How to Grow Sweet William in Australia

Sweet william (Dianthus barbatus) is a cottage flower in the pink family, Caryophyllaceae, grown for its dense, flat heads of fragrant flowers. They come in red, pink, white and bicolours. Those crowded heads of many small flowers set it apart from carnations and pinks, its relatives in the same genus. It flowers across most of Australia, and timing is what most gardeners get wrong. Sown in the wrong month, the traditional types give a year of leaves first.

Three mistakes that matter

1. Expecting flowers in the first summer. Traditional sweet william is a biennial (a plant that grows leaves in its first year and flowers in its second). A spring sowing often gives only a leafy clump. Sow from late summer to autumn for spring flowers, as set out in When to plant.
2. Wet, heavy soil. Plants rot at the crown in ground that stays wet, and rust is worse in poorly drained beds. Choose free-draining soil or a raised bed, as set out in Site, soil and setup, and water the soil rather than the leaves.
3. Crowding the plants. Clumps packed tightly stay damp in the middle, which suits rust and grey mould. Space plants 20 to 30 cm apart, as set out in Sowing Seeds, so air moves between them.

When to plant

Seed germinates in 7 to 14 days in soil of about 18 to 25°C. What decides the flowers is the cold that comes after: most sweet william needs a spell of winter cold (vernalisation) before it flowers. Seed sown from late summer to autumn grows into a clump that flowers in spring. Strains bred to flower in their first year, such as 'Wee Willie', can flower about 20 weeks after a spring sowing.

At the hot end of a window, raise seedlings in trays in part shade and plant them out as the weather cools. In cool gardens, have autumn sowings planted out by Anzac Day so their roots settle before the hardest frosts.

Site, soil and setup

Give sweet william full sun, at least six hours a day; in hot inland gardens it welcomes light afternoon shade. It grows best in light, free-draining soil enriched with compost, neutral to slightly alkaline, at about pH 6 to 7.5.

Do not replant sweet william where earlier plants rotted or wilted. The fungi behind wilt persist in the soil, so give new plants fresh ground.

Container growing

The dwarf 'Wee Willie' is the pick for pots and window boxes, and 'Single Standard Mixed' suits larger containers. Use a pot of at least 10 L with drainage holes; a 30 cm pot holds about 13.5 L. Potting mix can carry Legionella bacteria, so wet it first, wear gloves and a mask, and wash your hands afterwards.

• Watering: water at the base whenever the top 5 cm of mix is dry.
• Feeding: potted plants need more than beds. Water in fish and seaweed blend at 25 mL per 9 L fortnightly from the first flower buds.
• Position: full sun, moved into light afternoon shade during hot spells.

Sowing Seeds

Seedlings raised in trays and planted out give the most even stand, but seed can also go straight into the bed.

Starting in trays

• Fill trays or punnets with seed raising mix and sow the seed 5 mm deep.
• Keep the mix moist but never wet; seedlings usually appear in 7 to 14 days.
• Plant out once seedlings have true leaves and stand 5 to 10 cm tall, usually 6 to 8 weeks after sowing.
• Harden off the seedlings first (set them outside for a little longer each day so they adjust to sun and wind).
• Water them in with seaweed solution at half strength, 15 mL per 9 L.

Sowing direct

• Rake the soil fine, sow the seed 5 mm deep in short rows and firm the soil over it.
• Thin seedlings to 20 to 30 cm apart, or 15 to 20 cm for 'Wee Willie'.
• Check the row for snails at dusk until the seedlings are growing strongly.

All four mixes are sown the same way; they differ in height and in how soon they flower.

Care by growth stage

Sweet william is a light feeder, and compost dug in at planting carries it most of the way. Moist roots and dry leaves matter more than fertiliser.

If growth stalls in poor soil, scratch in a light dose of all-purpose controlled release granular (NPK around 14-5-10) at the label rate.

