Kalm St. Johnswort (Hypericum kalmianum)
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
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 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.
Some cultivars grown in Minnesota:
|Cultivar||Bright yellow flowers||Small rounded plant habit||Improved hardiness||Long bloom period|