Hypericum kalmianum

Kalm's St. John's Wort is a perennial, herbaceous, deciduous or evergreen shrub or small tree that, at maturity, can reach a height of 3 feet. It is drought tolerant and more loose and open in form than other of the more than 400 species of hypericum. It is also the most cold-hardy of the hypericum. Kalm's St. John's Wort is a flowering shrub with numerous 1 inch yellow flowers over a month long blooming period in mid to late summer. It also has a woody root system that is shallow and spreading. Prune if needed after bloom in late summer. Genus name comes from the Greek words hyper meaning above and eikon meaning picture in reference to the practice of hanging flowers from this genus above images, pictures or windows. The species name honors Peter Kalm, a student of Linnaeus, who reportedly discovered this plant in the wild in North America in the mid-1700s.

The plant prefers full sun, wet to moist conditions, and well-drained soil containing calcareous sand or limestone. Shallow water is tolerated if it is temporary. It also tolerates a range of soil types, drought, and dry soil. The leaves of this St. John's Wort species are toxic to domesticated farm animals, particularly those with white or thin fur because these plants contain a toxin that increases sensitivity to sunlight, causing irritation of the skin. Consumption of these plant species can also irritate the gastrointestinal tract of these animals.

Insects, Diseases, and Other Plant Problems: No known diseases or insect pests.

VIDEO created by Grant L. Thompson for “Trees, Shrubs, and Woody Vines for Landscaping” a plant identification course offered by the Department of Horticulture at Iowa State University.