Common St. Johnswort identification and control

Why it’s a noxious weed

Common St. Johnswort spreads easily to new sites and it can cause various health problems to animals. Eating Common St. Johnswort makes the skin very sensitive to sunlight. This can cause hair loss, severe burns and blistering, and lead to infection. It contaminates hay and remains dangerous to eat once dried. Common St. Johnswort is widespread and difficult to control. 

Plant description

Common St. Johnswort grows in full sun on disturbed, well drained sites such as roadsides, meadows, overgrazed range and logged areas.

Common St. Johnswort is an upright perennial herbaceous plant which grows 1 to 2.5 feet in height. It has taproots, short rhizomes (roots that spread outwards and can grow new stems) and branched stems. Stems are reddish, single or many, woody at the base, and branching up and outward.

Leaves are 1 to 2 inches long, attach directly to the stem with no stalk and are opposite (connect to the stem in pairs). They are lance-shaped with pointed tips and each leaf is spotted with tiny transparent dots.

Flowers are yellow, star-like and have 5 petals with tiny black dots on the edges. Flowers are clustered 25 to 100 at the end of the stems. Blooms from June – September. 

Be aware of look-alike plants

Common tansy, a Class C noxious weed, and tansy ragwort, a Class B noxious weed, may be mistaken for St. Johnswort. The state noxious weed board has a useful visual comparing the flowers of these species. 

When in doubt, take photos and report them on iNaturalist.

What to do if you find it. 

Property owners in King County are not required to control common St. Johnswort, as it is so widespread. We are not tracking infestations. We can provide advice on how to control common St. Johnswort, but there is generally no legal requirement to do so.

The King County Noxious Weed Control Board encourages property owners to remove common St. Johnswort where possible, and to avoid introducing it to new landscapes.

Control methods 

We recommend using a combination of methods to control noxious weeds. In areas with few weeds, it is important to act quickly before they become harder to control. Make a long-term plan as it often takes several years to get rid of most weeds. Start in the least infested areas first and then move into more heavily infested areas. 

Manual control

Pulling is effective for new or small infestations and needs to be repeated several times Make sure to wear gloves and avoid touching your eyes. Repeated tilling is effective for croplands. 

Mechanical control

Mowing will not kill existing plants but can prevent seed set if done regularly throughout the season.

Cultural control

Site-specific range management (including encouraging beneficial plant species) will help prevent weed regeneration.

Biological control

In rare occasions, biological control may be the best option. The Klamath weed beetles Chrysolina spp. and the St. Johnswort moth Aplocera plagiata feed on the leaves. The St. Johnswort root borer Agrilus hyperici larvae feed within plant roots. These insects take many years to have a significant impact on an infestation. They can only reduce a population, not eradicate it. Only recommended for large infestations or where other methods are not feasible. For more information, please visit WSU Extension Integrated Weed Control Project.

For more information or a site-specific control recommendation in King County, contact the noxious weed program. For information in other counties in Washington state, contact your local weed board or your local cooperative extension office.  

Disposal instructions

Do not leave plants onsite since they may reroot. If pulled, bag the plants to put into the garbage. 

Noxious Weed Disposal - Washington State Noxious Weed Control Board

Are your weeds piling up? - Noxious Weeds Blog