Whorled Watermilfoil
Myriophyllum verticillatum
Whorled Watermilfoil (Myriophyllum verticillatum) is a submerged aquatic plant belonging to the family Haloragaceae. It is one of the most widely distributed freshwater aquatic plants in the Northern Hemisphere, recognized by its distinctive whorled leaf arrangement and feathery, finely divided foliage that sways gracefully underwater.
• The genus name Myriophyllum derives from the Greek words "myrios" (countless) and "phyllon" (leaf), referring to the plant's highly dissected, numerous leaf segments
• The species epithet "verticillatum" refers to the whorled arrangement of leaves around the stem
• Commonly found in ponds, lakes, slow-moving rivers, and ditches across temperate regions worldwide
• Plays a critical ecological role in freshwater ecosystems as both a habitat provider and oxygenator
Taxonomy
• Native range extends from Western Europe through Central Asia to eastern Siberia, and across much of North America
• The genus Myriophyllum comprises approximately 60–69 species globally, with the highest diversity found in Australia
• M. verticillatum is considered a circumboreal species, thriving in temperate to cool-temperate freshwater habitats
• Fossil evidence of Myriophyllum pollen and fruits has been found in Eocene and Miocene deposits, indicating the genus has existed for at least 30–50 million years
• The species has been introduced to parts of the Southern Hemisphere, including New Zealand and southern South America, where it can become invasive outside its native range
Stems:
• Slender, cylindrical, typically 1–3 mm in diameter
• Green to brownish-green, often with a reddish tinge in strong light
• Branched, with roots forming at lower nodes
Leaves:
• Arranged in whorls of 4–6 (typically 5) around the stem at each node
• Submerged leaves are finely dissected into 14–24 pairs of linear, thread-like segments (pinnae)
• Each leaf is 1.5–4 cm long overall
• Leaf segments are 5–20 mm long, approximately 0.3–0.5 mm wide, and taper to a fine point
• Emergent leaves (above water) are less divided and may be entire or only slightly toothed — a key distinguishing feature from the invasive Eurasian watermilfoil (M. spicatum)
Flowers:
• Small, inconspicuous, and borne in emergent terminal spikes 2–10 cm long
• Flowers are arranged in whorls along the spike above the water surface
• Each flower is 4-petaled (on female flowers) or 8-staminate (on male flowers)
• The species is typically monoecious, with female flowers positioned below male flowers on the same spike
• Flowering period: June to September in temperate regions
Fruit & Seeds:
• Fruit is a schizocarp that splits into 4 small nutlets (mericarps)
• Each mericarp is approximately 1.5–2 mm long, ovoid, and tuberculate
• Seeds are small and sink to the sediment, where they can remain viable for extended periods
Turions:
• Produces specialized overwintering buds called turions (hibernacula)
• Turions are compact, thickened shoot tips that detach from the parent plant in autumn
• They sink to the bottom and remain dormant through winter, regenerating in spring
• This is a key survival and dispersal strategy for the species
Habitat:
• Found in still or slow-moving freshwater bodies including ponds, lakes, reservoirs, canals, ditches, and sluggish rivers
• Prefers nutrient-rich (eutrophic to mesotrophic) waters
• Typically grows at depths of 0.5 to 3 meters, occasionally deeper in clear water
• Tolerates a wide pH range (6.5–9.0) and moderate alkalinity
• Can grow in both soft and hard water conditions
Water Quality:
• Thrives in waters with moderate to high nutrient levels
• Sensitive to extreme pollution but tolerates moderate eutrophication
• Requires relatively clear water for optimal growth; heavy turbidity limits light penetration and restricts growth
Ecological Role:
• Provides critical habitat and shelter for fish, invertebrates, and aquatic insects
• Dense beds serve as nursery habitat for juvenile fish
• Produces oxygen through photosynthesis, contributing to dissolved oxygen levels in the water column
• Stabilizes sediments and reduces shoreline erosion through root networks
• Serves as a food source for waterfowl, particularly ducks, which consume the turions and seeds
