Great St. Johnswort
Hypericum ascyron
Last reviewed: September 2026
Great St. Johnswort is an upright forb that blooms with bright yellow flowers from June through August in wet meadows, stream banks and marsh edges across the northeastern and midwestern United States.
Toxicity unknown
Great St. Johnswort (Hypericum ascyron) is not listed by the ASPCA Animal Poison Control Center (opens in new tab) or the other sources we check, so we have no verdict for it either way. Absence from those lists is not a clearance. If your pet shows symptoms after eating any plant, call the ASPCA at (888) 426-4435.
The USDA PLANTS Database (opens in new tab) does not flag Great St. Johnswort as toxic to livestock.
Light
full sun
Water
medium
Size
24"–48" H × 12"–24" W
Bloom
Jun, Jul, Aug
Native to
CT, IA, IL, IN and 13 more states
Pollinators
bees, butterflies, beetles
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Great St. Johnswort is a tall native perennial that produces bright yellow flowers with prominent stamens from June through August. This robust plant forms upright clumps and thrives in full sun with moderate water needs.
Growing Great St. Johnswort
Grow Great St. Johnswort in full sun and medium water. Mature plants reach 24 to 48 inches tall and spread 12 to 24 inches wide. Expect low maintenance to keep it looking intentional in a front yard.
How to care for Great St. Johnswort, by region
How to care for Great St. Johnswort where it is actually grown: year-round basics plus what to do each season, region by region, with the sources for each.
Care guidance last updated: October 2026
Erie Drift Plain (includes Buffalo, NY)
Year-round
Great St. John's wort likes full sun to light shade and average to moist soil.
- Full sun to light shade suits it.
- Average to moist soil works. A nursery source lists mesic to wet soil.
- It spreads by underground rhizomes and can form small colonies, so give it room.
It can reach 2.5 to 6 feet tall, so keep it toward the back of a front bed.
Fall
Seed pods turn from bright green to dark brown as they ripen.
- Leave the pods up if you want seed or fall interest.
- The clump slowly widens by rhizomes.
Winter
This is a plant that comes back each year, so winter is a quiet time.
- Ripe dark brown seed pods can stay on the stems or be cut back.
- The clump spreads by rhizomes, so the same spot refills next season.
Spring
In spring, keep the soil average to moist and give new growth room.
- Average to moist soil is what it prefers.
- Rhizomes can spread underground into small colonies.
- Seed germinates within 4 weeks if sown at 70F, per a nursery source.
Summer
Yellow flowers appear June through August, and each plant blooms for only a couple of weeks.
- Keep soil average to moist through dry spells.
- It is listed as deer resistant.
- Even a short bloom is enough for bumblebees and other insects to visit.
Sources: Missouri Wildflowers, great St. John's wort, North American Native Plant Society, Hypericum ascyron
In an HOA neighborhood
Great St. Johnswort is an HOA-friendly choice. Maintenance level: low. Tidiness: 3 out of 5.
Works well in: mid zone, foundation.
- Tall height may appear unstructured in formal settings
- Winter dormancy leaves bare stems
Wildlife value
The cheerful yellow blooms attract bees, butterflies, and beetles throughout the summer months. This native plant provides important nectar resources during peak growing season.
Common questions
- When does Great St. Johnswort bloom?
- Great St. Johnswort typically flowers from June through August, producing bright yellow, star shaped blooms on tall upright stems.
- What kind of light does Great St. Johnswort need?
- It grows best in full sun but can tolerate part sun to part shade, especially where soil moisture stays consistent through the growing season.
- How much water does Great St. Johnswort require?
- This plant prefers medium to consistently moist soil and does well near wet meadows, ditches, streambanks and other seasonally saturated sites.
- What pollinators visit Great St. Johnswort flowers?
- Bees, butterflies and beetles regularly visit the flowers for pollen, since Hypericum species do not produce nectar.
- What growth habit does Great St. Johnswort have?
- It is a herbaceous forb that dies back to the ground each year, sending up new upright stems in spring.
- Is Great St. Johnswort toxic to livestock?
- The USDA PLANTS Database does not flag Great St. Johnswort as toxic to livestock.
- Is Great St. Johnswort HOA-friendly?
- Great St. Johnswort is generally an HOA-friendly choice in the right placement, with low maintenance.
These answers were drafted with an AI model from this plant's data. Verify care details with a local expert.
Native range data from vouchered herbarium specimen records and regional native plant society lists. Pollinator and host plant associations compiled from GBIF, iNaturalist, and published ecological literature.
If something looks wrong
Common plant problems, and what to check first
These come up on almost any plant. Each guide lists the likely causes in the order they are worth ruling out, with one thing to check before you act on any of them, and links to the source it came from.
- It is dying right after planting
- Leaves turning yellow
- Wilting even though I watered it
- Brown crispy leaf tips
- Bugs on the plant
Ready to grow Great St. Johnswort? Add it to your garden in Pollinator Patch to track it and get season-by-season care guidance.