Management

• Biological Control
• Herbicidal Control
• Hand Pulling
• Mowing
• Tilling
• Prescribed Burning
• Grazing Control
• Cultural Control
• Revegetation

Biological Control

Currently there are five insect species available for control of common St. Johnswort (see Table 1). This weed has the distinction of being the first target weed species in 1945 and 1946 for control using biological control insects. The Klamath weed beetles, Chrysoloina hyperici and C. quadrigemina, were very successful in reducing populations of common St. Johnswort in Australia and California. Adult beetles lay eggs on the leaves in the fall or spring. The eggs hatch in six to seven days and the larvae feed on the leaves and flowers, and can completely defoliate plants. Heavy larval feeding in the fall reduces winter survival of common St. Johnswort. Both insects do well in mountainous, open, sunny and warm areas, but C. hyperici prefers more moist conditions than C. quadrigemina. Neither insect does well in shaded barren or excessively rocky locations.

The St. Johnswort root borer, Agrilus hyperici, is a beetle that deposits eggs on the stems from the soil level up to about eight inches (20 cm) in July and August. The larvae feed in the roots from July to May of the following year. They consume the root tissues resulting in stunted plants and reduced flower production. They establish best in dry, mountainous areas. On damp sites, larvae are susceptible to fungal attack. They will attack common St. Johnswort plants growing in the shade.

The St. Johnswort moth, Aplocera plagiata (also known as Anaitis plagiata) deposits eggs on the foliage in July and the inchworm-like larvae feed on the leaves and flowers through September. Large populations of this insect can defoliate plants and inhibit flower and seed formation. They do well in dry, open areas with sandy, rocky soils, and soils with limestone parent material. They do not thrive in areas receiving high rainfall.

Herbicidal Control ¹

On pastures and rangeland, common St. Johnswort can be temporarily suppressed by aminopyralid, metsulfuron, picloram, or 2,4-D (see Table 2). Metsulfuron should be applied to actively growing plants in the spring, summer or fall at a rate of 1 ounce product (Escort^® or Cimarron^®) per acre. A non-ionic surfactant is needed in the spray solution at 0.5 percent by volume for metsulfuron to be effective. Picloram should be applied at 1 to 2 quarts product (Tordon^®) per acre to actively growing plants before bloom. The higher rate should be used to eradicate small populations. To reduce large populations, 1 quart picloram combined with 1 quart 2,4-D (4EC) per acre applied to actively growing plants before bloom is recommended. Pre-bloom foliar applications of 2,4-D at 2 quarts per acre with repeated applications to re-growth can also be used. The ester formulations of 2,4-D are more effective than amine formulations. Aminopyralid should be applied at a rate of 5 to 7 ounces (Milestone^®) per acre to plants before bloom. Glyphosate applied at 1 to 2 quarts per acre will kill common St. Johnswort on cropland or where revegetation is planned.

Hand Pulling

Hand pulling, grubbing, and hoeing may be effective on small populations of common St. Johnswort if it is applied persistently. Hand pulling that removes the short rhizomes will be most effective. Follow-up treatments will be necessary where a persistent seed bank exists.

Mowing

Mowing if applied before bloom will reduce flowering and seed production. Mowing will promote increased vegetative production and will not reduce populations.

Tilling

Common St. Johnswort is not normally a problem in cultivated crop fields because it is controlled by tillage procedures that clean crop fields of weeds. However, because it produces short rhizomes, it is possible to spread common St. Johnswort within a crop field and between fields. An application of glyphosate to common St. Johnswort plants that regenerate from rhizomes or seeds following tillage will reduce it on tilled fields. Cleaning tillage equipment of soil that may contain seeds or rhizomes is recommended after use on fields where common St. Johnswort has been growing and before use on weed-free fields.

Prescribed Burning

Fire favors seed germination and vegetative growth and by itself will not control common St. Johnswort. Prescribed fire on land infested with common St. Johnswort should be combined with other control treatments such as herbicides or biological control.

Grazing Control

Grazing to control common St. Johnswort is not recommended because of the potential for livestock poisoning. Prescribed grazing to maintain the health and competitiveness of pasture and rangeland plant communities is important in preventing and retarding invasion, and to increase the effectiveness of other control treatments.

Cultural Control

Common St. Johnswort seedlings are not strong establishers in healthy productive plant communities. Also, plant competition reduces the invasiveness of common St. Johnswort and increases the effectiveness of control applications. Therefore, practices that increase the competitiveness of desirable plant species such as conservation crop rotation and conservation cover will make the environment less hospitable for common St. Johnswort survival.

Revegetation

Species selected for revegetating disturbed sites and common St. Johnswort infestations should be appropriate for management objectives, adapted to site conditions, and competitive with the weed. Management objectives will determine if introduced or native species are seeded and the combination of species in the seed mix. The environmental conditions of the site including precipitation, soil texture and depth, slope and aspect, will affect species establishment. Refer to Montana Plant Materials Technical Note 46, "Seeding Rates and Recommended Cultivars," and Extension Bulletin EB19 "Dryland Pasture Species for Montana and Wyoming" for seeding rate guidance and revegetation species selection. State and area resource specialists can help determine the most appropriate, site-specific species mix and timing of seeding.

In most cases, herbicidal suppression of common St. Johnswort is needed for revegetation of infested lands. The herbicides listed in Table 2 will control common St. Johnswort and reduce competition during the establishment period with little or no injury to emerging grass seedlings. This is especially important for species that are slow to establish like many of the native grasses. However, where herbicides have been applied, chemical carryover should be assessed prior to planting permanent vegetation.

¹ Any mention of products in this publication does not constitute a recommendation by the NRCS. It is a violation of Federal law to use herbicides in a manner inconsistent with their labeling.

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Last Modified: 07/27/2007