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Weed Identification

perennial sowthistle

Family

Asteraceae

Scientific Name

Sonchus arvensis

Other Common Names:

field sowthistle
corn sowthistle
gutweed swine sowthistle

Habit

Seedlings have 0.2-0.3 inches long by 0.1-0.2 inches wide, ovate cotyledons. Mature plants have a vast horizontal root system and hollow, succulent, branching stems reaching 7 ft tall.

Leaves

Found mostly on the lower half of the stem, leaves are lance-shaped 2.5-16 in long and alternate in arrangement. Lower leaves have winged curled back stalks and are deeply lobed with soft prickly margins. Stalkless upper leaves clasp the stem and have 2-7 backward pointing lobes on each side. Margins of upper leaves are not as deeply lobed but they have teeth tipped with spines.

Identifying Characteristics

Yellow flower heads measuring more than 1 in across, stems below and flower bracts covered with yellow hairs and milky juice in stems and leaves. Even small numbers of plants can severely reduce crop yields due to its ability to compete for light, space, nutrients, and moisture.

Flower Seed Head

Each plant bears several flower heads made up of bright yellow ray florets and dark-green floral bracts. Stems below flower heads and floral bracts are covered with yellow hairs.

Seed Fruit

A single seeded achene. The seeds 3.0 mm long and less than 1 mm wide are reddish brown, and rectangular. Each plant is capable of producing 4,000 seeds that may remain viable in the soil for several years. Dispersal is aided by the pappus made up of several 0.4-0.5 in long white hairs.

Where Found

Throughout Canada and the United States in cultivated crops, pastures, and meadows.

Growth Habit

upright and nonwoody

Thorns or Spines

present

Approximate Flower Diameter

Varies: 
quarter
half dollar
larger

Dominant Flower Color

Varies: 
yellow
orange

Flower Symmetry

radial symmetery

Leaf Hairs

Varies: 
has hairs
no hairs

Leaf Shape

Varies: 
lance
oval

Leaf Arrangement

Varies: 
alternate
rosette

Leaf Margin

Varies: 
lobed
serrated

Leaf Structure

simple

Leaf Stalk

Varies: 
none
shorter than leaf

Stem Hairs

Varies: 
has hairs
no hairs

Stem Cross Section

round or oval

Milky Sap

present

Root Structure

taproot

Life Cycle

perennial

Ochrea

not present

Plant Type

Herb