Delphinium grandiflorum Summer Star | Siberian larkspur

Delphinium grandiflorum ‘Summer Star’ | Siberian Larkspur

A compact and charming perennial delphinium that flowers already in the first year without vernalization. ‘Summer Star’ produces an abundance of vivid blue blooms held above neat mounds of finely divided foliage. Ideal for borders, mixed containers, and cutting gardens, this Siberian larkspur combines strong garden performance with elegant, long-lasting color.

The plants stay compact and bushy, making them perfect for smaller gardens or pot displays, while still giving the lush, classic delphinium look. Continuous blooming throughout summer with regular deadheading.

Plant Information

Botanical name: Delphinium grandiflorum
Variety: Summer Star
Common name: Siberian Larkspur
Plant type: Perennial (first-year flowering)
Height: 35 cm / 14″
Spread: 25–30 cm
Habit: Compact, well-branched
Exposure: Full sun – partial shade
Hardiness zone: 3b–7b
Heat zone: 6–1
Seed form: Raw seed
Best uses: Bedding, borders, cut flowers, mixed containers, landscape plantings

Growing Information

Sowing period: December–May
Sowing method: 2–3 seeds per plug or cell
Germination: 14–18 days at +20–22 °C; keep substrate evenly moist and cover seed lightly.
Growing on:
Transplant 6–8 weeks after sowing into 9 cm pots or 1 L containers. For larger pots, plant 3 seedlings per container. Avoid deep planting and ensure good air circulation.
Fertilization: Moderate — begin feeding at 200 ppm N (2-1-2 ratio). Avoid high salt buildup.
Finishing temperature: +7 °C – +24 °C (avoid > +29 °C; may cause flower abortion).
Light: Requires good light; avoid short-day conditions below +13 °C to prevent rosetting.
Soil / Media: Well-drained, fertile soil with pH 5.8 – 6.5.

Garden Care

Prefers fertile, well-drained soil in full sun. Protect from strong winds.
Feed every two weeks during the growing season.
Deadhead spent flower spikes to encourage side shoots and repeat bloom.
Cut back to ground level in late autumn.
Can be propagated from basal cuttings in early spring.

Crop time:

Spring sowing: 16–20 weeks to flowering