How to Grow Sweet William Seeds

Grow Guide #2327
Family: Caryophyllaceae
Binomial name: Dianthus barbatus
Life Cycle: Biennial, usually grown as an annual

This 'How to Grow' guide details everything a home gardener needs to know to plant, grow and care for Sweet William (Dianthus barbatus).

When to Sow Sweet William Seeds

Sweet William can be grown year-round in most climates. Avoid planting in extremely hot or cold weather which can affect germination and growth. Use the table below to identify the best time of year to sow Sweet William in your climate.

  JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
Cool
Temperate
Sub-Tropical
Tropical
Arid

Preparation

Sweet William plants are best grown in full sun. Choose a location that will receive at least 6 hours of full sun each day.

Sweet William plants need a well drained soil enriched with plenty of organic matter. Prepare soil by weeding it thoroughly, digging it over to loosen it and adding aged animal manure or compost. Keep the area free of weeds until planting. Learn more about preparing soil for planting here.

Sweet William plants can be grown in containers. If possible choose a variety that’s recommended for container growing. Use a good quality potting mix and make sure your container is large enough for mature plants; a minimum of 10 litres is recommended for Sweet William. During the growing season, keep in mind that container grown plants may need additional fertiliser to encourage healthy growth.

How to Sow Sweet William Seeds

Sweet William seeds do not require any treatment (eg soaking, stratification) before sowing.

Sweet William seeds grow best when they are raised in trays or other containers and transplanted to the garden once established.

  1. Fill trays, punnets or jiffy pots with a good quality seed-raising mix, or use soil starter pellets.
  2. Sow seeds 4mm deep.
  3. Keep soil moist but never wet or dry.
  4. Seeds should germinate in around 7-14 days at a soil temperature of 18-22°C.
  5. Transplant seedlings to the garden once they have their first true leaves and are large enough to handle (usually 5-10cm tall).
  6. Plant out, spacing plants 15-30cm apart.

Optional: In cool climates Sweet William seeds can be sown indoors 6 weeks before the last expected frost. Grow them in a warm position with plenty of natural light.

How to Grow Sweet William

Sweet William plants may need watering during the growing season. Water when the soil is dry about 5cm below the surface (test this by scratching away a little soil with your finger). Water deeply in the early morning or late afternoon. Avoid watering the leaves of plants to avoid fungal diseases. Learn more about watering here.

If soil was well prepared no extra fertiliser should be necessary. In poor soil or to give your plants an extra boost, application of a high-potassium fertiliser or one formulated for flowering plants can be beneficial:

  • Apply slow release fertiliser at the recommended rate when transplanting or when seedlings are 5-10cm tall.
  • Apply liquid fertiliser at the recommended rate and frequency while plants are fruiting or flowering.

Pinch out the growing tips of Sweet William plants to encourage denser growth with stronger stems and more flowers. Using sharp secateurs or snips remove the top set of leaves, cutting just above a set of lower leaves.

Sweet William plants should flower in approximately 120-140 days.

Deadhead Sweet William flowers regularly during the growing season. Using sharp secateurs or snips cut fading or dead flowers off just above a set of leaves. Removing old flowers regularly will encourage plants to produce more flowers. Learn more about deadheading flowering plants here.

Sweet William is usually grown as an annual; if growing as a biennial prune back in late autumn. When plants have finished flowering prune them back to neaten them and encourage strong new growth. Using sharp secateurs or snips, cut individual stems just above a set of lower leaves.

If growing Sweet William for cut flowers, use sharp snips or secateurs to cut the longest stems possible, removing the lower leaves and placing the stems immediately in a clean bucket of water. Learn more about cutting and conditioning homegrown flowers here.

Common Problems when Growing Sweet William

Like all plants, Sweet William is susceptible to some pests, diseases and other problems. Below is a list of the most common problems gardeners encounter when growing Sweet William plants:

  • Aphids
    Aphids are small (2-4mm long) sap-sucking insects that congregate on the new shoots or the undersides of leaves. They can cause leaves to wilt or become discoloured, and also excrete honeydew which can attract ants and other insect pests. To manage aphids, remove them by spraying with a garden hose, apply a soap or alcohol spray, or encourage predatory insects to your garden. Read more about aphids here.
  • bacterial leaf spot
    Bacterial leaf spot is a disease that causes irregularly shaped brown spots on all above-ground parts of a plant. The spots at first appear to be wet but become dry and scab-like over time. Leaves and flowers can fall prematurely. Water plants at soil level (not on the leaves), dispose of fallen leaves and fruit and practice crop rotation.
  • strawberry with botrytis
    Grey mould (Botrytis sp.) is a fungal disease that causes flowers to become mouldy and fruit to rot. Spores are transported by wind and can survive in soil or on green waste. The fungus spreads most in cool, damp weather. Prune off affected flowers and fruit, water plants at soil level (not on leaves) and if necessary spray with an appropriate fungicide or homemade spray.
  • Wilt
    Fusarium wilt and verticillium wilt are diseases caused by soil-borne fungi. The fungi enter a plants’ roots and prevent water and nutrients from moving through plants. Leaves and branches will wilt, dry off and die and leaves may yellow. There is no cure for wilt but choosing resistant varieties, disposing of affected plants and soil, practicing good garden hygiene and crop rotation will all help to prevent it spreading.
  • Rust fungus
    Rust (Puccinia sp.) is a fungal disease that causes brown to orange raised spots or patches to appear on foliage. Fungal spores are spread by wind or water to neighbouring plants, especially in temperatures of 10-20C and when humidity is high. To manage rust, space plants to avoid overcrowding, grow them in the recommended amount of light (eg full sun), do not over fertilise crops, remove dead plants and practice crop rotation. Read more about rust fungus here.
  • Slugs and Snails
    Slugs and snails are molluscs that feed on tender leaves and shoots, mostly at night, leaving slimy trails behind them. Control them by removing their hiding places, keeping free range poultry, collecting them by torchlight or by placing traps. Read more about slugs and snails here.
  • thrip on a flower
    Thrips are black, beige or white flying insects (<1.5mm) with larvae that suck tissue from leaves and petals, leaving behind very small white or transparent markings. While not usually causing serious damage, the marks affect the look of flowers and foliage and thrips can also transfer pathogens from one plant to another. Wash thrips from affected plants using a garden hose, encourage predatory mites and lacewings with companion planting, or spray with soap, eco-oil or neem oil.

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