CIS 1108 Eurasian watermilfoil (Myriophyllum spicatum) Identification and management in Idaho by Timothy S. Prather, Sandra Robins, Sandy Daniel, and Karen Laitala Origin and Entry Description Eurasian watermilfoil is an invasive, submersed Table 1 describes the leaves, stems, and flowers of aquatic plant that forms very dense mats of veg- Eurasian watermilfoil and of other local water- etation on the water’s surface, interfering with milfoils, both native and introduced. For illustra- water recreation and inhibiting water flow. tions of each, see figure 1. Eurasian watermilfoil is a native of Europe, Asia, Eurasian watermilfoil, also called spike watermil- and northern Africa. Reports suggest that it was foil, is in the watermilfoil family, Haloragaceae. introduced to the United States in the late 1800s, It is a submersed, rooted perennial plant with but it was first documented in the eastern United smooth stems that branch near the water sur- States in the 1940s. Eurasian watermilfoil is face. The stems may reach lengths of 10 feet or 1 3 found throughout the United States, from Florida more, and are usually ⁄16- to ⁄16-inch thick. to Quebec in the East and from California to The leaves of Eurasian watermilfoil are finely British Columbia in the West. divided and occur in whorls of four. Each leaf is It spreads rapidly, primarily by fragmentation of 0.5 to 2 inches long, with 12 to 21 pairs of fine, plant parts. Plant fragments grow roots, stems, thin leaflets. These leaflets give milfoil its distin- and leaves as they float along in water. Because gushing feathery appearance. The leaves rarely Eurasian watermilfoil is easily spread by frag- extend above the water surface and collapse ments, transport on boats and boating equip- around the stem when removed from the water. ment is believed to play an important role in its Eurasian watermilfoil closely resembles the dispersal and the contamination of new water native northern watermilfoil (Myriophyllum sibir- bodies. icum). One somewhat variable distinguishing Eurasian watermilfoil can form large, floating characteristic is the number of leaf divisions: mats of vegetation on the surface of lakes, rivers, northern watermilfoil has fewer leaf divisions (5 and other water bodies, preventing light from to 12) than do the non-native species (12 to 23). reaching native aquatic plants and impeding The inflorescence is a terminal spike, 2 to 8 inch- water traffic. The plant thrives in areas that have es long, which is often pink. It is held erect above been subjected to various kinds of natural and the water during flowering from June to manmade disturbance. September, but becomes horizontal as fruits ripen. At the time of flowering, the upper part of University of Idaho • College of Agricultural and Life Sciences Table 1. A comparison of watermilfoils and similar species. Species Leaves Stems Flowers Eurasian watermilfoil Arranged in whorls of 4, rarely 5, Smooth stems branch abundantly Emergent flower stalks spike-like, (Myriophyllum around the stem at nodes near the water surface in water 3- 2-8 inches long spicatum) Generally has 12 or more pairs of 9 feet deep Small pinkish flowers are in the leaf divisions Usually 5 feet long but may reach axils of upper bract-like leaves lengths of 10 feet or more Introduced No emergent leaves Floral bracts as long as fruits or 0.5-2 inches long Flower stem width almost doubles longer below the inflorescence and curves Upper bracts subtending flowers Collapse around the stem when to lie parallel to the water surface out of water entire and ovate Shoot tip tassel-like No turions produced Lower bracts subtending flowers (winter buds) often pectinate (like teeth of a comb) Flower stems have 2 or 3 distinct black scales at the nodes between the flowers Northern watermilfoil Arranged in whorls of 4 or 5 around Stems rarely branch near the Emergent flower stalks spike-like, (Myriophyllum sibiricum) the stem at the nodes water surface in water more than 1.5-4 inches long Generally has 12 or fewer pairs of 3 feet deep Small pinkish flowers are in the Native leaf divisions No change in stem width below the axils of upper bract-like leaves 5 3 inflorescence; flower stems remain ⁄8-1 ⁄16 inch long Floral bracts shorter than fruits or, slender and erect near the water rarely, equaling them in length Leaves sturdy and maintain their surface spread-leaf shape when out of Upper bracts subtending flowers water Often whitish entire and ovate-oblong No emergent leaves Shoot tip knob-like Lower bracts subtending flowers Turions present in fall mostly serrate (winter buds) 0 to 2 indistinct, brown or black scales at the nodes between the flowers Whorl-leaf watermilfoil Arranged in whorls of 4 or 5 around Stems rarely branch near the Emergent flower stalks spike-like (Myriophyllum verticilla- the stem at the nodes water surface in water more than Upper flowers with yellow-green tum) 3⁄4-1 3⁄4 inches long 3 feet