Sunflower

Description

Sunflower, Helianthus annuus, is an herbaceous annual plant in the family Asteraceae, grown for its seeds. The plant has a thick, hairy, erect stem which gives rise to a large flower head. The plant has large, broad lower leaves which are oval and arranged alternately on the stem and smaller, narrower upper leaves which are attached individually to the stem. The flower head is a large disc reaching 10–30 cm (4–12 in) in diameter which is made up of 16–30 individual florets which are yellow-gold in color. The outer florets are sterile and produce the outer petals of the flower head, while the inner florets will mature into the seeds in the central disc. Sunflowers are annual plants, harvested after one growing season and can reach 1–3.5 m (3.3–11.5 ft) in height. Sunflower may also be referred to as girasole and originates from North America.


Uses

Sunflower seeds can be eaten either fresh or cooked or used to extract oil which is widely used in cooking. The seeds are commonly harvested for bird seeds. Sunflowers are often grown as ornamental plants due to their large, attractive flower head.


Propagation


Requirements
Sunflowers are generally very easy to grow and thrive in areas with long hot summers which promote flowering. They grow best at temperatures between 21 and 25.5°C (70–78°F). Sunflowers can be very large and require plenty of space although there are some varieties which have been specially bred to be compact for growing in smaller spaces or for growing in containers. Sunflowers will grow in a variety of soils as long as they are not waterlogged and can be grown successfully at a pH ranging from 6.0 to 7.5. A pH above 6.0 is not recommended. They should be grown in an area that receives full sunlight and it is preferable to provide them with some shelter from wind which can damage the stems e.g. along a fence or wall. Sunflowers originate from the dry prairie regions of North America and are drought resistant once established. Their easy to grow nature makes them ideal plants for growing with children.

Planting
Sunflowers are most easily grown by direct seeding outdoors. In areas with a long growing season, they should be planted in late Spring after all danger of frost and when the soil has warmed through. However, in areas with short growing seasons, the seeds can be planted a week or two before the last frost date as the plants are tolerant to cold Prepare the soil for planting by digging to loosen the soil and break up any large clumps. Plant seeds 2.5–5.0 cm (1–2 in) deep and space them 10–15 cm (3–4 in) apart. If planting in rows, allow 0.75 m (~30 in) between each one. An application of a balanced fertilizer at planting promotes the development of a strong root system. The seeds should sprout in 7–10 days. When the seedlings are about 15 cm (6 in) tall with two pairs of true leaves, thin the plants to a final within row spacing of 60 cm (2 ft). Sow more seeds every 2–3 weeks for a continuous bloom over the summer.

General care
Although sunflowers are drought resistant, frequent, deep watering promotes the development of a strong root system. The plants do not require fertilizing, indeed, addition of nitrogen promotes vegetative growth and will delay flowering. The stems of the plants can easily reach in excess of 1.8 m (6 ft) in height and will benefit from some support. Bamboo stakes work well.

Harvesting
Sunflowers are ready to harvest when the back of the flowers begin to turn yellow and the flower head droops. Cut the stem about 5 cm (2 in) below the head and hang up to dry in a warm, well ventilated place for several weeks. When the heads are thoroughly dried, remove the seeds with a brush or by hand. Allow the seeds to dry out for a further few days before storing.


References

Darby, H. & Halteman, P. (2011). Sunflower. University of Vermont Extension. Available at: http://northerngraingrowers.org/wp-co.... [Accessed 16 April 15]. Free to access.

Lee, C. (2014). Sunflower for seed. University of Kentucky Cooperative Extension Service and University of Kentucky College of Agriculture, Food and Environment. Available at: http://www.uky.edu/Ag/NewCrops/intros.... [Accessed 16 April 15]. Free to access.

Myers, R. L. (2002). Sunflower. Thomas Jefferson Agricultural Institute. Available at: http://www.agmrc.org/media/cms/sunflo.... [Accessed 16 April 15]. Free to access.

