Weed Information: Epilobium hirsutum (willowweed, hairy)
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Epilobium hirsutum (willowweed, hairy)

Family: Onagraceae

Description

A relatively new and extremely aggressive weed of wetlands in the Pacific Northwest but long known as a weed in the Northeastern U.S. This species is often confused with Chaemerion angustifolium (Epilobium angustifolium) which can be distinguished by the fact that it is much less hairy, often occurs on drier sites, and its sepals are free to the base.

Characteristics

Flower Color
  • Red (or pink)
  • Purple
  • Red-purple
Inflorescence Type (How the flowers are arranged on the plant)
  • Terminal raceme
Number of Petals
  • 4 petals
Petal Separation
  • Completely free to base
  • United at base only (<25% length)
Petal Tip
  • Bilobed
  • Rounded
  • Square
Flower Symmetry
  • Flowers regular
Flower Length (Head length in the Asteraceae)
  • 1.0 cm (.40 inch)
  • 1.5 cm (.6 inch)
  • 2.0 cm (.8 inch)
Flower Width (Head width in the Asteraceae)
  • 2.0 cm (.80 inch)
  • 2.5 cm (1.0 inch)
  • 3.0 cm (1.2 inches)
  • 3.5 cm (1.4 inches)
  • 4.0 cm (1.6 inches)
Number of Sepals
  • 4 sepals
Sepal Separation
  • United at base < 25% length
Sepal Tip
  • Pointed
Flower Sex
  • Bisexual (perfect)
Number of Styles
  • 1 style
Number of 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
  • 4 carpels
Ovary Position
  • Inferior (epigynous)
Fruit Type
  • Capsule, > 3-celled
Fruit Length at Maturity
  • 8.0 cm (3.2 inches)
  • 9.0 cm (3.6 inches)
  • 1.0 dm (4.0 inches)
  • 1.5 dm (6.0 inches)
Fruit Width at Maturity
  • 3 mm (.12 inch)
  • 4 mm (.16 inch)
  • 5 mm (.20 inch)
  • 6 mm (.24 inch)
Burs
  • Fruit or seed NOT bur-like

Leaf Arrangement
  • Alternate
  • Opposite
Leaf Type
  • Simple (including lobed leaves)
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)
  • 4 - 10 cm (1.60 - 4.0 inches)
  • 1 - 2 dm (4.0 - 8.0 inches)
Leaf Width
  • 1 - 2 cm (.40 - .80 inch)
  • 2 - 4 cm (.80 - 1.6 inches)
Leaf Shape (simple leaves only)
  • Lanceolate
Leaf or Leaflet Margin
  • Entire
Leaf or Leaflet Blade Base
  • Acute (cuneate)
Leaf or Leaflet Tip
  • Acute
Leaf or Leaflet Blade Surface
  • Puberulent
  • Pubescent
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)
Plant Height at Maturity
  • 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)
Wind Dissemination
  • Fruit, seed, or spore wind borne
Moisture Regime
  • Aquatic
  • Semi-aquatic
  • Seasonally saturated
Chlorophyll
  • Present

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

Root or Vegetative Propagule
  • Fibrous
  • Rhizomatous

US State
  • Connecticut
  • Illinois
  • Kentucky
  • Massachusetts
  • Maine
  • Michigan
  • New Hampshire
  • New Jersey
  • New York
  • Ohio
  • Pennsylvania
  • Wisconsin
  • West Virginia
  • Indiana
  • Maryland
  • Rhode Island
  • Vermont
  • Oregon
  • Washington
Canadian Province or Territory
  • British Columbia
  • New Brunswick
  • Nova Scotia
  • Ontario
  • Prince Edward Island
  • Quebec

  • How to Know the Weeds, 1972, (ISBN# 0-697-04880-2) See page: 110
  • Northwest Weeds, 1990 (ISBN 0-87842-249-8) See page: 89
  • Weeds, 1955 (1980) Walter Conrad Muenscher, ISBN# 0-8014-1266-8 See page: 319
  • Weeds of the Northeast, 1997 Cornell University ISBN# 0-8014-8334-4 See page: 254
  • Weeds of the Northern United States and Canada, 1964 Montgomery See page: 117
  • 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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