through summer, when it spreads across calm, sheltered bays and the edges of cattail stands, and when it produces small white three-petaled flowers with a yellow center from June through August [4]. By late summer, watch for small dense green buds ? turions ? developing along the runners before they detach and sink to the bottom to overwinter [2].

At-a-glance cues:

1. Leaves round to heart-shaped, about half an inch to two and a half inches across, floating flat on the water surface [1]
2. Underside of leaves is purplish-red and distinctly spongy along the midvein ? visible when you flip a leaf [4]
3. Small white flowers with exactly three rounded petals and a yellow center, held just above the water, June through August [4]
4. Long, unbranched roots hanging freely in the water ? the plant is not rooted to the bottom sediment [1]

Leaves:

1. Arrangement: basal ? all leaves arise from a central rosette at the water surface [5]
2. Shape: round (orbicular), heart-shaped (cordate), or kidney-shaped (reniform); often wider than long [5]
3. Size: 12?60 mm long ? 13?63 mm wide (roughly half an inch to 2.5 inches) [5]
4. Margin: smooth and untoothed (entire) [4]
5. Venation: parallel to slightly arched lateral veins; aerenchyma (spongy, air-filled tissue) concentrated on each side of the midrib on the underside [5]
6. Upper surface: green, smooth, and leathery [4]
7. Underside: purplish-red or dark green, distinctly spongy along the central vein [4]
8. Leaf stalk (petiole): 40?60 mm long, submerged [5]

Stems:

1. Habit: herbaceous, free-floating; the plant is not woody [1]
2. Stolons: slender, cord-like horizontal runners extend from the central rosette and give rise to new daughter rosettes [4]
3. Flower stem: upright, emerging above the water surface [5]
4. Roots: unbranched, threadlike, extending up to about 12 inches (30 cm) into the water column; not anchored in bottom sediment under typical conditions [1]
5. Turions: small, dense overwintering buds produced along stolons in late summer; detach in fall and sink to overwinter on the bottom [2]

Flowers:

1. Color: white petals with a yellow center at the base of each petal [4]
2. Petals: 3, thin and delicate, up to 10 mm (about 3/8 inch) long [5]
3. Shape: radially symmetrical, cup-shaped; flowers about ? to 1? inches across [4]
4. Arrangement: held above the water surface on individual stalks 0?90 mm long [5]
5. Bloom time: June through August [4]
6. Sex: dioecious ? male and female flowers occur on separate plants; male flowers have 9?12 stamens, female flowers have 6 carpels [5]

Fruit and seeds:

1. Fruit type: small, fleshy, berry-like capsule that opens when ripe [5]
2. Seeds: small, enclosed within the fleshy fruit; rarely viable in North American populations [2]
3. Ripening: late summer to fall [5]
4. Primary dispersal structure: turions ? small, dense, vegetative buds produced along runners in late summer; they sink to the bottom in fall and float up as new plantlets the following spring when water temperatures rise above about 59 ?F (15 ?C) [2]
5. A single plant can produce 100 to 150 turions per season, each capable of surviving on the bottom for up to two years [1][2]

Habitat:

European frog-bit grows in quiet, shallow water with little to no wave action. It thrives in protected bays, sheltered coves, marshes, ponds, drainage ditches, and slow-moving sections of rivers and streams [1]. In the Great Lakes region, it tends to establish first in coastal wetlands where stands of cattail or common reed (phragmites) break incoming waves and provide the sheltered conditions it needs [3]. The plant usually floats freely but can root in the bottom sediment in water shallower than about two feet [1].

It prefers slightly acidic to neutral water (pH 6.5?7.8), calcium-rich conditions, and moderate nutrient levels. It performs poorly in nutrient-poor water and does not tolerate significant wave action or saltwater [2].

Native to Europe and northern Asia, European frog-bit was brought to Ottawa's Central Experimental Farm in 1932 as an ornamental plant. It escaped into the Rideau Canal by 1939 and spread south and west through Ontario's waterways and the St. Lawrence River system into the northeastern United States [1]. In the United States it has been confirmed in Michigan, New York, Vermont, Ohio, New Jersey, Pennsylvania, Maine, Wisconsin, and Washington [1].

Reproduction:

European frog-bit spreads almost entirely without seeds. In North American populations, seeds are rarely viable, and the plant relies instead on two vegetative structures: stolons and turions [2].

Each plant sends out horizontal runners (stolons) from which new rosettes sprout and can break away as independent daughter plants, allowing a single plant to spread across an area roughly a yard in diameter within one growing season [2]. In late summer, plants produce small dense buds called turions along their runners. These buds detach in fall, sink to the bottom, and survive the winter there. When water temperatures warm in spring ? above about 59 ?F (15 ?C) ? turions float back up to the surface and develop into new rosettes [2]. A single plant can produce 100 to 150 turions in one season, and each turion can remain viable on the bottom for up to two years [1][2].

The plant spreads between water bodies when fragments or turions cling to boat hulls, propellers, trailers, waders, or waterfowl. Once turions settle on the bottom of a water body, removal of surface plants alone will not prevent re-establishment ? new plants will float up the following spring [2]. This is why cleaning all watercraft and gear before leaving any water body is so important for preventing new introductions.

Impact and Damage:

European frog-bit poses little direct risk to human health. However, dense mats on the water's surface can interfere with boating, fishing, and swimming in sheltered bays and along docks. Thick growth can also clog water intakes and make shoreline access difficult [1][3].

