How to Grow Delphinium
'Summer Skies' from Seed
The sky-blue Pacific Giant — colossal 150–180cm spikes carrying up to 80cm of clear sky-blue florets with white bee centres; the most dramatic vertical element in the traditional cottage garden border; a long-tongued bumblebee specialist and a perennial that, cut correctly after its first July flowering, reliably produces a second blue flush in September
Delphinium elatum 'Summer Skies' is the sky-blue member of the Pacific Giant series — a breeding programme developed in California specifically to produce delphiniums with stronger stems, larger individual florets, and more impressive overall spike dimensions than anything previously available from seed. The Pacific Giants represent the pinnacle of delphinium breeding for garden display, and 'Summer Skies' is among the most beautiful of the series: clear sky-blue florets — neither pale nor violet, but a precise mid-blue — each with a prominent white centre known as the "bee," arranged in a dense column that can reach 150–180cm in height and carry flower spikes up to 80cm long on a single stem.
Growing delphiniums to this scale from seed requires attention to a few specific requirements — the fridge treatment before sowing, cool germination temperatures, early staking, and slug protection throughout the season — but none of these is technically difficult. In return, the plant provides the most dramatic vertical element available in the traditional cottage garden border: a column of pure sky-blue that rises above everything around it, provides nectar exclusively accessible to long-tongued bumblebees, and can produce a second flush of flowers in September if managed correctly after the first blooming.
Quick Facts at a Glance
Plant Type
Hardy Perennial H2 — dies back in winter, returns stronger each year
Colour
Sky-blue florets with white 'bee' centre — large bowl-shaped flowers
Height
150–180cm; flower spike up to 80cm — back-of-border
Season
June–July; second flush September with correct management
Warning
All parts highly toxic if ingested — wear gloves; keep from children
Difficulty
3 out of 5 — rewards the right care
Understanding the Variety
'Summer Skies' belongs to the Pacific Giant Hybrid series — a group bred at the University of California to have colossal, strongly upright spikes that hold their form in the border. Each individual floret is large (4–5cm) with a distinctive white or pale centre called the "bee" — so named for its resemblance to a small insect nestled in the flower. The florets are arranged in a tight column, creating the characteristic tall, dense spike that is the defining visual feature of a mature delphinium.
The Fridge Trick — Why It Matters
Delphinium seeds can enter a state of heat dormancy that prevents germination at normal room temperatures. Placing the sealed, unopened seed packet in the salad drawer of a domestic fridge for 7–14 days before sowing breaks this dormancy and significantly improves germination rates. After removing from the fridge, sow immediately — the cold treatment has already begun the germination process. Keep sown seeds at 15–18°C for germination; a warm windowsill above 20°C can induce secondary dormancy and prevent germination entirely.
⚠ Toxicity — All Parts Poisonous
All parts of Delphinium elatum are highly toxic if ingested by humans, dogs, cats, or livestock — causing symptoms including nausea, heart irregularities, and in severe cases, respiratory failure. Wear gloves when handling plants or seeds, as the plant can also cause skin irritation on contact. Keep seed packets away from children. This does not reduce the value of the plant in the garden, but requires basic precautionary awareness.
The Second Flush Secret — Cutting for September Flowers
Delphiniums are one of the few tall perennials that can be reliably manipulated into a second flowering season. Immediately after the main flower spike fades in July, cut all stems down to ground level and feed the plant with a balanced fertiliser. New shoots emerge from the crown within 2–3 weeks, and a second, slightly smaller flush of sky-blue flowers appears in September — often coinciding with autumn sunshine when the blue stands out particularly brilliantly against the changing seasonal backdrop.
Sowing & Growing On
Fridge the Seeds First — 7–14 Days Before Sowing
Place the unopened seed packet in the salad drawer of your fridge for 7–14 days before the planned sowing date. Remove and sow immediately. Keep at 15–18°C throughout germination — cooler than most seeds. This two-step process (cold treatment then cool germination) is the single most important practice for reliable delphinium germination.
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Cold treat in fridge 7–14 days, then sow January–March on the surface of compost at 15–18°C. Surface sow onto moist seed compost — do not bury. Germination in 14–21 days at the correct temperature. Move seedlings to bright, cool conditions immediately after germination to prevent leggy growth.
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Pot on into individual 9cm pots when seedlings have 2–3 true leaves. Handle by the seed leaves. Grow on at 12–15°C in bright conditions. Do not rush — cool, steady growth produces stronger plants than warm, fast growth.
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Harden off and plant out May–June, 60–75cm apart, in rich deep soil. Delphiniums are heavy feeders. Prepare planting positions with generous compost dug in. Full sun, sheltered from strong winds. Space 60–75cm apart to allow good air circulation — close planting increases powdery mildew risk.
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Insert strong stakes when plants reach 30cm — before they need them. At 150–180cm, these are the largest-flowered plants in most gardens. Bamboo canes or hazel rods of at least 150cm are needed. Tie loosely at 30cm intervals as the stem grows. Staking after the plant falls is too late and damages stems.
