Sweet William is one of those old cottage-garden plants that rewards a little planning. Grow it as a biennial: sow in early to midsummer so the plants bulk up in their first year and bloom hard the following spring. Give them a gritty, well-drained bed sweetened with a bit of lime — they hate sitting in cold wet soil over winter, which is the single most common way gardeners lose them. Surface-sow or barely cover the seed since light helps germination, keep it evenly moist until you see sprouts, then ease off. Pinch young plants once to encourage branching, and don't crowd them; good airflow keeps them happy. If you let a few go to seed they'll often self-sow and quietly perpetuate themselves, which is a nice way to keep a patch going since the original plants tend to peter out after a couple of seasons.
The petals are genuinely edible and carry that signature clove-like spice the whole Dianthus clan is known for — strip away the bitter white heel at the base and scatter them over salads, fruit, or sugar them for a sweet garnish. Historically Sweet William and its relatives turned up in herbals as a mild tonic and nervine, sometimes steeped into cordials or vinegars, though it never carried the weight of serious medicinal herbs and is best treated today as a gentle culinary flavoring rather than a remedy. The scent alone earns its keep — brush past a clump on a warm evening and you'll understand why generations of gardeners kept finding room for it.
Rust
Orange to brown raised pustules on the undersides of leaves, with yellow spotting on the upper surface. Heavy infections cause leaves to yellow and drop.
Remove and destroy infected leaves. Avoid overhead watering and improve air circulation. Apply a sulfur or copper-based fungicide if it spreads. Clear plant debris in fall.
Fusarium wilt
Sudden wilting, brown streaks in stems, yellowing leaves. Affects sweet basil varieties most.
No cure. Remove and destroy plants. Do not replant in the same spot for 3+ years. Choose resistant varieties.
Botrytis (grey mould)
Grey fuzzy mould on petals and stems, worst in cool wet conditions.
Remove affected parts immediately. Improve air circulation. Avoid overhead watering. Apply copper fungicide if severe.
Crown Rot
Lower leaves yellow and wilt; the base of the rosette turns brown and soft, sometimes with white fungal threads at the soil line. Plants collapse in wet conditions.
Remove and destroy affected plants. Improve drainage and avoid overhead watering. Do not mulch directly against the crown. Space plants for airflow and avoid replanting in the same wet spot.