Best Herbicide For St John’s Wort: Top 15 Options

St John’s wort is a perennial broadleaf weed that spreads aggressively through creeping roots and seeds, forming dense patches in pastures, fence rows, roadsides, and disturbed ground. It is toxic to livestock and can cause photosensitivity, weight loss, and death when consumed in quantity. Because it is a broadleaf species, selective herbicides control it without harming most grasses.

Effective St John’s wort control depends on hitting the plant while it is actively growing and moving energy to its roots. Late spring to early summer is the ideal window for post-emergent treatments, when the plant is flowering and most vulnerable. Pre-emergent products prevent new seedlings from establishing but do not kill established plants.

St John’s wort is often found alongside desirable pasture grasses, so product selection must account for surrounding vegetation and grazing restrictions. Some products carry long soil residual activity that can affect sensitive crops for months, and others are not registered for use in grazing systems.

In this guide, we review the best herbicides for St John’s wort by examining selective broadleaf products, application timing, and safe-use practices for long-term control.

Best Herbicide For St John’s Wort: Top 15 Options

1. 2,4-D Amine – Best Overall Herbicide For St John’s Wort

2,4-D amine is the best overall herbicide for St John’s wort because it selectively controls broadleaf weeds without harming most pasture grasses. It is a synthetic auxin that causes uncontrolled cell division and twisting. It is absorbed through leaves and translocated throughout the plant.

2,4-D amine is used in pastures, fence rows, roadsides, and non-crop areas to control St John’s wort and other broadleaf weeds. It works best when applied to actively growing weeds in late spring or early summer before they set seed. It is often tank-mixed with picloram or dicamba for broader spectrum control.

This product is easy to apply with a backpack or boom sprayer and requires careful calibration for even coverage. It is not safe around desirable broadleaf plants, so drift control is essential in mixed landscapes.

2. Picloram – Best For Long-Term Pasture Control

Picloram is a selective broadleaf herbicide with long soil residual activity that controls St John’s wort and other deep-rooted broadleaf weeds in pastures and rangeland. It is absorbed through leaves and roots and translocated throughout the plant. It is highly effective on tough perennial species.

Picloram is labeled for pastures, rangeland, fence rows, and non-crop areas. It controls St John’s wort and other broadleaf weeds without harming established grasses. It works best when applied to actively growing weeds in spring or fall.

This product has long soil residual activity that can affect sensitive broadleaf plants for months or years. It is not suitable for use where legumes or sensitive crops will be planted soon after application.

3. Aminopyralid – Best For Grazing Systems

Aminopyralid is a selective broadleaf herbicide that controls St John’s wort and other deep-rooted broadleaf weeds in pastures and rangeland. It is absorbed through leaves and roots and translocated throughout the plant. It is highly effective on tough broadleaf species.

Aminopyralid is labeled for pastures, rangeland, fence rows, and non-crop areas. It controls a broad spectrum of broadleaf weeds without harming established grasses. It works best when applied to actively growing weeds in spring or fall.

This product has significant soil residual activity that can affect sensitive broadleaf plants for months or years. It is not suitable for use where legumes or sensitive crops will be planted soon after application.

4. Dicamba – Best For Tough Broadleaf Weeds

Dicamba is a selective broadleaf herbicide that controls St John’s wort and other tough broadleaf weeds in pastures and crop fields. It is absorbed through leaves, roots, and stems, making it effective under a wide range of conditions. It is often tank-mixed with 2,4-D for broader spectrum control.

Dicamba is used in pastures, fence rows, corn, and small grains to control St John’s wort and other broadleaf weeds. It has some soil residual activity that extends control of germinating broadleaf weeds. It works best when applied to actively growing weeds in spring or fall.

Dicamba is volatile and can drift long distances, damaging sensitive crops and garden plants. It is regulated in many states, with restrictions on timing and buffer zones.

5. Triclopyr – Best For Woody And Tough Broadleaf Weeds

Triclopyr is a selective broadleaf herbicide that targets woody plants and tough broadleaf weeds including St John’s wort. It is absorbed through leaves, stems, and roots, depending on the formulation. It is available in ester and amine forms.

Triclopyr is labeled for pastures, rights-of-way, and non-crop areas. It is often tank-mixed with 2,4-D or clopyralid for broader spectrum control. It works best when applied to actively growing weeds during mild weather.

This product has some soil residual activity in ester formulations. It can injure desirable broadleaf plants if applied carelessly, so careful targeting is essential in mixed landscapes.

6. Clopyralid – Best For Asteraceae And Tough Broadleaf Weeds

Clopyralid is a selective broadleaf herbicide that targets weeds in the Asteraceae family and other tough broadleaf species, including St John’s wort. It is absorbed through leaves and roots and translocated throughout the plant. It leaves most grasses unharmed.

