When to plant Petunia in Subtropical Australia
Subtropical zone — applies to Brisbane, Gold Coast, Sunshine Coast, Coffs Harbour, Lismore and similar areas. Growing elsewhere? Temperate · Cool · Tropical · Arid.
Raise petunia seedlings from May to September, sowing the tiny seed on the surface or barely 3 mm deep and pressing it into moist mix rather than burying it — light aids germination. In subtropical gardens around Brisbane and the Sunshine Coast, this late-autumn-to-early-spring window sets plants up to flower before the summer wet. Seed germinates in 7–14 days with warmth. Transplant seedlings 25–40 cm apart in rows 25–30 cm apart once they can be handled. Expect the first blooms about 80–90 days from sowing, then a long run of colour.
Sowing calendar — subtropical zone
Green months are your sowing window: May to September.
Growing notes
Petunias are grown as a warm-season bedding flower, but in the humid subtropics the sweet spot is the drier, milder stretch from late autumn into spring. Sow under cover, plant out as things warm, and you will get months of colour before the summer downpours arrive.
The main failure mode here is the wet season itself. Heavy rain batters the soft trumpet flowers to mush and drives grey mould (botrytis) through crowded plantings, so once the reliable summer storms set in, petunias tend to look ragged. Plant them somewhere with good airflow and morning sun, and consider pots or hanging baskets under eaves around the Gold Coast and Coffs Harbour where you can shelter them from the worst of the rain.
Getting the seed up is the fiddly part. It is dust-fine and needs light to germinate, so sow onto the surface, mist rather than drench, and keep it warm. Once growing, feed regularly with a balanced liquid fertiliser, as petunias are hungry and flower far better for it. Pinch young plants back to encourage bushiness rather than leggy stems, and deadhead or trim spent growth to keep the flush going. Watch for budworm caterpillars chewing the buds through the warmer months.
Petunia at a glance
| Sowing depth | 3 mm |
|---|---|
| Plant spacing | 25-40 cm |
| Row spacing | 25-30 cm |
| Germination | 7-14 days |
| Days to maturity | 80-90 days |
| Frost tolerance | Half Hardy |
| Method | Plant Seedlings |
Common questions
- When should I sow petunias in the subtropics?
- Raise seedlings from May to September, through late autumn into early spring. This gives plants time to establish and flower before the humid summer wet, which is hard on petunias. Transplant them out once frosts have passed and the weather is warming.
- Why is my petunia seed not coming up?
- Petunia seed is extremely fine and needs light to germinate, so it should sit on the surface, not be buried. Sow onto moist mix, press it in gently, and keep it warm and evenly moist. Covering the seed with soil is the usual reason it fails.
- How do I stop my petunias getting leggy?
- Pinch out the growing tips of young plants to encourage branching from the base. Feed regularly and give them full sun, as shade also causes stretching. Trimming straggly plants back through the season prompts fresh, bushy growth and more flowers.
- Do petunias survive the wet season in Brisbane?
- They struggle. Heavy summer rain damages the flowers and encourages grey mould in crowded plantings. Grow them in the drier cooler months, give them airflow and morning sun, or plant in baskets under eaves where you can shelter them from downpours.
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