Petunias are the classic summer bedding flower, and for good reason: no other annual gives so much colour for so little money, flowering non-stop from early summer until frost in hanging baskets, window boxes, pots and beds. Modern varieties come in almost every colour, including striped, doubled and near-black blooms.
Petunias are tender and need warmth, so they are started indoors and planted out after the last frost — a petunia put out too early will sit and sulk, or be killed outright by a cold night. Once they are established in sun and fed regularly, they are remarkably tough, coping with heat and drought better than most bedding plants, and a weekly deadhead keeps them blooming for months.
Petunias at a glance
| Growing factor | What petunias need |
|---|---|
| Sun | full sun |
| Soil pH | 6.0–7.0 |
| Days to germination | 7–14 |
| Days to maturity | 70–90 |
| Spacing | 12–18 inches apart |
| Water needs | moderate |
| Cold tolerance | tender |
| Family | Solanaceae |
Starting seeds
Petunias need a head start indoors — sow under cover and transplant out only after the last frost. Start seeds indoors a few weeks before your last frost date: in Zones 3–5 that means sowing in late winter, in Zones 6–8 early spring, and in Zones 9–10 late winter to very early spring. Zones 11–13 are frost-free, so indoor sowing can happen almost any time, with the cooler months giving the strongest plants; confirm exact timing with your local extension service. Seeds germinate in 7–14 days in warm, moist seed mix. Use clean trays or modules, a fine seed mix, and a bright windowsill or grow lights — leggy seedlings are almost always a light problem, not a seed problem. Harden seedlings off for a week before planting out, moving them outside for lengthening spells each day. Our seed starting calculator works out your indoor sowing date, and the grow light calculator checks whether your setup is bright enough.
Transplanting and planting out
Transplant seedlings out after the last frost, on a cloudy day or in the evening so they do not wilt in the sun. Space plants 12–18 inches apart — that spacing is what keeps air moving and disease down — and water each plant in well. Our plant spacing calculator works out exactly how many plants fit your bed, and the square foot garden planner helps lay them out.
Soil and feeding
Petunias grow best in fertile, well-drained soil with a pH of 6.0–7.0. Dig in compost before planting — it feeds the soil and helps it hold moisture without getting soggy — and mulch after planting to keep the roots cool and the weeds down. Feed during the growing season with a balanced fertiliser at the label rate, roughly once a month while plants are actively growing, and stop feeding as the season ends. Our fertiliser calculator works out the right amount for your bed, and the compost ratio calculator helps you make good compost to dig in.
Watering
Petunias have moderate water needs: keep the soil evenly moist while plants are establishing and flowering, and water deeply rather than little and often so the roots grow down. Water at the base in the morning so the foliage dries during the day, mulch to hold moisture in, and watch for wilting in hot, dry spells. Our watering calculator gives a starting point for your bed size, and the watering schedule generator can build a routine around it.
Sunlight and temperature
Petunias need full sun — at least six hours of direct sun a day. Petunias are tender — frost kills them or damages them badly, so they are grown as warm-season plants in most zones and planted out only once frosts are over. In frost-free Zones 11–13 they can grow for much of the year. Use the frost date calculator to find your last frost date. Our sunlight calculator can check whether your spot gets enough sun.
Pests and diseases
The pests and diseases to watch for on petunias are aphids, hornworms, flea beetles, whiteflies. Aphids, whiteflies and flea beetles can all attack petunias, and hornworms may chew the leaves in some regions. Check the undersides of leaves weekly and pick off any caterpillars you find. Check plants weekly, looking under leaves as well as on top, and act early. Hand-pick larger pests, knock aphids off with a strong spray of water, and remove badly affected leaves. Keep the area weed-free and avoid wetting the foliage when you water, since damp leaves invite fungal disease. Our pest identifier helper can help identify what you are seeing, and the plant disease checker helps narrow down leaf and stem problems.
Companion plants
Good companions for petunias are lavender, marigolds, garlic, chives. Lavender, marigolds, garlic and chives are good companions: marigolds help deter soil pests, and lavender draws pollinators to the bed. Our companion planting finder can suggest more pairings for your bed plan.
Harvesting
Petunias are grown for flowers, so harvesting is really deadheading. Snip off faded flowers, including the seed pod behind them, at least once a week — a petunia allowed to set seed stops flowering. If plants get leggy and bare in midsummer, cut them back by a third and feed them: they will regrow and flower again within a few weeks. In autumn, pull plants up once frost kills them. Our harvest date calculator can estimate your picking date from your sowing date, and the vegetable yield estimator helps you work out how much a bed will produce.
Common problems and fixes
| Problem | Likely cause | Fix |
|---|---|---|
| Leggy plants with few flowers | not enough sun, or never pinched | Give full sun, pinch young plants, and cut back leggy growth in midsummer. |
| Sticky leaves and sooty mould | aphids or whiteflies | Spray pests off with water and remove badly affected leaves. |
| Holes in leaves | hornworms or flea beetles | Pick caterpillars off by hand and check plants at dusk. |
Leggy plants with few flowers
Petunias in shade stretch and flower poorly, and plants that are never pinched grow into a few long, bare stems. Pinch the tips of young plants to make them branch, and do not be afraid to cut a leggy plant back hard in midsummer — it will regrow bushier.
Sticky leaves and sooty mould
Sticky leaves mean sap-sucking insects are feeding above them, and the sticky honeydew then grows black sooty mould. Wash the pests off with a strong spray of water, check the undersides of leaves, and the plants usually recover quickly once the insects are gone.
Holes in leaves
Big ragged holes usually mean a hornworm is hiding on the plant, while tiny shot-holes are flea beetles. Hornworms are large and easy to spot once you look for them — pick them off by hand and drop them in soapy water.
Frequently asked questions
How long does it take to grow petunias?
From planting to maturity takes 70–90 days. Seed germination takes 7–14 days, and the total time depends on whether you start from seed, a nursery plant, or a bulb or cutting.
How far apart should I plant petunias?
Space petunias 12–18 inches apart. That spacing gives each plant room for air movement, which keeps disease down, and room to reach its full size without crowding its neighbours.
What soil pH do petunias need?
Petunias prefer a soil pH of 6.0–7.0. Test your soil before amending it — most garden soils are close enough, and guessing with lime or sulphur can do more harm than good.
Do petunias need deadheading?
Yes, most varieties flower far better with regular deadheading. Snip off spent flowers including the seed pod behind them, and the plant keeps producing new blooms instead of putting energy into seed.
Can petunias survive frost?
No. Petunias are tender and frost kills them. Plant out only after your last frost date, and treat them as annuals in most zones — in frost-free Zones 11–13 they can live longer.