How to Grow Petunias in Australia

Petunia (Petunia × atkinsiana, often sold as Petunia × hybrida) is a half-hardy annual in the nightshade family, Solanaceae, grown for months of flowers. It shrugs off summer heat, suits beds, pots and hanging baskets, and in mild climates can flower almost all year. The seed is as fine as dust, and the step most gardeners get wrong is burying it.

Three mistakes that matter

1. Burying the seed. Petunia seed needs light to germinate, so seed covered with mix often never comes up. Press it onto the surface of damp seed raising mix and leave it uncovered, as set out in Sowing Seeds.
2. Planting in shade. In shade petunias grow spindly and carry few flowers. Give them the sunniest bed, pot or balcony rail you have (see Site, soil and setup).
3. Watering over the flowers. A heavy spray bruises the soft petals, and flowers that stay wet in cool, damp weather invite grey mould. Water the soil at the base early in the day instead.

When to plant

The seed needs warmth to germinate, about 20 to 25°C, so it is sown in trays under cover. Start trays 8 to 12 weeks before you plan to plant out. Plant out once frosts are over and the soil holds about 16°C.

In the tropics and subtropics, time sowings so plants flower through the dry, mild months, since wet-season rain spoils the blooms.

Site, soil and setup

Choose the sunniest spot you have, with at least six hours of direct sun a day. Petunias grow in most soils as long as water drains away freely, and do best at a pH of 6.0 to 7.0.

Where you can, plant into a fresh bed or fresh potting mix each year. Grey mould survives on old plant debris and in the soil.

Container growing

Compact types about 30 cm tall, such as 'Alderman Violet', 'Fire Chief' and 'Red & White Star', suit pots, window boxes and hanging baskets. Give one plant a pot at least 20 cm wide and deep, about 5 L. Three plants fill a 25 to 30 cm pot or basket holding at least 10 L of fresh potting mix.

• Watering: baskets and small pots can need water daily in summer, so water whenever the top few centimetres feel dry.
• Feeding: water in fish and seaweed blend at 25 mL per 9 L every fortnight. From the first flowers, add liquid potash at the label rate, fortnightly.
• Position: the sunniest spot you have, out of reach of heavy rain and sprinklers that batter the flowers.

Sowing Seeds

Raise petunias in a tray under cover and transplant them, because the seed is too fine to sow straight into garden soil.

Starting under cover

• Fill a clean tray with fine seed raising mix, firm it lightly and water it well before sowing.
• Mix the seed with a pinch of dry sand, scatter it over the surface and press it in without covering it.
• Cover the tray with cling wrap pierced with a few holes, and keep it in bright, indirect light at the temperature in When to plant.
• Mist with a spray bottle to keep the surface damp; seedlings usually appear in 1 to 3 weeks.
• Remove the wrap as soon as seedlings show, and move the tray to bright light at about 16 to 18°C.

Transplanting

• Prick out seedlings (lift each one into its own cell or small pot) once they have three true leaves.
• Harden off for about a week before planting out (moving them outdoors for longer each day so they adjust to sun and wind).
• Plant out seedlings 5 to 10 cm tall, about 30 cm apart, at the depth they grew in the cell.
• Pinch off the top 2.5 cm at planting so each plant branches from low down.
• Water in with seaweed solution at half strength (15 mL per 9 L).

The varieties differ mostly in colour, but two of them need a thicker sowing.

Care by growth stage

Sun, steady water at the roots and a feed that adds potassium once buds form keep petunias flowering. Weeks count from sowing.

If you prefer a conventional feed: mix a controlled release all-purpose granular (NPK around 14-5-10) into the soil at planting, at the label rate. Water in an all-purpose soluble (NPK around 23-4-18 to 27-5-9) at the label rate fortnightly during leafy growth. From the first buds, switch to a flowering soluble (NPK around 15-4-26 to 15-5-20) at the label rate fortnightly.

Deadheading

Removing faded flowers stops seed forming, which keeps plants blooming and looking tidy.