Deadheading

Cutting off spent heads keeps plants tidy and often brings a second, lighter flush. Prompt deadheading also helps plants carry on for another year.

• Young plants: pinch out the growing tip once, just above a pair of leaves, for bushier plants with more stems.
• Single heads: cut each faded head off just above a pair of leaves.
• Mass plantings: shear the whole planting back once the main flush fades.
• For seed: leave a few heads to ripen and dry on the plant.
• After flowering: cut stems back to the leafy base to neaten plants you keep for another year.

Companion planting

A sunny, well-drained cottage bed sown in autumn suits sweet william and several other spring cut flowers.

Grows well with

• Cornflowers: another autumn-sown cut flower that flowers in spring, for mixed bunches.
• Calendula: shares the autumn sowing window and a sunny, free-draining bed.
• Violas: low cool-season flowers to edge the front of a bed of taller sweet william.
• Foxgloves and bellflowers: tall partners for the long-stemmed cutting types.

Keep apart

• Sunflowers and corn on the north side: their summer shade leaves sweet william thin and short of flowers.
• Leafy crops watered daily by sprinkler: constantly wet leaves invite rust and grey mould.

Harvesting Blooms

Cut stems for the vase when only one in ten to one in five flowers in each head has opened.

• Cut in the cool of early morning, when stems are full of water.
• Take the longest stems you can, cutting just above a pair of leaves.
• Strip the lower leaves and stand the stems straight away in a clean bucket of water.
• Wear gloves for big bunches, as the plant can irritate sensitive skin.
• For seed, cut heads once the capsules are dry and starting to split.

Shake the black seed from the dry capsules and store it in a labelled paper envelope somewhere cool, dry and dark. Seed from a mix comes up in mixed colours. Keep cut stems away from cats and dogs, which can be upset by chewing them.

Common problems

No flowers in the first year. See the first point in Three mistakes that matter.

Seed does not come up. Seed sown too deep, a surface left to dry, or soil far outside the temperature in When to plant are the usual causes. Read more

Seedlings collapse at soil level. Damping off (seedlings killed by soil-borne organisms, mainly Pythium species and Rhizoctonia solani) strikes most often in wet seed raising mix. Read more

Plants rot at the base. Crown rot follows wet, poorly drained soil, the second point in Three mistakes that matter; lift the bed or move plants to pots.

Clumps die out in the centre. Older plants thin from the middle; an occasional feed can delay it, but it is usually the sign to sow again.

Pests and diseases

Snails and a few leaf diseases cause most of the trouble, and the diseases are worst in crowded, damp plantings.

Garden snail (Cornu aspersum) and grey field slug (Deroceras reticulatum). They feed at night on seedlings and soft leaves, leaving holes and slime trails. Collect them by torchlight, set shallow beer traps, or run copper tape around wooden or plastic pots and beds. Read more

Aphids (family Aphididae). These small sap-suckers cluster on new shoots and under leaves, leaving sticky honeydew. Hose them off first; if they persist, spray insecticidal soap late in the day, away from open flowers. Read more

Rust (Puccinia species). Raised brown to orange spots form on the leaves, spreading fastest in humid weather. Pick off affected leaves and pull out badly affected plants.

Grey mould (Botrytis species). Flowers and old heads turn brown and mouldy in cool, damp weather. Cut off affected heads and put them in the bin, not the compost.

Prevention first

• Space plants as set out in Sowing Seeds so air moves between them.
• Water at soil level, as set out in Care by growth stage.
• Clear spent heads and fallen leaves promptly.
• Check seedlings for snails at dusk for the first few weeks.

Tips

1. Let a few plants self-sow at the edge of a bed, then lift the seedlings while small and move them where you want them.
2. Near bushland on the NSW tablelands and in the alps, deadhead before seed sets. Sweet william has naturalised in the Orange district and near Perisher.
3. Plant a block near the vegie patch: bees and butterflies visit the flowers.