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Protect from slugs rigorously — especially in spring. Emerging delphinium shoots are among the most slug-attractive plants in the garden. Protect with grit, wool pellets, or regular evening patrols from the moment new growth appears in spring. Slug damage in early spring can remove the entire season's flowering.
Growing On & Care
Long-Tongued Bumblebee Specialist
The nectar of delphinium is hidden deep within the floral spur — a long tube that extends behind the petals. Only insects with sufficiently long tongues can reach it. In Britain, this means primarily the Garden Bumblebee (Bombus hortorum) and other long-tongued bumblebee species, which are among the most important pollinators of deep-throated flowers. A delphinium in full flower is visited almost exclusively by long-tongued bumblebees — a specific and genuinely valuable contribution to supporting these specialised pollinator species.
As a Cut Flower
Harvest delphinium spikes when the lowest third of florets are open and the upper florets are still in bud — the stem will continue opening in the vase. Immediately after cutting, hold the cut end under water and re-cut to prevent air locking in the hollow stem. Place in deep water. Vase life 5–7 days. Dried: hang stems upside down in a warm, dark, well-ventilated location. Delphiniums hold colour well when dried and the spikes make exceptional dried flower arrangements.
Watering and Feeding
Delphiniums are thirsty, hungry plants that cannot be neglected during active growth. Water deeply at the base of the plant every two to three days in dry weather. Avoid wetting the foliage if possible — damp foliage in warm conditions encourages powdery mildew. Feed with a balanced liquid fertiliser monthly through the growing season. After cutting back for the second flush in July, an immediate feed is essential to fuel the regrowth.
Companions in the Border
The vertical blue spikes of 'Summer Skies' pair beautifully with the flat horizontal plates of Achillea (yellow or white provides a classic complementary contrast), the soft white faces of Cosmos 'Purity' (which bridge the rigid blue spikes with a relaxed informality), and the frothy white umbels of Ammi majus (which soften the lower stems of delphiniums that can appear leggy as the height increases). All three companion options are available from the Bishy range.
Powdery Mildew
Powdery mildew is the most common delphinium problem — a white powdery coating develops on the leaves, particularly in dry conditions with poor air circulation. It is rarely fatal but affects plant vigour. Prevent by spacing plants adequately (60–75cm), watering at the base rather than overhead, and avoiding high-nitrogen feeding that produces soft, susceptible growth. If it appears, remove affected leaves and ensure good airflow around plants.
Winter and Perennial Management
After the second flush fades in September–October, cut all stems to 10–15cm above ground level. The crown overwinters below ground, protected by the short stub of stem. In spring — often not until May — new shoots emerge. Do not mistake this late emergence for plant death. Mark the position in autumn so you do not accidentally dig up the crown before the new shoots appear. Well-established Pacific Giant plants improve over three to four years as the crown develops.
Sowing & Flowering Calendar
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
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| ❄️ Fridge treat |
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| 🌱 Sow indoors |
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| 🌿 Plant out |
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| 🌸 Flowers |
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| 🌸 2nd flush |
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Common Problems & Solutions
| Problem | Likely Cause | What to Do |
|---|---|---|
| Seeds not germinating | Too warm; skipped fridge treatment | Delphinium seeds require cool (15–18°C) germination after a cold pretreatment. Room temperature above 20°C can induce secondary dormancy, preventing germination entirely. If seeds have not germinated in 21 days at warm temperatures, place the tray in the fridge for 7–10 days, then return to cool conditions — this can rescue a failed sowing. |
| Stems snapping in wind | Staked too late; supports inadequate | Insert stakes when plants reach 30cm — well before they need them. Use bamboo or hazel at least 150cm tall. Tie at multiple points (every 30cm) rather than just at the base. In exposed positions, use a hazel ring or circular support structure that allows the stem to move naturally without snapping. |
| Slugs destroying new shoots | Spring slug pressure | Emerging delphinium shoots are extremely attractive to slugs. Apply protection the moment you see new growth in spring — before damage occurs. Grit, wool pellets, or copper tape around individual plants are all effective. Regular evening inspections and hand-picking through March–May are essential. |
| No second flush | Cutting too late; no feeding after cut | Cut immediately when the first flush fades — not after two weeks of leaving it. Feed immediately after cutting with a balanced fertiliser. The second flush depends on new growth being initiated promptly and given sufficient nutritional support. |
Plant Specifications
The sky-blue giant — 150–180cm of pure blue that rises above everything and returns stronger each year
Fridge the seeds for 7–14 days. Sow at 15–18°C. Stake at 30cm height. Protect from slugs from the first green shoot. Cut to the ground immediately after July flowering and feed. Watch September produce a second column of sky-blue. Wear gloves. Keep it away from children and pets. Repeat every year as the crown grows larger and the spikes taller.
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