Key Facts
- Perennial herb with bright yellow five-petaled flowers and prominent stamens
- Blooms in summer in temperate climates
- Spreads by seed and by creeping roots and rhizomes to form dense patches
- Tolerates poor dry soils and prefers well drained sites
- Considered invasive or a noxious weed in many regions
- Causes photosensitization in grazing livestock if eaten
- Used medicinally in humans and interacts with many prescription drugs
Klamath Weed / St. John’s Wort (Hypericum perforatum) is a perennial herb with bright yellow five petaled flowers that appear in summer. It tolerates poor dry soils and spreads by seed and by creeping roots to form dense patches, which makes it useful for low care sunny sites but problematic where invasives are a concern.
Identification
Look for clusters of small bright yellow flowers with prominent stamens and opposite leaves that can appear translucent when held to light. Mature plants commonly reach about 12 to 40 inches tall and form colonies that spread 12 to 40 inches or more.
Where to grow and best uses
Full sun and well drained soils suit this species best. It tolerates poor, dry, sandy or rocky ground so it can work as a rough groundcover on slopes or disturbed sites where a low, flowering perennial is wanted. Avoid planting where it could escape into natural areas because it is considered invasive or noxious in many regions.
Planting and spacing
Flowers appear in summer. Set plants out in spring so they can establish before flowering. Space transplants roughly 30 to 100 cm apart 12 to 40 inches depending on how quickly you want a continuous patch.
Watering and soil
Choose a sunny, well drained spot. Plants tolerate dry conditions and require only occasional irrigation once established. Waterlogged soils reduce vigor and make the plant less healthy.
After flowering care and spread control
Prevent seeding if you want to limit spread. Remove flower heads before they set seed and pull or dig plants to remove roots and rhizome fragments. Mechanical removal can create new plants if root pieces are left in the soil, so follow up is essential. Check local extension guidance for recommended herbicide or biological control options where it is listed as a noxious weed.
Toxicity and medicinal notes
Ingestion by grazing livestock can cause photosensitization and related injuries. Extracts are used medicinally in humans and are known to interact with many prescription medicines. Handling can cause skin sensitivity in some people. Confirm pet risks and specific medical questions with a veterinarian or poison control source before allowing animals access or using extracts.
Frequently asked questions
- How fast does Klamath Weed/St. John’s Wort grow?
- It spreads by abundant seed and by creeping roots and rhizomes. Plants can form dense patches over a few seasons and persist where winters are mild.
- Is it invasive?
- Yes. It is considered invasive or a noxious weed in many regions. Confirm local status before planting.
- Can I use it as a groundcover?
- Yes in sunny well drained, poor soil sites where containment is acceptable. Avoid using it near natural areas where escape would cause ecological harm.
- Is Klamath Weed toxic to pets?
- Risk to livestock from photosensitization is documented. Risk to dogs and cats is less clear and should be checked with a veterinarian or local poison control service.
- How do I control established patches?
- Remove plants before seed set, dig out roots and rhizomes, and monitor the area for regrowth. Mechanical control can spread the plant if root pieces remain, so persistent removal and local extension advice are recommended.
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