Penstemon serrulatus

RHS Plant Profile
cascade penstemon

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cascade penstemon RHS / Joanna Kossak
cascade penstemon RHS / Joanna Kossak

Other common names

coast penstemon, serrulate beardtongue, serrulate penstemon

Synonyms

Penstemon diffusus

Plants for pollinators
Herbaceous Perennial

A semi-evergreen subshrub to 50cm, with glossy, oval to lance-shaped, dark green leaves. Broad, dense, one-sided clusters of narrowly bell-shaped, pinkish-purple flowers are produced in late summer

Position

Full sun

Soil Types

Chalk, Loam, Sand

Max Height

0.1-0.5 metres

Max Spread

0.1-0.5 metres

Size

Time to Maturity
2–5 years
Max Spread
0.1-0.5 metres
Max Height
0.1-0.5 metres

Growing Conditions

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

Position

Full sun
Aspect
West–facing or South–facing
Exposure
Exposed or 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)
H6

Colour & Scent

Season Stem Flower Foliage Fruit
Spring
Summer
Autumn
Winter

Botanical Details

Family
Plantaginaceae
Native to GB/Ireland
No
Foliage
Semi evergreen
Habit
Bushy
Genus
Penstemon may be deciduous or evergreen perennials or subshrubs, with simple, narrow leaves and racemes or panicles of bell-shaped or funnel-shaped flowers in summer and early autumn
Name Status
Correct
Plant Range
W North America

How to Grow

Cultivation

Grow in very gritty, sharply-drained soil in full sun. In cold areas, protect plant overwinter with a dry winter mulch. See penstemon cultivation for further advice

Propagation

Take softwood cuttings in early summer or semi-ripe cuttings in midsummer. Divide in spring or propagate by seed in late winter or spring

Suggested planting locations and garden types

  • Cottage and informal garden
  • City and courtyard gardens
  • Coastal
  • Gravel garden
  • Wildlife gardens
  • Flower borders and beds

Pruning

Deadhead after flowering

Pests

Maybe susceptible to slugs, snails, aphids and leaf and bud eelworm

Diseases

May be susceptible to grey moulds, powdery mildews and fungal leaf spot