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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 Wilhelm and Rericha’s Flora of the Chicago Region.

 

  • 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
  • Other 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.

Pollinators and 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, and tolerances:

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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