• Pinch once: if you missed it at planting, take off the top of each plant when it reaches about 15 cm.
• Deadhead often: pinch off each faded flower with the green base below it, where the seed develops.
• Large-flowered types: 'Snowball White' needs the most attention, while many small-flowered types shed spent blooms cleanly.
• Pots and baskets first: they are closest to the eye and gain the most from regular deadheading.
• Leggy plants: when stems grow long with few flowers, cut each back by half, leaving leaves on every stem.
• End of season: pull spent plants and bin them rather than composting them (see Site, soil and setup).

Companion planting

The best partners want the same sun and free drainage, and fill the gaps around petunias in a bed or basket.

Grows well with

• Lobelia and white sweet alyssum: low, spilling plants that suit the edges of a mixed hanging basket.
• Parsley: tucked into the sides of a basket, it shares the same potting mix and watering.
• Cineraria 'Silver Dust': its silver leaves set off blue petunias in a dry summer bed.

Keep apart

• Tomatoes, capsicums and eggplants: tomato russet mite attacks all of them as well as petunias.
• Tall plants on the north side: their shade makes petunias spindly and short of flowers.

Harvesting Blooms

Cut stems whenever plants are crowded with blooms: spring to late autumn in the south, the mild, dry months in the tropics and subtropics.

• Cut each stem just above a set of leaves, which also acts as a light prune.
• Strip any leaves that would sit under the water, as they rot quickly.

Pick in the cool of the morning or evening, and stand stems in water as you cut.

Common problems

Seedlings collapse at soil level. Damping off (Pythium species and Rhizoctonia solani) rots young stems, most often in wet, cold seed raising mix. Read more

Seedlings grow tall and pale. Too little light after germination stretches them, so move the tray into brighter light straight away. Read more

Long, bare stems in midsummer. Older plants stretch and flower less as the season runs on; cut them back as set out in Deadheading.

Flowers spoil after rain. Heavy rain bruises soft petals, and large-flowered types suffer most, while multiflora types such as 'Fire Chief' cope better.

Pests and diseases

Petunia shares several pests with its relatives tomato and tobacco, and most trouble comes in hot, dry or cool, damp spells.

Native budworm (Helicoverpa punctigera). Budworm caterpillars attack many ornamentals, and can bore into buds so they fail to open or chew holes in open petals. The look-alike cotton bollworm (Helicoverpa armigera) is managed the same way. Pick caterpillars off at dusk, or spray young ones with Bacillus thuringiensis at the label rate.

Tomato russet mite (Aculops lycopersici). Lower leaves turn dull grey or yellow and die from the bottom up, fast in hot weather. The mites need a hand lens to see; spray insecticidal soap or wettable sulphur at the label rate. Use sulphur only below 24°C, and not within 3 weeks of an oil spray.

Thrips (Frankliniella occidentalis and others). Some petunia varieties draw thrips, whose feeding leaves silvery flecked scars and deformed flowers. On some petunias, thrips carrying tomato spotted wilt virus leave small dead spots where they feed; pick those leaves off. Read more

Grey mould (Botrytis cinerea). Fading flowers turn mouldy in cool, damp weather, and the fungus spreads from ageing petals. Pick off affected flowers and water at soil level.

Garden snail (Cornu aspersum) and slugs. They feed on tender leaves and shoots at night and leave slime trails. Collect them by torchlight and clear away their daytime hiding places. Read more

Prevention first

• Water and deadhead as set out in Care by growth stage and Deadheading.
• Raise your own plants from seed, which is not known to carry tomato spotted wilt virus.
• Check seedlings for mites and thrips before planting them out.
• Keep weeds down around beds: budworm moths lay eggs on weeds, and thrips breed in them.

Tips

1. Million bells (Calibrachoa) look like small petunias but belong to a different genus, and are grown from cuttings because they set little seed.
2. In hot, windy weather, shade young transplants from the midday sun for their first few days.
3. Grow scented types such as 'Red & White Star' and 'Fire Chief' beside a path or window.
4. Self-sown seedlings may not match their parents, so check your council's advice before letting petunias drop seed near bushland.