Common St. Johnswort (Hypericum perforatum)
Common St. Johnswort is an herbaceous perennial that typically grows 1–2½ feet tall, though it may reach up to 5 feet under favorable conditions. It produces one or multiple erect stems from a woody crown. Stems are round, hairless, light green, and often have two small longitudinal ridges. Lower stems may become woody at the base and branch toward the top.
Leaves are arranged opposite each other, oblong in shape, about 1 inch long, and sessile (attached directly to the stem). When held up to the light, numerous tiny transparent glands are visible, giving the leaves a “perforated” appearance.
Flowering occurs from early to late summer. Flowers are bright yellow, about ¾ inch across, and arranged in flat-topped clusters (cymes) at the ends of stems. Each flower has five yellow petals, five green sepals, three styles, and numerous protruding stamens (often more than 20), giving the flower a star-like appearance.
Common St. Johnswort spreads by both seed and an extensive root system that includes a deep taproot and lateral rhizomes. Seeds are produced in three-celled capsules and are small, black, round, and shiny. Seeds can remain viable in the soil for more than 50 years.
This species is toxic to livestock and wildlife, causing photosensitivity and other health issues when consumed in significant quantities.
Quick ID Guide
Life Cycle:
• Herbaceous perennial
• Deep taproot with spreading rhizomes
Height:
• 1–2½ feet typical
• Up to 5 feet under ideal conditions
Stems:
• Round, hairless
• Light green
• Two longitudinal ridges
• Woody at base
Leaves:
• Opposite arrangement
• Oblong, about 1 inch long
• Hairless
• Tiny translucent glands (visible in light)
Flowers (Early–Late Summer):
• Bright yellow
• 5 petals• Numerous stamens
• Flat-topped clusters
Seeds:
• Small, black, shiny
• Produced in 3-celled capsules
• Long-lived in soil
Management Options
Prevention and cultural control strategies should be emphasized whenever possible.
There are two biological control agents available for common St. Johnswort; however, infestations in Sublette County are not always dense enough to support sustainable populations. Agents present in the county include leaf-feeding moths (“inchworms”) such as Aplocera plagiata and leaf-feeding beetles such as Chrysolina quadrigemina.
Small infestations may be controlled through careful hand-pulling or digging. The taproot must be removed below the crown, and as much of the lateral rhizome system as possible should be extracted to prevent regrowth. Wear gloves and long sleeves when handling the plant, as it is poisonous outside of medicinal use.
Mowing at regular two-week intervals can help deplete root reserves. Grazing is not recommended, as Common St. Johnswort is toxic to livestock and wildlife.
Larger infestations may require herbicide applications. Spring and early summer treatments prior to bloom are generally most effective. Always read and follow the herbicide label before application. Contact Sublette County Weed & Pest for assistance with product selection, rates, and application timing.