Associated Species:
• Commonly co-occurs with other submerged macrophytes such as Potamogeton spp. (pondweeds), Ceratophyllum demersum (hornwort), and Elodea canadensis (Canadian waterweed)
• Often found alongside emergent vegetation like Typha (cattails) and Phragmites (common reed) at the margins
Reproduction & Dispersal:
• Reproduces both sexually (via seeds) and vegetatively (via turions and stem fragments)
• Vegetative fragmentation is the primary mode of local spread — broken stem fragments can root and establish new colonies
• Turions detach in autumn, disperse by water currents, and germinate in spring
• Seeds are dispersed by waterfowl (endozoochory) and water currents
• This dual reproductive strategy makes the species highly effective at colonizing new habitats
Light:
• Prefers full sun to partial shade
• Requires at least 4–6 hours of direct sunlight daily for vigorous growth
• In shaded conditions, growth becomes sparse and leggy
Water:
• Submerged aquatic plant — must be fully or mostly underwater
• Optimal water depth: 30 cm to 1.5 meters
• Prefers still or very slow-moving water
• Water temperature range: 10–25°C for active growth
Soil & Substrate:
• Roots in soft, nutrient-rich sediment at the bottom of the water body
• Recommended substrate: a mix of aquatic planting media, loamy soil, or clay
• Can be planted in aquatic baskets filled with heavy garden soil topped with gravel to prevent soil washout
Planting Method:
• Plant stem cuttings or turions directly into the substrate at the pond bottom
• Alternatively, anchor stem fragments to a weight and allow them to root naturally
• Space plants 30–60 cm apart to allow for spreading
• Best planted in spring (April–May) when water temperatures begin to rise
Maintenance:
• Requires minimal maintenance once established
• May need periodic thinning if growth becomes overly dense
• Remove decaying foliage in autumn to prevent excessive organic buildup
• In garden ponds, contain growth using planting baskets to prevent it from outcompeting other species
Hardiness:
• Extremely cold-hardy; turions survive freezing temperatures and ice cover
• USDA Hardiness Zones 3–10
• Dies back to turions in winter; regenerates from turions and root crowns in spring
Propagation:
• Stem cuttings — the easiest and most reliable method; simply push cut stem ends into moist substrate
• Turions — collect in autumn and press into substrate for spring germination
• Division of established clumps in spring
Fun Fact
Whorled Watermilfoil is often confused with its notorious relative, Eurasian Watermilfoil (Myriophyllum spicatum), which is one of the most aggressive invasive aquatic plants in North America. However, there is a reliable way to tell them apart: • M. verticillatum typically has 5 leaves per whorl, while M. spicatum usually has 4 • The emergent (above-water) leaves of M. verticillatum are less divided — often entire or only slightly toothed — whereas M. spicatum's emergent leaves remain finely dissected • M. verticillatum produces prominent turions (overwintering buds), which M. spicatum does not The turions of Whorled Watermilfoil are a remarkable adaptation: • These compact, starch-rich buds can survive being frozen in ice for months • They are calorie-rich and are actively sought out by waterfowl such as coots and ducks during winter • Studies have shown that turions can pass through the digestive tracts of birds and still remain viable — making waterfowl a key long-distance dispersal vector Whorled Watermilfoil is also a natural water purifier: • Its dense root system absorbs excess nutrients (particularly nitrogen and phosphorus) from the water column • It competes with algae for nutrients, helping to reduce algal blooms and improve water clarity • This phytoremediation capacity has led to its use in constructed wetlands and natural water treatment systems The genus Myriophyllum has a fossil record stretching back to the Eocene epoch (~50 million years ago), with fossilized fruits and pollen found in ancient lake sediments across Europe and North America — evidence that these graceful underwater plants have been shaping freshwater ecosystems since long before the age of modern birds and mammals.
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