Hypericum, commonly known as St. John's wort, is a popular garden plant and cut flower most recognizable by its flowers with prominent stamen and colorful berries. There are over 400 species that can be annual, perennial or semi-evergreen depending on where they are grown. Hypericum is a beautiful and rewarding plant and most gardeners can be successful with a small amount of knowledge in how to grow it properly.
Hypericum is generally hardy in zones 3 to 8, depending on the species and can be found throughout most of the United States. While hypericum prefers full sun, it can be grown in a partially shady location, but may not produce as many flowers as it would if given more light. Soil for planting should be well drained and not boggy, except if you choose to plant hypericum elodes, which can grow in moist and marginal areas.
Caring for Hypericum in the Garden
Hypericum are relatively easy plants to care for. Space your plants adequately at planting, usually two to three feet apart when planting to allow for proper air circulation around the foliage. Do not over water the plants. If possible, water in the morning before the sun is hot. Test the soil by sticking your finger into the ground up to the second knuckle. If the soil is moist, check again in the afternoon or evening. If the soil is dry, water thoroughly around the base of the plant and avoid getting water on the leaves. This may cause burn and encourage disease formation and spread.
Pruning and Cutting
Always use sharp and clean pruners or shears when pruning any plant. Prune hypericum selectively in late winter or early spring when the plants are dormant. Remove any branches that might be crossing and rubbing against one another creating lesions on the wood. Maintain the plant's shape and size as you desire during this time. Hypericum bloom on new growth so prune the plant to its base about every three years to encourage maximum flowering. You may cut the berries for decorative purposes. Choose the most colorful and plump berries on the plant and cut to desired length just before a set of leaves.
Pests and Diseases
Hypericum is prone to a few insect and disease issues that can be avoided or treated. The most common problem with hypericum is powdery mildew. This mildew disease looks like a white film is coating the leaves of the plant. It most often occurs when hypericum is not getting enough light, is getting too much moisture, or both. To avoid this problem, plant your hypericum in the proper location (sunny, well-drained soil, and plenty of air circulation) and refrain from watering your plant overhead.
Aphids are a common pest issue with hypericum. They are a small soft-bodied insect that pierce plant leaves with their proboscis and suck the sap out of plant cells. This will inhibit photosynthesis and interrupt other metabolic processes of the plant. Both powdery mildew and aphids can be treated with a horticultural oil or you can make your own using cooking oils and dish detergent.
While there are several cultivated varieties of hypericum that are safe to plant in your garden, one species, hypericum perforatum, is considered a highly invasive noxious weed in many parts of the United States. Some common names for this weed are amber, rosin weed, klamathweed and goatweed. It spreads by underground rhizomes and by seed in abandoned pastures and along roadsides. This species is native to northern Africa, Europe and parts of Asia and was most likely brought to the U.S. for ornamental and medicinal purposes. Hypericum perforatum may have a showy yellow flower similar to that of a cultivated variety.