Round-leaved St Johns-wort: complete guide

Hypericum nummularium is a robust alpine plant native to Pyrenees and southwestern Alps. Member of Hypericum family prized for cheerful bright yellow flowers and compact form making it ideal for rock gardens border edging and alpine troughs. Grows as low dense shrub reaching 30-50 cm height with rounded leathery foliage beautifully complementing brilliant yellow blooms appearing July through September.

This plant appeals strongly to alpine gardeners and mountain flora enthusiasts because of exceptional hardiness USDA zone 4 and ability to thrive in less fertile soils where many ornamentals struggle. Botanical name nummularium references round coin-like leaves characteristic of this species. In native habitat it grows on rocky screes and alpine meadows where drainage is excellent and sunlight abundant.

For optimal performance position Hypericum nummularium in sunny location receiving 6-8 hours of direct sun daily. This promotes abundant flowering and maintains compact vigorous growth. Plant will still flower in partial shade though blooms will be less profuse and growth may become lanky. Avoid full shade which results in poor flowering and weak development.

Soil requirements are critical for success with this alpine species. Hypericum nummularium demands very well-drained gritty to sandy loam. It can actually thrive in quite poor stony soils making it particularly valuable for alpine troughs and rock gardens. Add coarse grit or lava gravel to heavy clay soils to enhance drainage. Neutral to slightly acidic pH 6.0-7.0 is ideal. Waterlogging is fatal to this plant so ensure water cannot accumulate around roots especially during winter months.

Watering is usually required only at planting time and during dry spells in growing season. Once established Hypericum nummularium is drought-tolerant and can survive extended periods without supplemental water. During growing period May through September water when top 2-3 cm of soil feels dry. Reduce watering in autumn and withhold it entirely during winter unless conditions are exceptionally dry.

Maintenance is minimal. Do not fertilize unless soil is severely impoverished. Excessive nitrogen promotes leafy growth at expense of flowers and can make plant soft and disease-prone. Lightly prune back after flowering to maintain compact habit though this is not essential. Remove dead wood in early spring. Plant spreads gradually eventually covering 60-80 cm. It responds well to pruning and can be kept more compact with selective cutting.

Hardiness is major advantage. Hypericum nummularium is reliably hardy to USDA zone 4 (-30 to -40 degrees Celsius) and can tolerate snow and severe cold without protection. In regions with very wet winters and heavy snowfall applying layer of grit over plant can prevent leaching of growing point.

This plant combines beautifully with other alpine and mountain species. Try pairing it with fellow yellow bloomers like Alyssum saxatile and various Sedum species for early-season displays. Later in season Hypericum flowers simultaneously with purple and pink Saxifraga and white Cerastium species. Yellow and white combination is particularly striking. Plant it also alongside fine-textured silver foliage plants such as Artemisia schmidtiana Nana and Achillea tomentosa for wonderful textural contrast.

Propagation is via seed or softwood cuttings in early summer. Sow seed directly onto gritty compost and cold stratify 4 weeks in refrigerator. Take cuttings from new growth in May-June insert into gritty compost and cover with plastic until roots develop. Many alpine nurseries offer young plants as well.

Plant Hypericum nummularium in groups of 3-5 specimens in rock gardens along border edges in alpine troughs or scree displays. It suits dry-tolerant gravel gardens and Mediterranean-style borders too. Throughout northern Europe its reliable performer requiring minimal special attention. Garden centers throughout Europe stock this reliable performer. Browse offerings at gardenworld.app where you can upload a front-yard photo and receive personalized plant recommendations from expert designers.