St. Johnswort

Clusiaceae or St. Johnswort Family
Hypericum perforatum

Growth Habit: Perennial, erect, numerous branches.
Leaves: Opposite, attached directly to stem approximately 1 inch long, transparent dots.
Stems: Woody at the base, 1 to 3 feet tall, rust colored with 2 ridges. Turns reddish brown and dies in late summer.
Flowers: Yellow, 3/4 inch in diameter, clustered.
Roots: Woody taproot can penetrate the soil to a depth of 4 to 5 feet. Lateral roots grow 2 to 3 inches beneath the soil surface and extend 3 feet.
Seeds: Produces between 15,000 and 33,000 seeds annually. Viability ranges from 6 to 10 years.
Reproduction: Vegetative buds from lateral roots and from seeds.
Habitat: Low elevational sites where the annual precipitation is between 15 and 30 inches.

Can be effective on small infestations if done consistently and persistently over the course of several years. Depletion of the seed bank is necessary for the complete eradication of an infestation.
Mowing: Mowing is not an effective method of control for this species, though if done prior to flower it will reduce the amount of seeds produced that season.
Biological Control: There have been five insect species released as bio-control agents for St. Johnswort in the U.S., and they have been very effective at reducing large infestations. Most of the insect species prefer open, sunny, warm areas. Contact the Missoula County Weed District for assistance with choosing appropriate species, monitoring and/or additional releases. 
Grazing: Grazing is not a recommeded method of control for this species due to its potential to poison livestock.
Herbicide: The following herbicides have been labelled for control of St. Johnswort. Always consult product labels and read them carefully to ensure correct species/land management usage and chemical application.

Trade Name:      Active Ingredient:  Rate: Efficacy:  Comments:
 Tordon 22k  picloram  1-1.5 pts/acre most effective if applied pre-flower stage Cannot use near surface water, shallow water, landscaped areas and current or future vegetable gardens
(Restricted Herbicide - need an applicator license to use.)
 Milestone  aminopyralid  5-7 oz/acre most effective if applied pre-flower stage Can be applied to waters edge;
cannot be used in landscaped areas and current or future vegetable gardens,
mulch or manure from treated areas should not be applied to vegetable gardens or composted for vegetable gardens
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