Putnam, D. H., Oplinger, E. S., Hicks, D. R., Durgan, B. R., Noetzel, D. M., Meronuck, R. A., Doll, J. D. & Schulte, E. E. (1990). Sunflower. In: Alternative Field Crop Manual. University of Wisconsin Extension, University of Minnesota Center for Alternative Plant and Animal Products and the Minnesota Extension Service. Available at: http://www.hort.purdue.edu/newcrop/af.... [Accessed 16 April 15]. Free to access.


Common Pests and Diseases

Alternaria leaf blight
Alternaria helianthi

Symptoms
Dark brown lesions on leaves surrounded by a yellow halo; lesions coalesce and become irregularly shaped and cause leaves to become blighted; plant becomes defoliated and dies
Cause
Fungus
Comments
Disease emergence favors hot weather and frequent rainfall; fungus may survive in crop debris or on suitable weed hosts; disease can be transmitted through infested seed
Management
Prune out infected leaves; use adequate plant spacing to reduce humidity around plants and promote good air circulation; disease can be controlled by application of appropriate foliar fungicide

Downy mildew
Plasmopara halstedi

Symptoms
Death of seedlings leading to reduced stand in field; if seedlings survive they may be chlorotic with thickened leaves; white cottony growth is present on leaf undersides; systemic infection causes stunted plant growth and reduced seed production
Cause
Fungus
Comments
Disease emergence favors high humidity; fungus can survive in soil for up to 10 years
Management
Plant sunflower varieties that are resistant to downy mildew; treat seeds with an appropriate fungicide prior to planting; foliar fungicides are ineffective at controlling systemic infections and are not recommended

Powdery mildew
Erysiphe cichoracearum

Symptoms
Powdery white patches which appear initially on lower leaves but which may spread to all above-ground parts of plants; white patches turn gray in color and black fungal fruiting bodies are visible; severely infected leaves may turn yellow and dry up
Cause
Fungus
Comments
Conditions which favor the development of the disease mach those that are favorable for the host plant; disease emergence is favored by periods of high humidity where leaves remain dry
Management
Allow adequate spacing between plants to promote good air circulation around foliage; plant sunflowers in an area that receives full sun for most of the day; remove and destroy all sunflower crop debris after harvest; applications of appropriate foliar fungicides can help control the disease but care should be taken as some labels do not allow seeds from treated plants to be used as food or feed

Septoria leaf spot
Septoria helianthi

Symptoms
Water-soaked circular or angular spots on leaves with a greasy, greenish appearance on lower leaves; lesions are usually gray with a darker margin; some lesions may have a narrow yellow border; tiny black fungal fruiting bodies may be present in the lesions
Cause
Fungus
Comments
Little is known about the survival and spread of the pathogen which causes the disease; spores are believed to be spread by splashing water; disease will develop rapidly during periods of moderate to warm weather with high rainfall
Management
Plant high quality seed which is free of diseases; rotate crop away from sunflower for a period of 3 years, especially if overhead irrigation is used; fungicides are rarely required for the treatment of Septoria leaf spot

Phoma blight
Phoma macdonaldii

Symptoms
Symptoms of the disease develop after flowering; large black lesions appear on stem and coalesce to form large blackened areas; dark colored irregularly shaped lesions appear on leaves and flowers; early infections can cause flowers to die; infected plants die prematurely and produce little seed; disease often affects plants in a circular pattern in the field
Cause
Fungus
Comments
Fungus survives in seeds or sunflower debris in the field; disease emergence favors periods of wet weather during flowering
Management
Rotate crop to a non-host (e.g. small grains) a period of 4 years; plant hybrids which are more tolerant of the disease; control stem weevil populations in sunflower fields

Verticillium wilt
Verticillium dahliae

Symptoms
Lower leaves developing mottled appearance; leaf tissue between veins turns yellow and then brown; infected leaves wilt, dry out and eventually die; stems of plants may become blackened close to the soil line; a cross section of the stem reveals blackened vascular system
Cause
Fungus
Comments
Fungus is soil-borne and enters plants through the roots, invading the vascular system; pathogen can be spread to uninfested fields through contaminated irrigation water or movement of infested soils
Management
Plant high quality, disease-free seed; avoid planting sunflowers in fields known to have been infested with Verticillium previously; plant resistant sunflower hybrids in areas where disease is known to be problematic