How to Grow Sweet William
Dianthus barbatus
Sweet William is a hardy biennial that forms a leafy rosette in its first year and produces fragrant, densely clustered flowers in the second. Sow in early summer for blooms the following year, in well-drained soil and full sun. Deadhead spent flowers to extend the display and encourage self-seeding. It is easy to grow and reliably hardy outdoors through winter.
In your Leaftide garden - Hardiness
- Hardy
- Season
- Any Season
- Harvest
- Continuous Harvest
- Spacing
- 20 cm
- Height
- 45 cm
- Sow depth
- 0.5 cm
- Min. soil temperature
- 8°C+
- Start from
- Seed
- Light
- Full sun to part shade
- Water
- Moderate
When to Start
Sow from late spring to midsummer, in trays or where it is to flower. It only grows leaves in its first year and flowers the following spring or summer.
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Growth Stages
Approximate days from start
Care Essentials
- Apply a balanced fertiliser when transplanting to final position and again in spring of the flowering year. Avoid high-nitrogen feeds which promote foliage at the expense of flowers.
- Sweet William is a biennial, forming a leafy rosette in its first year and blooming in the second. Deadhead spent flowers to extend the display and allow self-seeding for future plants. It thrives in well-drained soil in full sun.
Growing Conditions
- Min. soil temperature
- 8°C+
- Germination temperature
- 15–20°C
Pet Safety
- When to Start
- Sow from late spring to midsummer, in trays or where it is to flower. It only grows leaves in its first year and flowers the following spring or summer.
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Spacing & Planting
| Plant spacing | 20 cm |
| Row spacing | 30 cm |
| Sow depth | 0.5 cm |
| Mature height | 45 cm |
| Mature spread | 30 cm |
Pests & Diseases
Pest Aphids
Identification: Dense clusters of small green or black insects on flower stems and buds; sticky honeydew and sooty mould may follow.
- Blast off with a strong jet of water; repeat every few days until colonies are gone.
- Spray with diluted insecticidal soap or neem oil, covering stems and buds thoroughly.
- Apply a pyrethrin-based contact spray in the evening to protect pollinators.
Pest Slugs and Snails
Identification: Ragged holes in young leaves and seedlings; silvery slime trails on foliage and around the crown.
- Apply ferric phosphate slug pellets around young plants after transplanting.
- Use beer traps or copper tape around pot-grown plants.
- Metaldehyde pellets are banned outdoors in the UK; scatter ferric phosphate pellets sparingly instead.
Pest Carnation Tortrix Moth Caterpillar
Identification: Leaves webbed together with silk; small green caterpillars feeding inside the webbing.
- Manually remove and destroy webbed leaves and caterpillars by hand.
- Apply Bacillus thuringiensis (Bt) spray to affected plants.
- Spray with a contact insecticide containing deltamethrin or lambda-cyhalothrin.
Disease Fusarium Wilt Fusarium oxysporum f. sp. dianthi
Symptoms: Sudden wilting of stems despite moist soil; brown streaking inside the stem when cut; rotting at the stem base.
Treatment: Remove and destroy affected plants immediately — do not compost. There is no effective chemical cure once a plant is infected.
Prevention: Avoid waterlogged soil; rotate planting positions each year; use fresh compost for seedlings. Fusarium persists in soil, so avoid replanting dianthus in the same spot for at least 3 years.
Disease Leaf Spot Alternaria dianthi
Symptoms: Small purple or brown spots with pale centres on the leaves, sometimes with a yellow halo; spots may merge in severe cases.
Treatment: Remove affected leaves promptly. Apply a copper-based fungicide or mancozeb spray at the first sign of infection.
Prevention: Avoid overhead watering; space plants for good airflow; clear debris from the base of plants in autumn.
Growing Conditions
| Min. soil temperature | 8°C |
| Germination temperature | 15–20°C |
| Base growth temperature | 5°C |
| Min. daylength to start | 10h |
| Photoperiod sensitivity | day neutral |
Frequently Asked Questions
- How far apart should I plant Sweet William?
- Plant Sweet William 20cm apart with 30cm between rows.
- When does Sweet William flower?
- Sweet William flowers the year after sowing. It spends its first year growing leaves and flowers the following spring or summer.
- Is Sweet William frost hardy?
- Sweet William is classified as Hardy.