Clopyralid is labeled for pastures, rangeland, rights-of-way, and some crops. It is effective on St John’s wort, thistles, and other tough broadleaf weeds when applied to actively growing plants. It is often tank-mixed with 2,4-D for broader spectrum control.

Clopyralid has long soil residual activity that can affect sensitive broadleaf plants, including legumes and vegetables. Compost made from treated grass clippings has caused damage to garden plants in some cases.

7. Metsulfuron Methyl – Best For Low-Rate Control

Metsulfuron methyl is a selective ALS inhibitor that controls St John’s wort and other broadleaf weeds at very low application rates. It is absorbed through leaves and roots and translocated to growing points, disrupting cell division. It is highly effective on many broadleaf species.

Metsulfuron methyl is labeled for pastures, rights-of-way, and non-crop areas. It controls St John’s wort, thistles, and other broadleaf weeds when applied to actively growing plants. It works best when applied in spring or fall during mild weather.

This product has significant soil residual activity that can affect sensitive broadleaf plants for months. It is not suitable for use where sensitive plants will be planted soon after application.

8. Glyphosate – Best Non-Selective Option

Glyphosate is a non-selective herbicide that kills St John’s wort and most other vegetation it contacts. It is absorbed through foliage and translocated to roots, killing both top growth and the underground crown. It has no soil residual activity.

Glyphosate is used for fence line clearing, driveway cracks, and site preparation before planting. It works best when applied to actively growing St John’s wort in late spring or early summer, when the plant is moving energy to its roots. It is not selective, so it kills desirable plants on contact.

Resistance to glyphosate has developed in many weed species, so rotating to other modes of action helps preserve its effectiveness. It requires careful application to avoid drift onto desirable vegetation.

9. Glufosinate – Best Non-Selective Alternative To Glyphosate

Glufosinate is a non-selective herbicide that controls St John’s wort and other vegetation through contact action. It is absorbed through leaves and acts on contact, with symptoms appearing within hours. It is often used as an alternative to glyphosate where resistance is a concern.

Glufosinate is labeled for fence rows, rights-of-way, and non-crop areas. It has no soil residual activity, so it does not prevent new weeds from germinating. It works best when applied to small, actively growing vegetation with good spray coverage.

This product requires good spray coverage for effective control and works best on small weeds. It is less effective on large or stressed vegetation than systemic herbicides.

10. Diuron – Best For Long-Term Soil Residual Control

Diuron is a non-selective herbicide with long soil residual activity that controls St John’s wort and other vegetation in fence rows. It is absorbed through roots and inhibits photosynthesis in susceptible plants. It is highly effective on annual weeds and some perennials.

Diuron is labeled for fence rows, rights-of-way, and industrial sites. It controls annual grasses and broadleaf weeds when applied before or after germination. It works best when applied to bare soil or small, actively growing vegetation.

This product has significant soil residual activity that can affect desirable plants for months or years. It is not suitable for use where sensitive plants will be planted soon after application.

11. Bromacil – Best For Total Vegetation Control

Bromacil is a non-selective herbicide with long soil residual activity that controls St John’s wort and other vegetation in fence rows. It is absorbed through roots and inhibits photosynthesis in susceptible plants. It is highly effective on tough perennial species.

Bromacil is labeled for fence rows, rights-of-way, and industrial sites. It controls a broad spectrum of vegetation when applied to bare soil or actively growing plants. It works best when applied in late summer or early fall before soil freezes.

This product has significant soil residual activity that can affect desirable plants for months or years. It is not suitable for use where sensitive plants will be planted soon after application.

12. Hexazinone – Best For Total Fence Line Clearing

Hexazinone is a non-selective herbicide with long soil residual activity that controls St John’s wort and other vegetation in fence rows. It is absorbed primarily through roots and inhibits photosynthesis in susceptible plants. It is highly effective on tough perennial species.

Hexazinone is labeled for fence rows, rights-of-way, and non-crop areas. It controls a broad spectrum of vegetation when applied to bare soil around the base of established woody plants. It works best when applied in late summer or early fall before soil freezes.

This product has significant soil residual activity that can affect desirable plants for months or years. It is not suitable for use where sensitive plants will be planted soon after application.

13. Simazine – Best Pre-Emergent For Non-Crop Areas

Simazine is a pre-emergent herbicide that prevents St John’s wort seeds from germinating in non-crop areas. It is absorbed through roots and inhibits photosynthesis in emerging seedlings. It has no post-emergent activity on established plants.

Simazine is labeled for fence rows, rights-of-way, and industrial sites. It controls St John’s wort and other annual weeds when applied in early spring before seed germination. It works best when applied to bare soil or small, actively growing vegetation.

This product has significant soil residual activity that can affect desirable plants for months. It is not suitable for use where sensitive plants will be planted soon after application.

