Dahlia 'Knockout' (S-c) | dahlia 'Knockout'

Herbaceous Perennial

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A tuberous perennial to 80cm in height with toothed, pinnate, dark mahogany to purple-black leaves. Single bright yellow flowers with orange centres are borne from summer to autumn

Position

Full sun

Soil Types

Clay, Loam, Sand

Size

Time to Maturity: 1 year

Growing Conditions

Clay Loam Sand

Moisture: Moist but well–drained
pH: Acid or Alkaline or Neutral

Position

Full sun

Aspect: South–facing or West–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): H3

Colour & Scent

Season	Stem	Flower	Foliage	Fruit
Spring
Summer
Autumn
Winter

Botanical Details

Family: Asteraceae
Native to GB/Ireland: No
Foliage: Deciduous
Habit: Clump forming
Genus: Dahlia are tuberous rooted perennials with pinnately divided leaves and showy flowerheads, double in many cultivars, in summer and autumn
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: Semi-cactus dahlias have fully double flowerheads with narrow, pointed, straight or incurved ray florets broader at the base

How to Grow

Cultivation

Grow in full sun, in fertile, humus-rich, well-drained soil enriched with organic matter and general-purpose fertiliser. Pinch out growing tips to encourage bushy plants. Water freely in dry periods. Lift and store tubers in autumn to replant, or use as a source of cuttings, in spring. See dahlia cultivation and our video How to plant dahlia tubers and care tips

Propagation

Propagate by softwood cuttings taken in spring from shoots from stored tubers, or divide the tubers, ensuring each section has at least one viable bud

Suggested planting locations and garden types

City and courtyard gardens
Cottage and informal garden
Patio and container plants
Cut flowers
Flower borders and beds

Pruning

Deadhead to prolong flowering. Cut back to near ground level in autumn, once frost has damaged foliage, before lifting and storing for the winter

Pests

May be susceptible to aphids, capsid bugs, caterpillars, earwigs nibbling flowers, glasshouse red spider mite and slugs

Diseases

May be susceptible to powdery mildews in dry conditions, fungal rots including grey mould in wet conditions and in storage, and a virus that causes stunting, leaf markings and distortion