For native aquatic communities, the main problem is what frog-bit mats do to the water beneath them. By blocking sunlight, large mats dramatically reduce native submerged aquatic plants. In one documented case, a native aquatic plant ? bladderwort (Utricularia vulgaris) ? declined to just one-eighth of its normal density beneath a frog-bit mat, while populations in areas without frog-bit grew to thirteen times their baseline density [2]. Beneath dense mats, dissolved oxygen can drop to just 1.9 mg/L ? low enough to stress fish and other aquatic animals that require oxygen-rich water [2].

Dense mats have also been linked to shifts in underwater insect communities and to lower fish species diversity and reduced fish abundance in invaded areas [2]. The overall ecological impact is likely greatest in shallow, sheltered Great Lakes wetlands that are already stressed by other invasive plants ? invasive cattail and common reed create the wave-sheltered conditions that allow European frog-bit to move in and spread [2].

Studies of invasive aquatic plants broadly have documented approximately a 13% decrease in lakefront property values in areas with established infestations, reflecting reduced recreational use and water quality concerns [2].

Similar species:

Waterlilies (Nymphaea spp.) are the most commonly confused plants, because both have round, floating leaves with a similar general outline. Waterlily leaves are much larger, however ? typically around 12 inches across ? and their undersides are purple with a distinctive V-shaped notch where the leaf stem attaches at the base. Waterlily flowers are also much larger and have many overlapping petals, unlike the three-petaled flowers of European frog-bit.

American frog-bit (Limnobium spongia) is a native species and the most difficult look-alike to distinguish. Its leaves are also round to heart-shaped and float on the surface, measuring one to three inches across ? overlapping in size with European frog-bit but tending toward the larger end. Both species share a floating habit and similar leaf texture. American frog-bit is native to the eastern and southern United States and is not established in the Great Lakes region, so location is a useful clue ? but always check leaf and root details when in doubt.

Pond-lilies (Nuphar spp.) are native floating-leaf plants with much larger leaves, six to twelve inches long. Their most reliable distinguishing feature is their flowers: pond-lilies produce yellow, globe-shaped blooms, while European frog-bit's flowers are small, white, and three-petaled.

Water-shield (Brasenia schreberi) is a native floating-leaf plant with smaller, elliptical leaves that can look similar at a distance. The easiest way to tell it apart is how the leaf stem attaches: on water-shield, the stem connects to the center of the leaf's underside (called a peltate attachment), while on European frog-bit the stem connects at the notched base of the leaf. Water-shield leaves and stems are also coated in a thick, clear slime on their undersides ? a distinctive trait shared by no other floating-leaf plant in the region.

Monitoring and rapid response:

Signs to watch for:

1. Small round to heart-shaped leaves, half an inch to two and a half inches wide, floating in dense patches on calm water in bays, coves, or along the sheltered edges of wetlands ? easiest to spot June through September when the plant is actively growing
2. Small white three-petaled flowers with a yellow center, held just above the water surface, June through August
3. Purplish-red, spongy leaf undersides visible when you flip a leaf ? a reliable field identifier
4. Loose, tangled mats of floating rosettes connected by slender runners near cattail stands or other emergent vegetation

What you can do:

1. If you find a small patch on your shoreline, hand-pull or rake the entire plant ? leaves, stems, runners, and roots ? and bag all material for disposal on land; never leave pulled plants at the water's edge where fragments can drift back in
2. Work in early to midsummer, before the plant produces turions (usually before August); a follow-up visit later in the season helps catch any regrowth from missed fragments
3. After any water activity near an infestation, thoroughly clean all watercraft, trailers, waders, and gear before moving to another water body ? turions and fragments spread easily on equipment
4. For a natural area crew tackling a larger infestation, target small outlier patches first to stop outward spread; plan for multiple removal visits each season and expect follow-up work the next year, as turions already on the bottom will resprout
5. Shading with black shade cloth has been documented to suppress European frog-bit populations, and can be a practical option for small, confined areas such as a shoreline cove or dock access channel [2]

When to call a professional:

1. If the infestation covers a large area of open water or extends across a significant stretch of shoreline, physical removal alone is unlikely to be effective ? contact your local aquatic invasive species coordinator or CISMA (Cooperative Invasive Species Management Area) for guidance
2. If European frog-bit has spread across a shared or public water body, coordinated treatment involving all bordering properties is needed; a single-property removal will not prevent re-establishment from plants and turions elsewhere in the system
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. USGS Nonindigenous Aquatic Species Program. Hydrocharis morsus-ranae (European Frog-bit) Species Profile. nas.er.usgs.gov
2. Zhu B, Ottaviani CC, Naddafi R, Dai Z, Du D (2018). "Invasive European frogbit (Hydrocharis morsus-ranae L.) in North America: an updated review 2003?16." Journal of Plant Ecology 11(1):17?25. semanticscholar.org
3. Michigan State University Extension. "European frog-bit: Be on the lookout for this aggressive invasive plant." canr.msu.edu
4. Wisconsin Department of Natural Resources. "European Frog-bit (Hydrocharis morsus-ranae)." dnr.wisconsin.gov
5. Native Plant Trust. "Hydrocharis morsus-ranae (European frog's-bit)." Go Botany. gobotany.nativeplanttrust.org
6. Plants of the World Online (POWO). "Hydrocharis morsus-ranae L." Kew Science. powo.science.kew.org
7. ITIS (Integrated Taxonomic Information System). "Hydrocharis morsus-ranae L." TSN 503098. itis.gov

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