More than 400 plants share the name St. John's wort (Hypericum spp.). The common European St. John's wort (Hypericum perforatum), which grows in U.S. Department of Agriculture plant hardiness zones 3 through 8, can quickly overtake garden beds and spread into the wild. The more tame North American St. John's wort (Hypericum frondosum), which grows in USDA zones 5 through 8, is better behaved in the garden. These varieties grow as short-lived perennials for four to five years, but they require only minimal care during their brief lives.
Things You'll Need
- Isopropyl alcohol
Plant St. John's wort in a bed that drains well to minimize the chances of root rot. Choose a location that receives full, all-day sun or partial afternoon shade.
Space plants 18 to 24 inches apart and avoid planting new plants within the spread of the St. John's wort, especially in humid climates. Allow space between the plants for air circulation, which prevents fungal leaf diseases on St. Johns wort.
Water St. John's wort when the soil feels almost completely dry. Supply enough to water to thoroughly dampen the soil 6 inches deep, and then allow it to dry again before the next watering. St. John's wort is naturally drought-tolerant and doesn't tolerate wet or soggy soil.
Cut out dead or damaged stems from the plant at any time, using shears wiped down and disinfected with isopropyl alcohol. Cut damaged stems back to the nearest healthy wood. Trim off the the flowers before they wilt and set seeds if you're worried about the plant becoming invasive. Dispose of the flowers and seeds promptly so the plants don't invade the garden.
Pick caterpillars from St. John's wort plants by hand if they are stripping too many of the plants' leaves. Butterfly caterpillars usually only cause minor damage and don't require control.
Pull up and destroy any plants that develop mushy, rotten stems and roots, which are usually caused by soil nematodes. Plant new plants in a different garden bed to avoid exposure to the nematodes in the old site.
Spread a 2-inch layer of mulch over the bed in fall after the plants die back if your area experiences winter freezing. Cut back the plants to the ground in late winter if cold kills off the top growth. St. John's Wort usually grows back in spring and flowers on the new wood.