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Kalm's St. John's wort

Hypericum kalmianum

Common names:
Kalm's St. John wort, Kalm's hypericum, Kalms St. John wort, Kalm's Saint Johnwort

Content Detail

A sun-loving, low-growing, 2 to 3 foot high, native shrub found in moist, low areas in the upper Great Lakes. The long-blooming, bright yellow summer flowers have showy, pompom-like stamens. Best used en masse for full effect. This species is native to the Chicago Region according to Swink and Wilhelm’s Plants of the Chicago Region, with updates made according to current research. 

  • Family (English) Hypericum
  • Family (botanic) Hypericaceae
  • Tree or plant type Shrub
  • Foliage Deciduous (seasonally loses leaves)
  • Native locale Chicago area, North America
  • Size range Low-growing shrub (under 3 feet), Small shrub (3-5 feet)
  • Light exposure Full sun (6 hrs direct light daily)
  • Hardiness zones Zone 4, Zone 5 (Northern Illinois), Zone 6 (City of Chicago), Zone 7
  • Soil preference Moist, well-drained soil
  • Tolerances clay soil, Occasional flooding
  • Season of interest early summer, midsummer, late summer, early fall
  • Flower color and fragrance Yellow
  • Shape or form Round, Thicket-forming
  • Growth rate Moderate

Native geographic location and habitat:

C-Value: 10. Found in moist soils near ponds and dune areas around the Upper Great Lakes region.

Attracts birds, pollinators, or wildlife:

Birds and butterflies are attracted to flowers and seeds.

Bark color and texture:

Twigs are slender and reddish-brown. Older stems have exfoliating bark exposing a bronzy colored inner bark.

Leaf or needle arrangement, size, shape, and texture:

Alternate, 1/2 inch wide linear to oblong leaves are up to 5 inches long. They are medium bluish-green on the upper leaf with small glandular dots beneath.

Flower arrangement, shape, and size:

Bright yellow, five-petaled flowers with pompom-like stamens are long blooming.

Fruit, cone, nut, and seed descriptions:

Brown capsules, about 1/4-inch long, persist into winter.

Plant care:

Kalm’s St. John’s wort grows best in full sun in moist to well drained soils. It flowers on new wood so it should be pruned in spring.

List of pests, diseases, tolerance, and resistance:

Susceptible to leaf spots. Tolerant of salt, drought, and soil compaction.

Ames St John’s Wort (Hypericum kalmianum ‘Ames’):

A small, 2 to 2 1/2 feet high and wide, rounded shrub from Iowa State introduction program. It flowers in summer with 1 inch wide golden yellow flowers and is extremely hardy.

Gemo St. John’s Wort (Hypericum kalmianum ‘Gemo’):

A  2 to 3 feet high cultivar with narrow, willow-like leaves and small yellow flowers with bushy stamens.

Sunny Boulevard™ St John’s Wort (Hypericum kalmianum ‘Deppe’):

A 2 to 3 feet high and wide shrub with small, bright yellow flowers.

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