14. Diuron + Bromacil – Best Combination For Total Control

Diuron combined with bromacil provides both pre-emergent and post-emergent control of St John’s wort in fence rows and non-crop areas. Both ingredients inhibit photosynthesis, and the combination provides longer residual control than either product alone.

Diuron plus bromacil is labeled for fence rows, rights-of-way, and industrial sites. It controls a broad spectrum of vegetation when applied to bare soil or actively growing plants. It works best when applied in late summer or early fall before soil freezes.

This combination has significant soil residual activity that can affect desirable plants for months or years. It is not suitable for use where sensitive plants will be planted soon after application.

15. 2,4-D + Picloram – Best Combination For Pasture Control

2,4-D combined with picloram provides broad-spectrum control of St John’s wort and other tough broadleaf weeds in pastures. Both ingredients are synthetic auxins that cause uncontrolled cell division and twisting. The combination delivers faster top-kill than picloram alone.

2,4-D plus picloram is labeled for pastures, rangeland, fence rows, and non-crop areas. It controls St John’s wort and other broadleaf weeds without harming established grasses. It works best when applied to actively growing weeds in spring or fall.

This combination has long soil residual activity that can affect sensitive broadleaf plants for months or years. It is not suitable for use where legumes or sensitive crops will be planted soon after application.

Tips For Controlling St John’s Wort With Herbicides

• Identify the weed first: Confirm that the plant is St John’s wort and not another yellow-flowered species before choosing a product.
• Treat during flowering: Apply post-emergent herbicides when St John’s wort is flowering in late spring or early summer, when the plant is moving energy to its roots.
• Combine pre and post-emergent: Pre-emergent herbicides prevent germination, while post-emergent products kill existing plants. Combining both delivers the best long-term control.
• Watch grazing restrictions: St John’s wort is toxic to livestock. Check the label for grazing restrictions before treating pastures and hay ground.
• Rotate modes of action: Using the same herbicide group repeatedly selects for resistant weeds. Rotate between synthetic auxins, ALS inhibitors, and photosystem II inhibitors.
• Prevent seed production: Remove or treat plants before they flower to prevent new seeds from entering the soil seed bank.
• Monitor and re-treat: St John’s wort regrows from creeping roots and seeds. Scout treated areas several times per season and re-treat new growth while it is small.

Best Herbicide For St John’s Wort FAQ’S

1. What is the best herbicide for St John’s wort?

2,4-D amine is the best overall post-emergent herbicide for St John’s wort because it selectively kills broadleaf weeds without harming most grasses. Picloram and aminopyralid are best for long-term control in pastures.

2. Will 2,4-D kill St John’s wort?

Yes, 2,4-D kills St John’s wort when applied to actively growing plants. It is absorbed through leaves and translocated to roots, causing the plant to twist and die over one to three weeks.

3. When should I spray St John’s wort?

Spray St John’s wort in late spring or early summer when the plant is flowering and actively growing. Pre-emergent products go down in early spring before seed germination.

4. Is St John’s wort toxic to livestock?

Yes, St John’s wort contains hypericin, a compound that causes photosensitivity in livestock. Poisoning can lead to skin blistering, weight loss, and death. Remove livestock from infested pastures and treat before plants flower.

5. Does St John’s wort come back after spraying?

St John’s wort can regrow from creeping roots and seeds after a single treatment. Repeated applications over two to three seasons are usually needed for complete control.

6. Can I use vinegar to kill St John’s wort?

Vinegar can burn St John’s wort foliage but rarely kills the roots. It works best as part of an integrated program combining hand-pulling, mowing, and selective herbicides.

7. What is the difference between St John’s wort and other yellow-flowered weeds?

St John’s wort has five-petaled yellow flowers with prominent stamens and opposite oval leaves. Other yellow-flowered weeds like tansy and ragwort have different flower structures and leaf arrangements. Confirm identification before treatment.

Conclusion

2,4-D amine is the best overall herbicide for St John’s wort because it selectively kills broadleaf weeds without harming most grasses. Picloram, aminopyralid, dicamba, and triclopyr follow closely as broad-spectrum options for pastures, fence rows, and non-crop areas.

For long-term residual control, metsulfuron methyl, diuron, bromacil, hexazinone, and simazine prevent new St John’s wort seedlings from establishing and reduce the need for repeat applications. These products are best used where total vegetation control is acceptable and where sensitive plants are not nearby.

Effective St John’s wort control requires matching the product to the site, timing applications for late spring or early summer during flowering, and protecting surrounding vegetation. Rotating modes of action prevents resistance, and treating before plants set seed reduces future infestations.

Start your St John’s wort control program this season, wear protective equipment when handling plants, and plan for follow-up treatments each year. Consistent effort over two to three seasons delivers a clean pasture and long-term control.