When to plant Lobelia in Tropical Australia
Tropical zone — applies to Darwin, Cairns, Townsville, Broome, Mackay and similar areas. Growing elsewhere? Temperate · Cool · Subtropical · Arid.
Raise lobelia as seedlings from February to May, surface-sowing the dust-fine seed no more than 1 mm deep and pressing it onto damp mix rather than burying it. Germination takes 14 to 21 days in warmth, and plants reach flowering size in roughly 100 to 120 days. Transplant the tiny clumps out at 15 cm spacing, rows 15 to 20 cm apart, once they can be handled. In the tropical north lobelia is a cool-dry-season annual: it dislikes the heat and humidity of the Darwin and Cairns wet, so the drier months from late summer onwards give you the trailing blue edging it is prized for.
Sowing calendar — tropical zone
Green months are your sowing window: February to May.
Growing notes
Lobelia is a cool-climate favourite that only earns its place in the tropics during the milder dry season. Sow into seed trays rather than direct, because the seed is close to dust and washes away or buries too deep in open ground. Scatter it thinly on the surface, mist to settle it, and keep the tray bright and evenly moist until the threadlike seedlings appear.
Prick out in small clumps rather than single plants; lobelia sulks when its roots are disturbed alone. Grow on in a spot with morning sun and afternoon shade, which around Townsville and Cairns keeps the plants from cooking.
The common failure here is heat collapse and mildew. Once daytime temperatures climb and humidity builds, lobelia stops flowering, yellows and can rot at the base. The fix is to grow it strictly through the coolest, driest part of the year, water at the roots rather than over the foliage, and give each plant enough space for air to move. Treat it as a short-lived seasonal edging or hanging-basket filler rather than a long-term performer, and pull it out once the wet returns and the display fades.
Lobelia at a glance
| Sowing depth | 1 mm |
|---|---|
| Plant spacing | 15 cm |
| Row spacing | 15-20 cm |
| Germination | 14-21 days |
| Days to maturity | 100-120 days |
| Frost tolerance | Half Hardy |
| Method | Plant Seedlings |
Common questions
- Does lobelia grow in tropical Queensland?
- Only as a dry-season annual. Lobelia struggles in the heat and humidity of a Cairns or Townsville summer, so grow it through the cooler, drier months from February onwards and expect it to fade once the wet returns.
- How do you sow lobelia seed?
- Surface-sow the fine seed onto damp seed-raising mix no deeper than 1 mm and do not cover it, as it needs light to germinate. Mist to settle it and keep the tray bright and evenly moist for the 14 to 21 days it takes to sprout.
- Why is my lobelia dying in summer?
- Tropical summer heat and humidity are the usual culprits, causing the plants to yellow, stop flowering and rot at the base. Grow lobelia only in the cool dry season, water at the roots and give it airflow to slow mildew.
- How far apart should lobelia be planted?
- Transplant seedling clumps about 15 cm apart with rows 15 to 20 cm apart. This lets the low, trailing plants knit together as edging while keeping enough air between them to reduce fungal problems.
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