Sunflower 
 

Sunflower Biology

 Helianthus annuus
Family: Asteraceae (Daisy family).

Common names:
Sunflower, Annual sunflower, Common sunflower.

Confused with: Wild sunflower (Verbesina encelioides). These plants can be distinguished by:
• seedling leaf shape – sunflower leaves are a slightly flattened oval shape with a pointed end. Wild sunflower are a blade shape, with a tapering, pointed end.
• adult plants – sunflowers have dark green leaves and a single stem or may have some branching towards the top. Wild sunflowers are blue/green in colour and highly branched, with branches emerging from near the base.
• seeds – sunflower has a flattened, wedge shape black seed with grey stripes. Wild sunflower has a smaller brown seed. The inner seeds in the head are winged, with a prominent pale wing surrounding the seed

Description
: Seedlings – can emerge from very deep in the soil. Cotyledon leaves are oar-shaped, 15 – 20 mm long and 5 – 8 mm wide. The first true leaves are rounded, with a pointed tip and prominent, white central vein, 20 – 30 mm long and 10 – 15 mm wide. A strong, hairy central stem quickly develops, lifting these leaves above the ground.

Leaves – are large, alternate, spade shaped, with a pointed and prominent veins. They have serrated edges and are up to 30 – 40 cm long and to 35 cm wide, borne on stalks 15 – 25 cm long.

Plants – in cultivation may be large, to 2 m in height, terminating in a large, dinner plate sized flower head. Sunflowers have a long taproot. In waste areas, plants are often shorter, multi-branched, with much smaller leaves and many, much smaller flower heads.

Flower heads – may be up to 30 cm across, with a ring of bright yellow “petals” 2 – 4 cm long surrounding the centre of the flower head. The flower head arranged in a complex pattern of whorls and is surrounded by 2 layers of green, pointed bracts. Flower heads generally face the sun, following the sun’s path during the day. This pattern ceases as the heads mature. Heads become black at maturity as the flower parts drop off, exposing the seeds.

Seeds – are large, wedge shaped and flattened, 5 – 16 mm long, grey to black with longitudinal streaks.

Lifecycle/Biology:
An annual or biennial, summer growing plant which is relatively drought tolerant. Volunteer plants can be seen at most times of the year, following rain, and will flower from spring through to autumn.

Ecology:
Can occur on a range of soil types and situations, but prefers sandy soils and wetter places. Can be found on road sides, disturbed areas, waste area and fallow paddocks wherever sunflower is grown.

The problem:
Sunflower is a minor weed, with scattered plants occurring along roadsides wherever sunflowers are grown.

Distribution:
Found throughout most of Australia where sunflowers are grown. It can also be a garden escape.

Origin:
A native of North America.

Reference:
Plants of Western New South Wales, p. 679 - 680.

Compiled by:
Graham Charles
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