Helianthus annuus 'American Giant'

RHS Plant Profile
sunflower 'American Giant'

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Annual Biennial

A vigorous, erect annual growing to 4.5m tall, with oval to heart-shaped, hairy, toothed, mid-dark green leaves. Sturdy stems bear large, bright yellow flower heads in Summer followed by edible seeds

Position

Full sun

Soil Types

Chalk, Clay, Loam

Max Height

4-8 metres

Max Spread

0.1-0.5 metres

Size

Time to Maturity
1 year
Max Spread
0.1-0.5 metres
Max Height
4-8 metres

Growing Conditions

Chalk Clay Loam
Moisture
Moist but well–drained
pH
Alkaline or Neutral

Position

Full sun
Aspect
East–facing or 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)
H4

Colour & Scent

Season Stem Flower Foliage Fruit
Spring
Summer
Autumn
Winter

Botanical Details

Family
Asteraceae
Native to GB/Ireland
No
Foliage
Deciduous
Habit
Columnar upright
Genus
Helianthus can be tall, erect annuals, tuberous or rhizomatous perennials, with coarse simple leaves and large daisy-like flower-heads
Name Status
Unresolved

How to Grow

Cultivation

Sow seeds in humus-rich, moist but free-draining soil in full sun in Spring. Alternatively, seeds can be sown indoors, three weeks before the last frost, in modules then planted out.

Propagation

Propagate by seed or division. Divide established clumps every 2-4 years to both increase plants and retain vigour. See sowing seeds indoors for further advice

Suggested planting locations and garden types

  • City and courtyard gardens
  • Wildlife gardens
  • Cut flowers
  • Flower borders and beds

Pruning

Cut back after flowering

Pests

May be susceptible to slugs and snails

Diseases

May be susceptible to powdery mildew, sclerotinia diseases