Eurasian water-milfoil;
spike water-milfoil

Haloragaceae/Water-milfoil Family
pronounced: mirio-file-um / spi-ka-tum
from: myrios (G.): numberless
phyllon (G.): leaf
spica (L.): spike
“a plant with many leaf divisions, and a spike of flowers”

Eurasion Water-milfoil Mat

Eurasion Water-milfoil Stem

Eurasian water-milfoil is submersed. It tolerates a wide range of water conditions, and often forms large infestations. Eurasian water-milfoil stems are reddish-brown to whitish-pink. They are branched and commonly grow to lengths of six to nine feet. The leaves are deeply divided, soft and feather-like. Leaves are about two inches long. The leaves are arranged in whorls of three to six leaves about the stem. The flowers of Eurasian water-milfoil are reddish and very small. They are held above the water on an emersed flower spike that is several inches long.

Habit:

• Eurasian water-milfoil is a submersed, rooted, perennial
• its stems can “top out” in 20 feet of water, but the plant is most often found in water 0.5 to 3.5 m deep (Aiken et al. 1979)
• often forms large infestations; often is the most abundant submersed species in a locale
• spreads and reproduces mainly by regrowth of plant fragments; spreads locally by stolons
• will halt boat traffic on rivers; will fill a lake surface from shore to shore
• an aquatic weed worldwide
• seems to prefer lakes, ponds and slow-moving rivers and streams but can also grow in fast-moving water (Newroth 1985)
• tolerates a wide range of water conditions, including spring water and even brackish water of tidal creeks and bays with salinity of up to 10 parts per thousand (Beaven 1960)
• temperature tolerance: Eurasian water-milfoil is winter-hardy, able to overwinter in frozen lakes and ponds in northern states and Canada; but is also able to grow in shallow, over-heated bays

More information about Eurasion Water-milfoil.

Hydrilla Verticillata;
hydrilla, water thyme

Hydrocharitaceae/Frog’s-bit Family
pronounced: hid-ril-lah ver-ti-ki-lah-ta (long/short marks)
from: hydr (G.): water
verticillus (L.): the whorl of a spindle
“water plant with whorls of leaves”

Hydrilla Mat

Hydrilla Stem

Hydrilla is a submersed plant. It can grow to the surface and form dense mats. It may be found in all types of water bodies. Hydrilla stems are slender, branched and up to 25 feet long. Hydrilla’s small leaves are strap-like and pointed. They grow in whorls of four to eight around the stem. The leaf margins are distinctly saw-toothed. Hydrilla often has one or more sharp teeth along the length of the leaf mid-rib. Hydrilla produces tiny white flowers on long stalks. It also produces 1/4 inch turions at the leaf axils and potato-like tubers attached to the roots in the mud.

Habit:

• Hydrilla is a herbaceous perennial.
• Submersed, “obligate” (requiring a wet habitat).
• Forming dense stands of very long stems (25 ft.) in the water.
• Reproduces mainly by regrowth of stem fragments; also reproduces by growth of axillary buds(turions) and subterranean tubers; tubers can remain viable for more than 4 years (Van & Steward 1990).
• A single tuber can grow to produce more than 6,000 new tubers per m2 (Sutton et al. 1992).
• Hydrilla can grow in almost any freshwater: springs, lakes, marshes, ditches, rivers, tidal zones.
• Can grow in only a few inches of water, or in water more than 20 feet deep.
• Can grow in oligotrophic (low nutrient) to eutrophic (high nutrient) conditions.
• Can grow in 7% salinity of seawater (Haller 1974).
• Temperature tolerance: hydrilla is somewhat winter-hardy; its optimum growth temperature, 20-27o C (68-81o F); its maximum temperature, 30o C (86o F) (Kasselmann 1995).
• U.S. southern populations overwinter as perennials; northern populations overwinter and regrow from tubers.
• Can grow in only 1% of full sunlight.
• Low light compensation and saturation points and low CO2 compensation point make it a competitive plant because it can start growing in low light before other plants do (Van et al. 1976; Bowes 1977).

More information about Hydrilla.