PlantSowGrow
How to grow Petunia (Grandiflora Mixed)
Petunia × atkinsiana
Large trumpet flowers for pots, baskets and bedding, flowering non-stop through summer.
At a glance: Sow late January to mid February.
When to sow, plant and harvest
Timed for a typical UK garden (last frost mid-April). The app uses your own town's frost dates.
Growing Petunia (Grandiflora Mixed): what it needs
- Light
- Full sun · 6+ hours a day
- Watering
- Water daily in summer; baskets twice in heat
- Soil
- Any well-drained soil
- Spacing
- 25 cm
- Height
- 30 cm
- In pots
- Grows well in containers
- Feeding
- High-potash liquid feed weekly
- Hardiness
- USDA zones 2–11
- Difficulty
- Intermediate
Step by step
- Late January – mid February · 12 weeks before the last spring frost
Sow Seeds Indoors
Sow in trays or modules of seed compost on a warm windowsill or in a propagator. Surface-sow the dust-fine seed and keep at 20–25°C.
- 7–14 days after “Sow Seeds Indoors” · Late January – early February
Check Germination
Seedlings should appear in 7–14 days. Keep the soil moist. If sown outdoors, thin them to 25 cm apart once they have two true leaves.
- 14–21 days after “Check Germination” · Mid February
Pot On Seedlings
When seedlings have two true leaves, move each into its own small pot or module, holding them by a leaf, not the stem.
- Late April · 11 days after the last spring frost
Harden Off
Stand plants outside by day and bring them in at night for 7–10 days so they get used to outdoor conditions.
- Early May – early June · 3 weeks after the last spring frost
Plant Out
Plant out 25 cm apart once frosts are over. Water in well.
- 76–106 days after “Check Germination” · Early May – early June
First Flowers — Deadhead Regularly
Snip off faded flowers every few days to keep new ones coming for months. Take a photo for your journal!
- Mid October – early November · around the first autumn frost
Clear or Collect Seed
When frost blackens the plants, pull them up for the compost heap. Leave a few seedheads first if you want to save seed.