deep petals Emergent leaves 1-3 feet long Floral bracts very variable, from Native Generally has 9-17 pairs of leaf No change in stem width below the one to 10 times length of the fruits divisions inflorescence Bracts subtending flowers pecti- Turions nate or pinnate, never entire (winter buds) yellow-green Andean watermilfoil Arranged in whorls of 4 or 5 around Simple or branched Inflorescence commonly forked (Myriophyllum quitense) the stem at the nodes 1-3 feet long and submersed to emergent 3 ⁄8-1 inch long Subtended by small, whitish, pecti- Native nate (comb-like) bracts less than Pectinately divided into 13-21 1 thread-like segments ⁄8 inch long 2 3 Bracts ovate to oblong, ⁄8- ⁄8 inch long, and pectinate (comb-like) to lightly toothed 3 Emergent leaves less than ⁄4 inch long Figure 1. Comparison of watermilfoils and similar species. Eurasian watermilfoil Northern watermilfoil Whorl-leaf watermilfoil Andean watermilfoil Western watermilfoil (Myriophyllum spicatum) (Myriophyllum sibiricum) (Myriophyllum verticillatum) (Myriophyllum quitense) (Myriophyllum hippuroides) 2 Species Leaves Stems Flowers Western watermilfoil Arranged in whorls of 4 or 5 around Freely branched Emergent flower stalks spike-like the stem at the nodes (Myriophyllum hip- 4 inches to 2 feet long Yellowish-white or cream-colored puroides) Submersed leaves pinnately dissect- flowers ed into 13-23 thread-like segments Subtended by a bract-like scale Native Bract leaves linear to oblong, with serrate bracts 1 3 Parrotfeather Arranged in whorls of 4-6 around Stems stout, ⁄16 – ⁄16 inch thick Emergent flower stalks spike-like the stem at the nodes (Myriophyllum Submersed until time of flowering White flowers aquaticum) Has both emergent and sub- mersed leaves 6 inches to 3 feet long Subtended by a bract-like scale 3 Introduced Emergent leaves 1-1 ⁄8 inch long, with 9-18 linear segments on each side of the leaf Submerged leaves are more feath- er-like, 1-2 inches long, with 25-37 thread-like divisions Coontail Dense bulky whorls around the Long, stiff, brittle, with many Submersed, located in the leaf (Ceratophyllum stems branches axils demersum) Usually 5-12 leaves per whorl Inconspicuous Each leaf forks into thread-like Native segments with fine teeth along Note: one of the few one side of each segment genera of vascular plants that do not have roots White water-buttercup Alternate Submerged, sparingly branched Solitary and terminal on long Whitewater crowfoot 2 stalks (Ranunculus aquatilis) Of two types: Greater than ⁄16 inch thick 3 6 Submerged leaves finely divided 3 feet long White, ⁄8- ⁄8 inch wide Five sepals and petals Native Floating leaves broad, flat, 3-5, palmately lobed Fanwort Of two types: Branched Mostly in the axils of floating (Cabomba caroliniana) 3 leaves Submerged leaves fan-shaped, ⁄8-2 6.5 feet long inches wide, arranged in pairs on Submerged stems have a tubular Small, white flowers with a yellow the stem; leaves opposite with Native appearance base float on the water surface palmately finely dissected linear segments Floating leaves small, linear to oval, alternate, with the stem attached at the center of the leaf Common bladderwort Finely divided into segments Branched, up to 6 feet long Occur on upright stalks above water (Utricularia macrorhiza) Paired, bladder-like cavities Free floating or sometimes attached at leaf axils along stems attached to substrate Yellow, 2-lipped, with a 1-inch-wide Native spur Parrotfeather Coontail White water-buttercup Fanwort Common bladderwort (Myriophyllum aquaticum) (Ceratophyllum demersum) (Ranunculus aquatilis) (Cabomba caroliniana) (Utricularia macrorhiza) 3 the stem, below the inflorescence, is double the Detection width of the lower stem. The process for detection of Eurasian watermilfoil The flowers are grouped in whorls of four. The should begin with a risk assessment. A risk lower whorls are female flowers and the upper assessment will determine if a survey is necessary male. The lower flowers are surrounded by floral and can help determine when and where to sur- bracts that are often longer than the flowers and vey to maximize detection. Risk assessment and have pectinate (comb-like, fringed) margins. survey methods are described in the next two sec- Higher on the spike the bracts become shorter tions. than the flowers, broader than long, and have entire margins. Risk Assessment The fruit is a hard, segmented capsule containing Use table 2 to assess the risk of infestation to four seeds. water resources in your area. If a water body is high risk in the proximity category and high risk in one or more other categories you should con- Biology sider conducting a survey. Eurasian watermilfoil is found in lakes, ponds, Survey Methods rivers, irrigation canals, and other waterways in Once a risk assessment has been conducted and waters up to about 19 feet deep, depending on a water body is considered at risk, use the follow- light penetration.