Hypericum perforatum

Scientific Name Hypericum perforatum L.
Common Names English: Common St. Johnswort, St. John's Wort, Klamath weed; German: Tüpfel-Johanniskraut, Tüfel-Hartheu; French: millepertuis perforé; Spanish: Hierba de San Juan, corazoncillo, hipérico
Description Erect noxious perennial, to 1.2 m (47.24 inch) tall, with rhizomes and showy, bright yellow flowers.


Characteristic Features

Transparent (pellucid) perforations on the leaves and sepals, and black punctate glands on the leaves and stems. The punctuations are not as dense as in H. punctatum, which has much smaller flowers.

Young Plant

Cotyledons lanceolate to ovate, 1.5-3 mm (0.059 - 0.118 inch) long, 1-2 mm (0.039 - 0.078 inch) wide. Subsequent leaves opposite, oval to elliptic, increasingly larger. Underside leaf margins dotted with a few elevated black glands.


Lanceolate to ovate, 1.5-3 mm (0.059 - 0.118 inch) long, 1-2 mm (0.039 - 0.078 inch) wide.


Multiple from base, from thick rhizomes, to +1m (3.28 ft) tall, dichotomously branching (branches decussate), glabrous, glaucous. Midrib of leaf decurrent on stem and with black glandular punctations.


Opposite, decussate, sessile, lance-ovate to oblong or elliptic, entire, to +1.8 cm (0.71 inch) long, +7mm (0.275 inch) broad, entire, acute to obtuse, glabrous, glaucous (mostly below), with black glandular punctations on margins and a few scattered on the leaf tissue, perforate.

Propagation Organs


Inflorescence terminal bracteate cymes. Pedicels to 2 mm (0.078 inch) long, glabrous (or flowers sessile).
Flowers with petals (4)5, yellow, spreading, to 1.3 mm (0.051 inch) long, 6 mm (0.236 inch) broad, oblong, obtuse, with black glandular punctations.
Stamens many (+50), erect to spreading.
Filaments yellow, 7-8 mm (0.27 - 0.32) inch long, glabrous.
Anthers biglobose, styles 3, spreading, 6 mm (0.236 inch) long, glabrous, yellow.
Ovary superior, glabrous, 3.5 mm (0.14 inch) long, 2.1 mm (0.08 inch) in diameter, ovoid, 3-locular, ovules (seeds) many.
Sepals 5, linear, 5-6 mm (0.19 - 0.24 inch) long, 1.1mm (0.043 inch) broad, acuminate, glabrous, with perforations adjacent and parallel to midrib.

Flowering Period

May - September.


Capsules 3-chambered, ovate, not lobed, sticky-glandular, 5-10 mm (0.19 - 0.39 inch) long, with persistent styles 3-10 mm (0.118 - 0.39 inch) long, open longitudinally to release seed.


Shiny black to brown, nearly cylindrical, ~ 1 mm (0.039 inch) long, densely pitted, often coated with gelatinous material from the capsule that aids dispersal and may inhibit germination until it breaks down or leaches out in about 4-6 months.
Germination occurs from fall through spring.

Viability of Seeds

Up to 12 years.


Reproduces by seeds and vegetatively from rhizomes.



Pastures, fields, waste ground, roadsides, railroads.


Slightly acidic to neutral soils. Does not tolerate saturated soils.

Agricultural Importance

H. perforatum is an important weed in both natural and agricultural ecosystems. It contains the alkaloid hypericin, which causes photosensitisation in mammals which ingest it, resulting in blisters on weakly pigmented parts of exposed skin. St. John's Wort is a major weed of pastures because of its toxicity to stock and its ability to compete with desirable pasture species.


Useful non-chemical contribution to Integrated Weed Management

Releasing the leaf-feeding beetle Chrysolina quadrigemina on uninfested plant populations can eventually provide excellent long-term biological control. Cultivation readily controls plants on agricultural lands. Improving soil fertility and reseeding with desirable vegetation can help reduce or eliminate infestations in pastures. Mowing and overgrazing reduces seed production, but promotes vegetative spread from rhizomes.

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