Weed Identification

Australia > > St John's Wort

St John's Wort

Hypericum perforatum

Family: Clusiaceae.

Form: Herb

Origin: Native of Europe, western Asia, North Africa.

Flowers/Seedhead: Flowers: About 2 cm wide with 5 bright yellow petals. Flowers late spring and summer.

Description: Perennial rhizomatous herb with erect stems to 1.2 m high. Leaves hairless, paler on the lower surface, ovate to narrow-oblong to about 3 cm long. Stems with opposite weakly pronounced longitudinal ridges and with occasional black glands. Fruit sticky, containing many pitted seeds about 1 mm long. Roots to 1 m deep; rhizomes shallow producing many buds.

Distinguishing features: Distinguished by numerous translucent glands on the leaves (obvious when leaves are held to the light), and yellow flowers often with black glands on the petal margins (look like dots—see photo).

Dispersal: Spread by seed, growth of rhizomes and movement of cut sections of rhizomes.


Flowers with black gland dots on petal margins

Notes: There are two varieties, var. perforatum with broad leaves and var. angustifolium with narrow leaves but intermediates are also common. One plant will produce thousands of seeds and these may remain viable in the soil for many years. Introduced to Australia in 1800s, and still spreading, especially on roadsides and cleared land. Major weed in USA and Canada. In Australia biocontrol has been partly successful but work is still continuing. Hypericin is concentrated in oil glands and causes photosensitisation in light skinned stock especially sheep.

References:

    The Biology of Australian Weeds. R. Groves et al. (eds), Vol. 1, 1995, pages 149–167. Noxious Weeds of Australia. W. Parsons and E. Cuthbertson, 1992, pages 387–391.

Web References: Search Australian web sites for further information on this weed.

Fruit is sticky and 3-celled
Dense stand in paddock,
near Wyangala Dam, NSW, November

This weed has been included in the WEEDeck field guide as card H16

More information about WEEDeck is available from Sainty & Associates Pty. Ltd.

 

Australia > > St John's Wort

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