How to Grow Sunflowers in Australia

Sunflower (Helianthus annuus) is a fast-growing annual in the daisy family, Asteraceae, grown for cut flowers, seed and tall summer screens. Sown in the right months, it flowers in every Australian climate zone, usually 8 to 12 weeks from sowing. The usual mistake is sowing too early, into soil still too cold for seed to come up quickly and evenly.

Three mistakes that matter

1. Sowing into cold, wet soil. Seed can sit for weeks, then rot or be eaten before it emerges. Wait for the soil temperature in When to plant, and sow into moist, not soggy, soil.
2. Letting plants go dry as buds form. Water demand climbs steeply from about 10 days after buds show, and dry soil can then leave empty seed in the centre of the head. Step up watering as buds appear, following Care by growth stage.
3. Feeding for leaves, not stems. Too much manure or high-nitrogen fertiliser gives tall, soft growth that leans and a dense canopy that holds disease. Prepare the bed with compost, then follow the feeds in Care by growth stage.

When to plant

Push a soil thermometer 10 cm into the bed at 8 am on a few mornings in a row. Sow once it reads 12°C or more and is rising, with heavy frosts over.

In cool and alpine gardens, sow in early January only types that flower within about 10 weeks, such as 'Dwarf Sonnet', to beat April frosts.

Site, soil and setup

Sunflowers need full sun for at least 6 hours a day, as shade from budding onward reduces seed set. Open heads usually stop turning and face north-east, so plant them along the western side of the garden. Tall types also need shelter from strong wind.

Deep, fertile, free-draining loam with a pH of 6.0 to 7.5 suits them best. The taproot (main root) can be 40 to 50 cm deep when plants are only 30 cm tall, so avoid shallow soil.

Grow sunflowers in the same bed only once every three years, and not where white mould (Sclerotinia sclerotiorum) struck in the last five years.

Container growing

For pots, try the branching, pollen-free 'Dwarf Sonnet' (25 to 40 cm) or the double-flowered 'Teddy Bear' (40 to 60 cm).

Give one plant under about 60 cm a pot at least 25 cm across and deep, about 10 L. Plants of 60 to 80 cm, or shorter types with full-size heads, need a pot 35 to 40 cm across, about 20 to 35 L. Taller types need a pot at least 50 cm across, about 60 L. Fill pots with fresh potting mix, not garden soil.

• Watering: check pots daily and water when the mix is dry 2 cm down, until it drains from the base.
• Feeding: water in fish and seaweed blend at 25 mL per 9 L fortnightly until the first flowers open. Then switch to liquid potash at the label rate, fortnightly.
• Position: full sun, out of strong wind.

Sowing Seeds

Sow seed where plants will flower; raise seedlings in pots only to get ahead in cool areas.

Starting under cover

• Sow one seed per deep cell or small pot of seed raising mix, 15 to 25 mm deep, about twice the width of the seed.
• Keep pots warm and bright; seedlings usually appear in 5 to 10 days, but can take up to 30 days in cool conditions.
• Harden off seedlings before planting out (move them outdoors for a little longer each day until they cope with full sun and wind).
• Plant out while small, before roots circle the pot.

Sowing direct

• Sow two seeds per spot at that depth, spaced to suit the type in the table below.
• Keep the soil just moist until seedlings emerge.
• Once true leaves open, snip off the weaker seedling at soil level.
• Birds dig out fresh seed, so until seedlings are up, cover rows with wildlife-safe netting: white, taut, with mesh too small for a finger.

Care by growth stage

Weeks count from sowing; cool-weather sowings can take about two weeks longer to flower.

If you prefer a conventional feed, use these instead of the organic feeds. Mix a controlled release all-purpose granular (NPK around 12-3-8 to 14-5-10) into the bed at the label rate at sowing. Without the granular, water in an all-purpose soluble (NPK 23-4-18 to 27-5-9) at the label rate fortnightly during leafy growth. From the first open flowers, switch to a fruiting and flowering soluble (NPK 15-4-26 to 15-5-20) at the label rate fortnightly.

