Scouting for weeds: Common St. Johnswort

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

Editor’s note: The identification information is from the new field guide An IPM Pocket Guide for Weed Identification in Christmas Trees. For ordering information, call 517-353-6740 or visit: http://www.ipm.msu.edu/pdf/pocketGuidesLandsc07.pdf .

Common St. Johnswort flowers
Common St. Johnswort flowers

Common St. Johnswort (Hypericum perforatum L.)

Family. Clusiaceae (St. Johnswort))

Life cycle. Multistemmed, shrublike perennial.

Leaves. Opposite, narrow oval to oblong leaves have round to pointed tips and entire margins, and attach directly to the stem. Leaves are covered with small, transparent dots when viewed against the light.

Stems. Erect with numerous branches and up to 3 feet tall. Herbaceous stems are often reddish in color and become woody at the base with maturity.

Flowers and fruit. Bright yellow to orange-yellow flowers with five petals are approximately 0.75 inch wide, often with several black dots along the petal margins. Fruit are capsules with three cells containing numerous dark brown to black, cylinder-shaped seeds.

Reproduction. Seeds and short runners.

Black dots on petal
Black dots along the petal margin
of common St. Johnswort

Young plant
Young common St. Johnswort
plant

Transparent dots on leaf
Transparent dots on common
St. Johnswort leaf

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