What is common St. John's wort?

Common St. John’s wort is a bushy perennial from Europe that’s well known as an herbal supplement and less well known as a pasture weed. Good St. John’s wort identification matters most if you have horses or other livestock, because the plant is toxic to grazing animals.

It came over with early settlers and now grows in dry, sunny, disturbed ground across Pennsylvania. It isn’t on the state noxious weed list, but it does crowd out better forage in pastures and hay fields, and it’s hard to get rid of once it’s established.

In a regularly mowed lawn it rarely gets a foothold. The trouble spots are rough, sunny areas that only get mowed now and then.

St. John's wort identification: key features

Check a plant in early summer, when it’s in bloom:

• Leaves: small, oblong and smooth-edged, in opposite pairs with no stalk. Hold one up to the sky and you’ll see dozens of tiny translucent dots, as if the leaf had been poked with a pin. That’s where the name perforatum comes from.
• Stems: upright, branched near the top, often rusty red and woody at the base. Plants usually reach 1 to 3 feet.
• Flowers: bright yellow, about an inch across, with five petals and a starburst of many yellow stamens. Look for tiny black dots along the petal edges. Crush a flower or bud and your fingers may stain reddish-purple.
• Seed capsules: small, sticky and egg-shaped, turning brown in late summer. The dead reddish-brown stalks stand through winter.

Where it grows in Pennsylvania

St. John’s wort likes full sun and well-drained, poor soil. In central PA you’ll see it on old farm fields, pastures, roadside banks, rail lines and utility rights-of-way, gravel lots and shale or sandy ground that doesn’t hold much water.

It struggles in shade and in thick, vigorous grass, so it moves in where grazing, drought or disturbance has opened up the ground. Overgrazed horse paddocks and freshly graded banks that were never fully reseeded are common starting points.

St. John's wort look-alikes

Several yellow-flowered plants get confused with it:

• Spotted St. John’s wort is a native relative. It has smaller flowers, and its petals and leaves are speckled with black dots all over rather than mostly along the petal edges.
• Bird’s-foot trefoil has clusters of yellow pea-like flowers and leaves split into small leaflets.
• Yellow sweetclover has three-part leaves and tall, thin spikes of tiny yellow flowers.
• Yellow rocket flowers in spring, not summer, with four-petaled flowers and glossy lobed leaves.

The light test on the leaf is the fastest way to be sure.

How to get rid of St. John's wort

Hand and mechanical. For a handful of plants, pulling or digging works well if you get the crown and as much root as possible. Do it in late spring or early summer before the plants flower, when the soil is moist. Expect to return, because root pieces can resprout. Wear gloves, and bag any plants with seed capsules. Mowing on its own won’t kill it, but cutting before bloom cuts down seed production.

Cultural. This weed loses to thick, healthy grass. In pastures, avoid overgrazing, rotate paddocks, lime and fertilize according to a soil test, and reseed thin areas in late summer. On banks and rough areas, get good grass cover established and mow a few times each summer.

Chemical. For larger infestations, extension sources list 2,4-D for young plants and plants before bloom, along with pasture products containing metsulfuron or aminopyralid, applied to actively growing plants before flowering. Glyphosate can be used as a spot treatment where you plan to reseed, since it kills grass too. Some products labeled for pastures are not allowed on home lawns, and some carry grazing or haying waiting periods, so check those carefully.

Read and follow the product label, because the label is the law, and make sure it’s labeled for your site, whether that’s a pasture, lawn or roadside. Applying herbicide for hire in Pennsylvania requires a PA Department of Agriculture pesticide applicator certification, so a field-sized job should go to a licensed vegetation-management company.

Treated plants can become more appealing to animals while they’re dying, so keep livestock off sprayed areas for as long as the label requires, and longer if the plants are still standing.

Keeping it from coming back

The seeds can stay alive in the soil for years, so plan on several seasons of follow-up. Walk the area each June and pull or treat new plants before they flower.

Keep grass thick and stop letting it go to seed. Clean mowers and equipment after working in an infested field, and buy hay from sources you trust. On a bare or eroding bank, the long-term fix is good grass or groundcover across the whole slope so there’s no open, sunny ground to colonize.

Questions we get about common st. john's wort

Similar weeds

Spotted KnapweedCentaurea stoebeYellow SweetcloverMelilotus officinalisBird's-foot TrefoilLotus corniculatus