Iris 'Black Knight' (TB)

RHS Plant Profile
tall bearded iris 'Black Knight'

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Synonyms

Iris germanica 'Black Knight'

Herbaceous Perennial
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A highly attractive, clump-forming, rhizomatous, herbaceous perennial with upright, strap-like, green leaves and elegant, dark purple flowers with gold streaks on the lower petals, known as falls, in summer

Position

Full sun

Soil Types

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

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

Position

Full sun
Aspect
East–facing or South–facing or West–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)
H7

Colour & Scent

Season Stem Flower Foliage Fruit
Spring
Summer
Autumn
Winter

Botanical Details

Family
Iridaceae
Native to GB/Ireland
No
Foliage
Deciduous
Habit
Clump forming, Columnar upright
Potentially harmful
Harmful if eaten. Wear gloves and other protective equipment when handling. Pets: Harmful if eaten. For further information and contact numbers regarding pets, see the HTA guide to potentially harmful plants
Genus
Iris may be rhizomatous or bulbous perennials, with narrow leaves and erect stems bearing flowers with 3 large spreading or pendent fall petals, alternating with 3 erect, often smaller, standard petals, in late winter, spring or early summer
Name Status
Accepted
Horticultural Group

Horticultural Classification Codes

Horticultural Classification Codes are often given to genera which have a large number of species or cultivars. They are used to categorise cultivated plants based on specific, shared characteristics such as shape, colour, size or flowering time, and can be displayed as letters or numbers

For example:
Dahlia ‘Sunset Kate (Sin) = Single flower
Rosa ‘Marigold’ (HT) = Hybrid Tea

Tall Bearded irises are herbaceous rhizomatous perennials to 70cm or more, with flowers 10-20cm across, bearded on the falls, in late spring and early summer

How to Grow

Cultivation

Plant rhizomes just above soil level. Thrives in a sunny position, in ideally neutral soil with good drainage. See bearded iris cultivation

Propagation

Propagate by division of the rhizome shortly from mid-summer to early autumn. See dividing irises

Suggested planting locations and garden types

  • City and courtyard gardens
  • Coastal
  • Cottage and informal garden
  • Cut flowers
  • Flower borders and beds
  • Garden edging
  • Wall side borders

Pruning

Cut back old flower stems after flowering and remove dying foliage in autumn

Pests

May be susceptible to slugs and snails

Diseases

May be susceptible to aphid-borne viruses, bacterial soft rot and grey moulds; see Iris diseases