Companion planting

Because evidence for companion planting is limited, these pairings rest on shade and insects.

Grows well with

• Pumpkins, zucchini and cucumbers: plant them on the sunny side of a sunflower row, where their vines cover bare soil.
• Marigolds and cosmos: with sunflowers, they draw bees and beneficial insects into the patch.

Keep apart

• Low crops that need full sun: tall sunflowers shade them for part of each day.

Harvesting Blooms

Cut stems for the vase when a flower is just opening and its outer petals have lifted to stand out from the centre.

• Cut in the cool of early morning, as stems cut at midday tend to wilt.
• Strip leaves that would sit below the waterline, as they rot and feed bacteria.
• Stand stems in water as you cut, then move them to a clean vase with flower preservative.

With the water changed every 2 to 3 days and the vase out of direct sun, most stems last one to two weeks. Standard orange-yellow types usually last longest, while dark-petalled types can drop petals within a week.

For seed, keep watering and cover the head once the petals fall, to keep birds off. Use a paper bag or a bag with mesh too small for a finger.

Seed is ready when the back of the head turns from yellow to brown and the bracts (leafy scales around it) dry. This usually takes 4 to 6 weeks after flowering. Cut the head, hang it upside down in a dry, airy spot, and knock the loosened seed into a bucket.

Store dry seed in a labelled paper bag in a cool, dry place out of the sun. Dried heads can also go to the chooks.

Common problems

Stems lean or snap. Heavy heads catch strong wind, and roots in waterlogged soil may not hold a tall plant upright.

Buds distort or blacken at the rim. Frost while buds are forming can twist them and turn the edge of the head black or purple.

Birds strip ripening seed. Sulphur-crested cockatoos, galahs and other seed-eating birds take seed from uncovered heads as it ripens. Read more

Heads rot in wet weather. Rhizopus head rot (Rhizopus species) enters heads damaged by hail, birds or caterpillars when rain or heavy dew keeps them wet.

Pests and diseases

Pests strike at two stages: seedlings, then buds and heads from budding until harvest.

Common garden snail (Cornu aspersum). Snails and slugs feed at night and can strip a row of emerging seedlings, though larger plants are less affected. Read more

Rutherglen bug (Nysius vinitor). From spring, swarms often arrive from drying inland weeds to suck sap from stems and developing seed, so buds can wilt and seed fills poorly. Read more

Green vegetable bug (Nezara viridula). These shield-shaped bugs suck sap from upper stems and heads, and large numbers leave heads wilted or deformed. Read more

Corn earworm (Helicoverpa armigera). Caterpillars up to 40 mm chew florets and bore into heads from budding onward; look-alike native budworm (Helicoverpa punctigera) is managed the same way.

White mould (Sclerotinia sclerotiorum). In cool, wet or humid weather it rots stems and heads so plants can collapse, and it also infects beans, lettuce and potatoes.

Powdery mildew (Golovinomyces latisporus). A white, powdery coating spreads over the leaves late in the season, most often when days are humid and nights cool. Read more

Prevention first

• Clear weeds from late winter, as Rutherglen bugs breed on them, and pick off snails by torchlight while seedlings are small.
• From budding, shake heads weekly over a bucket of soapy water early in the morning to catch bugs, and wear gloves to hand-pick caterpillars.
• Keep to the spacing in Sowing Seeds, as airflow slows powdery mildew and sunflower rust (Puccinia helianthi).
• Where white mould struck, grow only resistant crops such as sweet corn, onions or sweet potato there (see Site, soil and setup).

Tips

1. Sow single-stem types every two to four weeks for a longer run of blooms; branching types flower for weeks.
2. Grow pollen-free types for vases, since they drop no pollen. They set little seed without pollen-bearing sunflowers nearby, so grow a seed type such as 'Giant Russian' for seed heads.
3. Cut spent heads before seed drops to avoid self-sown seedlings, as some types can turn weedy.