Weed Information: Myriophyllum spicatum (watermilfoil)
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Myriophyllum spicatum (watermilfoil)

Family: Haloragaceae

Description

The description as written here includes both Myriophyllum spicatum and Myriophyllum exalbescens as varieties. They can generally be distinguished by the number of leaflets with var. spicatum having more than twelve pair. It is important to distinguish between the two since var. spicatum is an extremely aggressive alien and var. exalbescens is native. See both pages 272 and 276 in Common Weeds of the United States.

Characteristics

Flower Color
  • White
Inflorescence Type (How the flowers are arranged on the plant)
  • Axillary cluster
Number of Petals
  • 0 (no petals)
  • 4 petals
Petal Separation
  • Completely free to base
Petal Tip
  • Rounded
Flower Symmetry
  • Flowers regular
Flower Length (Head length in the Asteraceae)
  • < 1 mm (< .04 inch)
  • 1 mm (.04 inch)
Flower Width (Head width in the Asteraceae)
  • < 1 mm (< .04 inch)
  • 1 mm (.04 inch)
Number of Sepals
  • 4 sepals
Sepal Separation
  • Completely free to base
Sepal Tip
  • Laciniate
Flower Sex
  • Unisexual (monoecious or dioecious)
Number of Styles
  • Styles lacking (male flowers only)
  • 4 styles
Number of Stamens
  • stamens lacking (female flowers only)
  • 4 stamens
  • 8 stamens
Stamens Attached to the Petals
  • Stamens NOT attached to the petals
Stamens Connate (united)
  • Filaments or anthers NOT united

Carpel Separation
  • Carpels united (or one)
Number of Carpels
  • 0 (male flowers only or plants producing spores)
  • 4 carpels
Ovary Position
  • Inferior (epigynous)
Fruit Type
  • Nutlets, 4
  • Nut
Fruit Length at Maturity
  • 1 mm (.04 inch) or less
  • 2 mm (.08 inch)
Fruit Width at Maturity
  • 1 mm (.04 inch) or less
  • 2 mm (.08 inch)
Burs
  • Fruit or seed NOT bur-like

Leaf Arrangement
  • Whorled 4 per node
Leaf Type
  • Compound even pinnate > 9 leaflets
Stipules
  • Lacking
Tendrils
  • Tendrils absent
Venation
  • Pinnate
Petiole (leaf stem) Blade Ratio
  • Petiole absent (leaf sessile)
  • Petiole < 25% total length
Leaf Length (blade PLUS petiole)
  • 1 - 2 cm (.40 - .80 inch)
  • 2 - 4 cm (.80 - 1.60 inches)
Leaf Width
  • 1 - 2 cm (.40 - .80 inch)
  • 2 - 4 cm (.80 - 1.6 inches)
Leaf or Leaflet Margin
  • Entire
Leaf or Leaflet Blade Base
  • Acute (cuneate)
Leaf or Leaflet Tip
  • Acute
Leaf or Leaflet Blade Surface
  • Glabrous (NO hairs)
Succulence
  • Not succulent

Milky Juice
  • Juice NOT milky (watery)
Woodiness
  • Herbaceous
Spines or Thorns
  • Spines or Thorns ABSENT
Aromatic (vegetative structures)
  • Not aromatic
Life Cycle
  • Perennial
Growth Habit (while flowering)
  • Erect, columnar (narrow)
  • Floating
  • Submerged (mostly or totally)
Plant Height at Maturity
  • 2 - 4 dm (8.0 - 16.0 inches)
  • 4 - 6 dm (1.3 - 2.0 feet)
  • 6 - 8 dm (2.0 - 2.6 feet)
  • 8 - 10 dm (2.6 - 3.3 feet)
  • 1 - 1.5 meters (3.3 - 5.0 feet)
  • 1.5 - 2.0 meters (5.0 - 6.6 feet)
  • 2 - 2.5 meters (6.6 - 8.3 feet)
  • 2.5 - 5 meters (8.3 - 16.6 feet)
Wind Dissemination
  • Not wind disseminated
Moisture Regime
  • Aquatic
Chlorophyll
  • Present

Stem Cross Section
  • Round
Flowering Stem Leaves
  • Stem uniformly leafy
Main Stem Branches
  • Main stem unbranched

Root or Vegetative Propagule
  • Fibrous
  • Rooting at the nodes

US State
  • Alabama
  • Arkansas
  • Connecticut
  • Delaware
  • Florida
  • Georgia
  • Illinois
  • Kentucky
  • Louisiana
  • Massachusetts
  • Maine
  • Michigan
  • Missouri
  • Mississippi
  • North Carolina
  • New Hampshire
  • New Jersey
  • New York
  • Ohio
  • Pennsylvania
  • South Carolina
  • Tennessee
  • Texas
  • Virginia
  • Wisconsin
  • West Virginia
  • Indiana
  • Maryland
  • Vermont
  • Alaska
  • Arizona
  • California
  • Colorado
  • Iowa
  • Idaho
  • Minnesota
  • Montana
  • North Dakota
  • New Mexico
  • Nevada
  • Oklahoma
  • Oregon
  • South Dakota
  • Utah
  • Washington
  • Nebraska
Canadian Province or Territory
  • British Columbia
  • New Brunswick
  • Ontario
  • Prince Edward Island
  • Quebec

  • Aquatic and Riparian Plants of the West, 2003 (ISBN# 1-879906-59-7) See page: 86
  • The Biology of Canadian Weeds: Canadian Journal of Plant Science Vol:59Pg.201
  • Common Weeds of the United States, 1971 (ISBN 0-486-20504-5) See page: 276
  • Gilkey`s Weeds of the Pacific Northwest, 1980 (ISBN 0-88246-039-0) See page: 201
  • Selected Weeds of Oregon, 1985 and 1989 Supplement (s) Oregon State Dept. of Agriculture, See page: 47
  • South Dakota Weeds 2002 South Dakota Dept. of Ag. 523 E. Capitol, Foss Bldg. Pierre, South Dakota 57501 See page: 220
  • Weeds of the Northern U.S, and Canada, 1999 Royer and Dickinson, (ISBN# 1-55105-221-0) See page: 386 - 387
  • Weeds of the United States and Their Control, Lorenzi 1987, ISBN# 0-442-25884-4 See page: 221
  • Listed as a `Weed` by the Biota of North America Program, 2008 x
  • Listed in the WSSA Composite List of Weeds X
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