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Extension > Garden > Yard and Garden > Trees and Shrubs >Kalm St. Johnswort

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Kalm St. Johnswort (Hypericum kalmianum)

Kathy Zuzek and Beth Berlin

Plant description

Mature height: 2 to 3'
Mature width: 2 to 3'
Growth rate: Slow
Plant form: Rounded
Deciduous or evergreen: Deciduous
Native range: Quebec to New York west to Ontario and Illinois
Native to Minnesota: No
Invasive in Minnesota: No

Culture

Light: Full sun
Soil texture: Sand, loam, clay
Soil pH: Prefers 5.0-7.5, tolerates higher
Soil moisture: Droughty to wet soils and excessively- to poorly-drained soils
Hardiness zone: 4 to 7
Pests and stresses: None serious.
Other: Tolerates drought, salt, soil compaction

Kalm St. Johnswort in the landscape

Kalm St. Johnswort is a small dense rounded shrub that can be used in border, foundation, mass, or specimen plantings in the landscape. Restricted only by its need for full sun and a zone 4 hardiness level, H. kalmianum is adaptable to many soil textures, soil moisture and drainage levels, and soil pH levels. This is a low maintenance species; pruning is rarely needed and few pests bother this plant.

The bright yellow flowers of Kalm St. Johnswort provide this species with its one season of interest: summer. In spring, narrow willow-like, 1-2" long leaves emerge on finely branched plants. Blue-green summer foliage provides a backdrop for the 5-petaled, 1- 1½", bright yellow flowers that are produced in July and August. Flowers are attractive to insects and butterflies and are excellent for cutting. Small, non-ornamental, 5-valved capsules can be seen in fall against muted yellow autumn foliage.

North America map showing native range of St. Johnswort

Native range in North America, USDA, NRCS Plants Database

green bush with yellow flowers

Kathy Zuzek, UMN Extension

'Ames' in bloom

close up of yellow flowers with long hairlike protrusions at center

Kathy Zuzek, UMN Extension

Flowers of 'Ames'

small brown bushes

Kathy Zuzek, UMN Extension

Fall color of 'Ames'

Some cultivars grown in Minnesota:

Cultivar traits:
Cultivar Bright yellow flowers Small rounded plant habit Improved hardiness Long bloom period
'Ames' X X X  
'Gemo' X X   X
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