Crops Sunflower (Helianthus annuus)

Seedling

The sunflower plant is an annual, forming a strong taproot, from which develop deeply-penetrating lateral roots. Solitary ornamental specimens, and some older crop types, may attain considerable heights (exceptionally up to 5 m), but modern crop cultivars may be less than 1 m tall (dwarf types) or 1.5 m (semi-dwarf) at maturity.

Plant in bud, from above

The stem, at first round and slender, later thickens and becomes angular and hairy; the large, dark green leaves are roughly heart-shaped, with a wrinkled surface and prominent veins. The leaves are individually stalked and arranged round the stem in such a fashion that light interception is maximised.

Flower head

In modern agricultural varieties there is usually only a single terminal flower-head, with the many fertile tubular florets forming a dark central disc, surrounded by a circle of bright yellow ray florets. The heads turn to follow the sun until the flowers are fully open, when they remain facing east.

Ripening seedhead

As the seeds ripen they darken, and the heads droop, the stem bending immediately behind the head. Sunflowers perform best on freely-drained and deeply-worked soils: seedlings will tolerate some frost but sowing into warm soil in late spring will promote rapid establishment and minimise bird damage in the seedbed.

Seeds

Seed rates of 8-10 kg per ha may be used to establish a population of about 10 plants per square metre in 40-50 cm rows, though dwarf types may benefit from thicker stands in 25 cm rows. Nitrogen requirement is modest at 50-75 kg per ha: soil phosphate and potash status should be good, and the crop has a requirement for boron. Some herbicides are available for early weed control, but an established crop should suppress most weeds. Flowering usually takes place during July. Bird damage to the seed-beds can be a serious problem, which may be minimised by growing the crop in large blocks; infection of the heads by the fungal disease Botrytis may cause losses in damper conditions, reliable control measures not yet having been determined. The crop may be direct-combined, possibly preceded by desiccation of the heads; yields of modern hybrids are usually 1.5-3.0 t seed per ha. The total area of sunflowers grown in Europe has been expanding steadily and seems likely to continue to do so; within that total, the industrial crop area is also expected to rise. It is likely that optimal results from the high-oleic acid types will be achieved when varieties bred specifically for European conditions become available. There is scope for development of uses of the seedcases (a by-product) as fuel, as a constituent of building materials, and as a feedstock for chemical processes.