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Petunias are tender perennials grown as annuals almost everywhere, and their season runs from an indoor sowing ten to twelve weeks before the last frost to the first frost of autumn. The long lead time is the thing that catches people out; petunia seedlings are slower than almost any other bedding plant for the first month, and a sowing made at the same time as marigolds or zinnias produces plants that are still tiny when everything else is ready to go out.
The other structural point is that there are two distinct halves to the summer. In the first half the job is to build a plant: pinching, potting up, establishing roots. In the second half the job is to keep it flowering: feeding weekly, deadheading if the type needs it, and cutting back once when the middle goes bare. Treating the second half like the first, or the first like the second, is where most seasons go wrong.
| Stage | Timing | Main jobs |
|---|---|---|
| Sowing | Ten to twelve weeks before last frost | Surface sow on damp compost, light and 75F (24C) |
| Transplanting seedlings | Two to three weeks after germination | Move to plugs or small pots at the first true leaves |
| Growing on | Four to eight weeks after sowing | Cooler and brighter, first pinch, steady half strength feed |
| Hardening off | Two weeks before planting out | Increasing outdoor exposure, no frost |
| Planting out | After the last frost, nights above 50F (10C) | Plant, water in, pinch again three weeks later |
| Early summer | Plants filling out | Weekly feed begins, tie in or spread trailing stems |
| Midsummer | Middle of the plant going bare | Cut back by a third, feed hard, water daily |
| Late summer | Regrowth flowering | Deadhead, keep feeding, watch for budworm |
| Autumn | Cool nights, shorter days | Stop cutting back; take cuttings if overwintering |
| First frost | End of the season | Clear out; do not reuse basket compost |
Late winter and early spring: sowing and transplanting seedlings
Sow indoors ten to twelve weeks before your last expected frost. The seed is dust fine and needs light to germinate, so it is scattered on the surface of damp compost and never covered. Held at 75F (24C) it comes up in seven to ten days. Cover the tray to keep humidity up until germination, then take the cover off immediately, because petunia seedlings damp off readily in still saturated air.
The first three weeks after germination are the slow part. The seedlings sit at the size of a pinhead and appear to do nothing, and the temptation is to feed them or to conclude the batch has failed. Neither helps. Keep them bright, keep them just moist, and they take off in week four.
Transplant at the first true leaves, handling the plants by a leaf and never by the stem, which is thin enough to crush between two fingers without noticing. From that point drop the temperature to around 60F (16C) and give as much light as you have. Warm and dark is what produces the drawn pale seedlings that never make a decent plant.
Spring: hardening off and planting out
Petunias are frost tender at every stage, so nothing goes out permanently until the last frost has passed and nights are reliably above 50F (10C). Plants put out into cold soil sit and go purple in the leaf rather than growing, and stay checked for weeks after the weather improves.
Harden off over ten to fourteen days: a sheltered spot for a couple of hours on the first day, building up to full days outside and then a night out, before planting. Skipping this step scorches the foliage of a plant grown soft under cover within one sunny afternoon.
Space bedding types eight to twelve inches (20 to 30 cm) apart in the ground. In a basket, three to five plants in a twelve inch (30 cm) basket is right, and crowding in more gives you a spectacular six weeks and then a congested plant with gray mold in the middle. Pinch at planting and again three weeks later, and water every two or three days for the first fortnight while the roots move out into the new compost.
Record your planting out date, the number of plants per basket and the position of each basket. When one hook consistently produces a better display than another, it is the position, and that is the single most useful thing your notes will tell you next season.
Summer: feeding, cutting back and the second flush
Weekly feeding starts about six weeks after planting, when the compost is exhausted, and it continues without a break to the end of the season. Water daily in a basket in hot weather, judging by the weight of the pot rather than the surface of the compost. Deadhead the large flowered types twice a week; the trailing types drop their own flowers and need none.
Somewhere in the middle of summer the plants will go bare in the center with flowers only at the tips. That is the moment to cut back by a third, feed hard and accept two to three weeks with fewer flowers. Plants cut back at this point flower well into autumn; plants left uncut do not recover and drift downhill from there.
Budworm damage typically appears in the second half of summer, showing as small round holes in unopened buds and dark grains of frass on the leaves below them. Check the shoot tips weekly through this period, because it is much easier to deal with at the start of an infestation than after the plant has stopped flowering.
Autumn: winding down and carrying plants over
Stop making hard cuts within six weeks of your first expected frost, because regrowth at that point is too slow to flower before the plant is killed. Keep feeding and deadheading instead and let the display run out naturally. Cool nights actually improve the color of petunia flowers, and a well fed plant often looks better in early autumn than it did in the heat of midsummer.
To carry a plant over, take four inch (10 cm) cuttings from clean non-flowering shoot tips before the first frost, strip the lower leaves, and root them in damp gritty potting mix on a bright sill above 65F (18C). They root in two to three weeks. Overwinter the young plants frost free and just moist, and pinch them once in late winter to build a branched plant for spring.
After the frost, clear the plants and tip out the container compost rather than reusing it. A season of heavy feeding in a basket leaves compost that is exhausted, compacted and often carrying the gray mold that was in the plant, and starting the next season in it costs more than a bag of fresh compost saves.