Eurasian watermilfoil was accidentally introduced from Eurasia in the 1940s. Two theories exist as to how it entered North America: (1) it escaped from an aquarium, or (2) it was brought in attached to commercial or private boats. A resort owner is thought to have introduced watermilfoil into the Tennessee Valley Authority reservoir system in 1953.
Eurasian watermilfoil, also called spike watermilfoil, is an emergent, herbaceous aquatic plant. Stems grow to the water surface, usually extending 3 to 10, but as much as 33, feet in length and frequently forming dense mats. Stems of Eurasian milfoil are long, slender, branching, hairless, and become leafless toward the base. New plants may emerge from each node (joint) on a stem, and root upon contact with mud. The grayish-green leaves of Eurasian watermilfoil are finely divided and occur in whorls of three or four along the stem, with 12-16 pairs of fine, thin leaflets about 1/2 inch long. These leaflets give milfoil a feathery appearance that is a distinguishing feature of the plant. Eurasian watermilfoil produces small yellow, 4-parted flowers on a spike that projects 2-4 inches above the water surface. The fruit is a hard, segmented capsule containing four seeds.
Plant: herbaceous aquatic plant; stems grow to the water surface, usually extending 3-10 ft. but as much as 33 ft. in length and frequently forming dense mats; stems are long, slender, branching, hairless and become leafless toward the base; new plants may emerge from each node (joint) on a stem and root upon contact with mud.
Leaves: bright green, finely divided and occurring in whorls of three or four along the stem, with 12-16 pairs of fine, thin leaflets about ½ in. long that give it a feathery appearance.
Flowers, fruits and seeds: produces small, yellow, four-parted flowers on a spike that projects 2-4 in. above the water surface; flower spikes often remain above water until pollination is complete; fruit is a hard, segmented capsule containing four seeds.
Spreads: vegetatively by rhizomes, fragmented stems and axillary buds that develop throughout the year. Although seeds are usually viable, they are not an important means of dispersal.
Look-alikes: many species of submerged aquatic plants including non-native invasive parrot-feather (Myriophyllum aquaticum), and native species such as Northern water-milfoil (M. sibiricum), coontail (Ceratophyllum dmersum) and water marigold (Megalodonta beckii).