Kalm’s St. John’s wort is a hardy shrub that thrives in full sun and well-drained soils, reflecting its resilience to a range of outdoor conditions. This plant features bright yellow flowers with a bushy array of stamens, creating an eye-catching display from mid to late summer. Its narrow, blue-green foliage provides a striking contrast against the flowers, making it a popular choice for ornamental use in gardens.
I. Appearance and Characteristics
Hypericum kalmianum, commonly called Kalm’s St. Johns wort or Kalm’s St. Johnswort, is a flowering plant in the St. John’s wort family Hypericaceae. It is native to the Great Lakes region in the northern United States and southern Canada. Hypericum kalmianum was named after its discoverer, Swedish botanist Pehr Kalm (1715-1779).
Hypericum kalmianum is a slender shrub that grows to a height of 20–60 cm (7.9–23.6 in). Its bark is whitish and papery. It has ascending four-edged branches that bear two-edged branchlets. The crowded bluish-green leaves are linear to obscure and 3–4.5 cm (1.2–1.8 in) long. One to ten yellow flowers are borne on terminal and open combs, with each flower measuring 2–3.5 cm (0.8–1.4 in) wide. The leaf-like sepals are oblong and 5–15 mm (0.2–0.6 in) long. The ovoid capsules typically have five carpels and styles, though they can occasionally bear three, four, or six. The capsules are 7–10 mm (0.3–0.4 in) long and 4–7 mm (0.2–0.3 in) thick.
The plant flowers between June and August.
Formerly included within Hypericum kalmianum is another species first formally described in 2016, Hypericum swinkianum. Both species are part of the Hypericum subsect. Centrosperma (H. sect. Myriandra). H. kalmianum can be differentiated from H. swinkianum by its narrower leaves, flowerheads averaging fewer than 7 flowers, and an affinity toward calcareous rather than acidic habitats. Additional species of Midwestern Hypericum closely related to these two may be yet to be described.
II. How to Grow and Care
Sunlight
Kalm’s St. John’s wort needs sufficient sunlight but doesn’t tolerate direct light. Grow the plant in the shade of other plants or on a balcony away from strong, hard light. Expose it to more sunlight in the spring and fall to help it grow, but in the summer, place the potted plants in half-shaded places and mist them with water to cool the temperature and increase humidity. Otherwise, symptoms like drying leaf tips may appear.
Temperature
Plants of the Hypericum genus are widely distributed. They like warm, moist environments; the best temperature range for the plants is 20 to 26 ℃, but they can tolerate temperatures as low as -10 ℃. They require little water and endures drought, but don’t tolerate waterlogged soil. Avoid watering so frequently that the soil gets waterlogged.
Watering
Originating from North America’s Great Lakes region, kalm’s St. John’s wort is accustomed to well-drained soils and periodic moisture. This species thrives with a balance of hydration, showing moderate drought tolerance once established. For optimal growth, watering should occur once every 1-2 weeks, allowing the soil to dry slightly between sessions. As an outdoor plant often found in garden landscapes, kalm’s St. John’s wort benefits significantly from mulching which helps retain soil moisture and regulates root temperature.
Soil
Kalm’s St. John’s wort can adapt to a wide range of soil types. It grows well in most common soils (except for heavy clay) and prefers sandy loam with good drainage. A slightly acidic loam with a 5.5-7.0 pH value is ideal. Kalm’s St. John’s wort is tolerant of barren soil and drought, but if the soil can’t drain water well, waterlogging can cause the roots to rot.
Fertilizing
Kalm’s St. John’s wort requires very little fertilizing. Usually, no additional fertilization is needed after the initial planting, unless obvious symptoms of slow growth appear. If that happens, apply some low-concentration balanced compound fertilizer (10-10-10 NPK ratio).
Planting Instructions
Plant kalm’s St. John’s wort in the early spring or early fall. Choose a cool, shaded place, such as within tree shade. Remember to clear the soil first and remove weeds.
Dig a planting pit slightly shorter than the diameter of the root ball and 3 times wider. If you’re planting more than one sapling, avoid planting them too close, or low light and poor ventilation may result in powdery mildew. Pile some soil around the root collar (the juncture of trunk and roots) to form a gentle slope, and this helps disperse water. After planting, press the soil firmly and water thoroughly. Note the distance between each.
Some plants in the Hypericum genus have subterraneous rhizomes that tend to expand in all directions. To prevent excessive expansion, it’s a good idea to fence the planting spots or set brick perimeters around them to control their growth.
Pruning
Kalm’s St. John’s wort requires very little fertilizing. Usually, no additional fertilization is needed after the initial planting, unless obvious symptoms of slow growth appear. If that happens, apply some low-concentration balanced compound fertilizer (10-10-10 NPK ratio).
Propagation
Kalm’s St. John’s wort can be propagated by seeding, cuttage, or plant division. To sow seeds yourself, sow them in a sand bed. Dust a thin layer of soil on them, water once, and sprout the seeds in temperatures of 10 to 12 ℃. Keep the potting soil moist. Transplant seedlings when they grow to 5 to 10 cm tall.
For St. John’s worts with dense branches, propagation is usually performed at the end of summer by soft-branch cuttage. Choose some robust, soft branches that sprouted in the current year. Cut 10 to 15 cm-long from the tip, making sure to keep at least 2 full leaves on each branch. Insert the cut branches into the soil and keep half the length aboveground. Keep the soil and the environment moist. When the seedlings reach 20 cm tall, transplant to pots. Seedlings need to grow to 70 cm tall to move outdoors or wait until the end of the next summer.
III. Uses and Benefits
Hypericum kalmianum is perfect as a specimen or massed in borders, rock gardens, slopes, and naturalized areas. Makes an excellent low hedge.