Kootenai County Seal, Administration Building, and Lake

Bill Hargrave,
Weed Superintendent

Email: kcnoxiousweeds@kcgov.us
P: (208) 446-1290
F: (208) 446-1282

Physical Address:
Click here for map.
10905 N. Ramsey Road
Hayden, ID 83835

Office Hours:
Mon - Fri 8:00a.m. - 4:30p.m.


BOTANICAL NAME Hypericum perforatum
WHAT DOES IT LOOK LIKE? A perennial that can grow 1 to 3 feet tall. A mature plant generally consists of one or more woody crowns which are attached to the root system. Large crowns can produce up to 30 erect, reddish, flowering stems yearly. The flowers have five bright yellow petals and deep purple dots along the jagged edges. The rust colored stems of St. Johnswort can be seen during the winter sticking above the snow.
WHERE DOES IT LIKE TO GROW? This agressive weed prefers poor soil and full sun and can be found mostly in meadows, dry pastures, rangelands, roadsides, and empty fields.
WHEN DOES IT BLOOM AND HOW DOES IT SPREAD? St. Johnswort blooms from May to late September with the peak flowering time in mid to late June. An average plant produces about 15,000 to 30,000 seeds. Seeds can be moved short distances by the wind and longer distances by cling to man, machinery and animals. The seeds can live in the ground from 6 to 10 years. This plant can also make new plants from its wide-spreading root system.

CAUTION Animals that eat St. Johnswort, and then are exposed to direct sunshine, develp severe sunburns that are seen as skin irritations in bald or white areas. Young cattle and sheep are most often affected.


The new growth of St. Johnswort is small and hard to find in the spring. During the winter the rust colored stems of St. Johnswort can be seen sticking above the snow. This is a good time to record and mark those infected areas so that you can find them in the early spring.
> The use of fertilizer and good watering habits has been shown to control the spread of St. Johswort.
> Cut and bag flower heads, if possible, to prevent plants from going to seed.
> Pulling or digging can be considered on new or small infestations. To identify a young plant, hold the leaves up to the light and they will appear to have pin holes in them. Due to the toxicity of the plant, wear gloves and avoid touching your eyes.
>The leaf eating beetle - ^Chrysolina hyperici^- has been very successful in controlling large infestations of St. Johswort in Kootenai County.
>The best time to apply herbicides is right after germination on new seedling, before any blossoms open.

WANT MORE INFORMATION? Contact the Kootenai County Noxious Weed Control Office at (208) 446-1290, visit our office a 10905 Ramsey Road in Hayden or email us at kcnoxiousweeds@kcgov.us
> Weeds of the West, 9th Ed.; 2001
> Northwest Weeds, R.J. Taylor, 1990
> USDA Poisonous Plant Research Laboratory website http://www.ars.usda.gov/Services/docs.htm?docid=9986
> Invasive Species website http://www.invasivespeciesinfo.gov/plants/stjohnswort.shtml
>Developed by Linda Ely, Kootenai County Noxious Weed Control, October 2009


Download a PDF
St. Johnswort (57.44 KB)
Information to identify and control St. Johnswort.