Delphinium grandiflorum 'Summer Blues' (Summer Series)

RHS Plant Profile
delphinium 'Summer Blues'

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Herbaceous Perennial

A low growing, clump-forming herbaceous perennial with shorter stalks formed with divided green leaves and spikes packed with sky-blue flowers in summer.

Position

Full sun

Soil Types

Chalk, Loam, Sand

Max Height

1-1.5 metres

Max Spread

0.5-1 metres

Size

Time to Maturity
2–5 years
Max Spread
0.5-1 metres
Max Height
1-1.5 metres

Growing Conditions

Chalk Loam Sand
Moisture
Moist but well–drained or Well–drained
pH
Acid or Alkaline or Neutral

Position

Full sun
Aspect
West–facing or South–facing or East–facing
Exposure
Sheltered
Hardiness

Hardiness Ratings

All ratings refer to the UK growing conditions unless otherwise stated. Minimum temperature ranges (in degrees C) are shown in brackets

  • H1A: under glass all year (>15C)
  • H1B: can be grown outside in the summer (10 - 15)
  • H1C: can be grown outside in the summer (5 - 10)
  • H2: tolerant of low temperatures, but not surviving being frozen (1 to 5)
  • H3: hardy in coastal and relatively mild parts of the UK (-5 to 1)
  • H4: hardy through most of the UK (-10 to -5)
  • H5: hardy in most places throughout the UK even in severe winters (-15 to -10)
  • H6: hardy in all of UK and northern Europe (-20 to -15)
  • H7: hardy in the severest European continental climates (< -20)
H5

Colour & Scent

Season Stem Flower Foliage Fruit
Spring
Summer
Autumn
Winter

Botanical Details

Family
Ranunculaceae
Native to GB/Ireland
No
Foliage
Deciduous
Habit
Columnar upright
Potentially harmful
Humans/Pets: Harmful if eaten. Wear gloves and other protective equipment when handling. For further information and contact numbers regarding pets, see the HTA guide to potentially harmful plants
Genus
Delphinium can be annuals, biennials or perennials, with palmately lobed basal leaves and showy bowl-shaped flowers in spikes, racemes or panicles
Name Status
Accepted

How to Grow

Cultivation

Will grow well in fertile, well-drained soil in full sun and apply a balanced liquid fertilizer every other week during the growing season. It will need shelter from strong winds; staking may be necessary. See staking perennials and delphinium cultivation for further advice

Propagation

Propagate by basal cuttings in early spring

Suggested planting locations and garden types

  • City and courtyard gardens
  • Cottage and informal garden
  • Cut flowers
  • Flower borders and beds

Pruning

Deadhead spent flower spikes back to fresh flowering side shoots and cut back to ground level in late autumn

Pests

May be susceptible to aphids, earwigs, delphinium leaf miner, delphinium moth caterpillars, slugs, snails and caterpillars

Diseases

May be susceptible to Powdery mildews, delphinium black blotch, crown rot and viruses