Kalm's Saint John's Wort Hypericum kalmianum

Kalm's St. John's Wort is a compact perennial, herbaceous, deciduous, or evergreen shrub or subshrub that, at maturity, can reach a height of 4 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.

This plant is native to Eastern Canada and the Northeastern and North Central United States. It is found along lakes, rivers, and cliffs in the wild. It may be found along Lake Michigan and Lake Huron.  The shrub is native to Illinois and is listed as endangered. In protected areas of Illinois, this shrub appears to be increasing. 

The genus name, Hypericum, comes from the Greek words hyper meaning above, and eikon meaning picture. This references the practice of hanging flowers from this genus above images, pictures, or windows. The species name, kalmianum, 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, including drought, and dry soil. Pruning, if needed to maintain the plant's shape, should be done after flowering has ended or in late fall; however, you should never cut the shrub back to the ground.

Various bees collect pollen from the flowers, and the shrub is browsed by White-tail Deer. 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, irritating the skin. Consumption of these plant species can also irritate the gastrointestinal tract of these animals. 

Kalm's St. John's Wort has a somewhat formal appearance and may be used in the landscape as a low hedge, border planting, or foundation planting. Planted for added interest in a Rock Garden or plant in mass.

Seasons of Interest:

Bloom:  Summer     Foliage:  Spring, Summer, and Fall     Fruits:  Fall, persist until Spring

Quick ID Hints:

• dense mounding, rounded, or ball-like broadleaf evergreen to a semi-evergreen shrub or subshrub up to 4 feet tall 
• leaves are opposite, narrow, linear, bluish-green, and dotted with small glands 
• 4-angled stems
• five-petaled golden yellow flowers that are 1.5 inches in diameter and borne on 3 to 7 flowered cymes during the summer and bloom for 1 month
• fruits or beaked, oval, brown seed capsules
• yellowish-brown, orangish-brown to reddish-brown bark

Insects, Diseases, and Other Plant Problems: This plant has 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.