Petunias are popular for adding a splash of winter colour but if you sow them in the garden, frost can kill seedlings as they emerge. You can nurture the seedlings by growing them indoors until they’re ready to be planted out.
To provide the required warmth, shelter and moisture the easiest option is to use a mini greenhouse. This features a tray with a lid that has vents so you can control airflow.
For successful germination the soil temperature needs to be 16°C which is easily checked with a thermometer.
Sow the seeds in the tray and use a spray bottle to mist the potting mix with water, creating humidity.
Replace the lid and position the mini greenhouse in a spot with lots of light but no direct sun.
TIP Some greenhouses are called propagation units and have a built-in heater to regulate the soil temperature.
Plant the seedlings
Once your seeds are ready to germinate it's time to plant them. Taking petunias from germination to garden bed takes about six weeks.
When the seedlings emerge you can remove the mini greenhouse cover and let the growing medium dry out slightly between waterings.
After about 14 to 21 days the seedlings can be transplanted to pots or a windowbox. Lift them carefully out of the growing medium with a stick, being careful not to disturb the roots or let them dry out.
The seedlings can be put outside on warm days to acclimatise but make sure they’re protected from direct sun and wind, bringing them back inside at night.
To plant them out in the garden, choose a spot with good drainage that gets plenty of light but not harsh afternoon sun and has good airflow.
Position petunias about 300mm apart and keep the area weed-free.
How to Grow Petunias in a Mini Greenhouse
Petunias like a moist, well-drained growing medium and the seeds don’t like to be covered in too much soil, so we took the seed cell tray out of the greenhouse and used the base for sowing. TIP The seedlings should emerge within a week.
Water in seeds by misting the mix with water using a spray bottle to settle the seeds in position, covering the growing tray with the lid of the mini greenhouse to keep the humidity high.