Kalm's St. John's-wort
Hypericum kalmianum · Hypericaceae
📷 Daderot · CC0
⚠️ Feuilles contiennent hypéricine et naphthodianthrones ; ingestion provoque troubles digestifs, photosensibilité chez chiens, chats, enfants.
The gardener's calendar
In autumn, ease off feeding and take the chance to clean up around kalm's st. john's-wort and prepare for winter.
ID card
- Common name
- Kalm's St. John's Wort
- Family
- Hypericaceae
- Type
- Deciduous shrub
- Mature height
- Approximately 1.2 m
- Flowering
- June to August, yellow flowers
- Foliage
- Deciduous, light green
- Hardiness
- Very hardy (zone 4-5)
- Origin
- Eastern North America
✓ Taxonomy verifiedSource: AI estimate · hardiness: reliability high.
Expert tips
The key tip
Hypericum kalmianum is a shrubby groundcover: pinch out young stems in May to force compact branching. Flowers in late summer = June-August, for a long time.
The common mistake
Letting it grow without intervention and not pinching: it becomes a sprawling shrub, poorly dense, and flowers less well.
The pro's trick
Pinch back to 15-20 cm in May to get a denser, more floriferous clump.
Seasonal watch
August-September: intense heat can scorch young leaves if the soil is too dry.
Care
Sunlight
Plant your Kalm's St. John's Wort in full sun for generous and abundant flowering. It also tolerates partial shade, but will flower less densely. Avoid heavily shaded areas that slow down fruiting.
Watering
Water regularly during the growing season (spring-summer), keeping the soil slightly moist but not waterlogged. Once established, the shrub tolerates dry periods well. Reduce watering in autumn and winter.
Soil
Kalm's St. John's Wort adapts to most soils as long as they are well-drained. It prefers slightly acidic to neutral substrates. Avoid heavily calcareous soils or areas prone to water accumulation.
Pruning and shaping
Prune lightly in late winter or early spring to maintain a compact form and encourage branching. Remove dead or poorly oriented branches. Moderate pruning is sufficient; this shrub has a naturally attractive growth habit.
Fertilizing
A balanced fertilizer application in spring is generally sufficient. In poor soil, add more in summer if growth seems to slow. Avoid excess nitrogen, which favors foliage at the expense of flowering.
Health and pests
Kalm's St. John's Wort is very robust. It may occasionally suffer from rust or scale insects in very humid or confined conditions. Improve air circulation and intervene quickly if needed. No major problems really weaken it.
Did you know?
Kalm's St. John's Wort is named after Swedish botanist Pehr Kalm (1716-1779), who explored the North American continent and collected numerous plants for Europe. The golden-yellow flowers produce spectacular stamens arranged in three distinct clusters, giving the shrub its striking graphic and luminous appeal during peak summer flowering.
Where to buy it
Best planted in autumn so the roots settle in before summer, or in early spring once the frost has passed. Pick a plant with healthy foliage and roots that are not exposed. Best time: March, October, November.
Add this plant to my garden →Plants from the same family
- Rose of sharonHypericum calycinum
- Tall tutsanHypericum inodorum
- Egyptian Saint John's WortHypericum aegypticum
- Hooker's St. JohnswortHypericum hookerianum
- Moser's St. John's wortHypericum moserianum
- Hypericum dummeriHypericum dummeri
Tips generated and checked by Viridia — adapt them to your local climate.