Petunia

Light and exposure

Petunias want sun, plenty of it. Give them the brightest spot on the balcony or terrace, where the midday sun reaches them unobstructed, with at least six hours of direct light a day. In shade, or even partial shade, they get leggy, flower poorly and soon look weak.

This holds just as true for classic petunias in a window box as for trailing surfinias in a hanging basket. A spot that feels sheltered and cosy can be a false economy if it lacks light, the plant pays for it in flowers. Wind itself is not a problem for petunias, unlike heavy rain, which batters the flowers.

Watering

In a pot or hanging basket, petunias get seriously thirsty once the warm weather sets in. The volume of soil is small, the roots heat up fast, and the plant loses a lot of moisture through its many flowers and dense foliage. In high summer, watering every day, sometimes twice a day for a basket in full sun all day long, is nothing out of the ordinary.

Water at the base, keeping the flowers dry, since they dislike standing moisture and brown easily. A petunia that wilts by the end of the day needs water right away, not tomorrow: with this plant, drought is punished fast, with dropped buds and a sharp drop in flowering.

Soil and repotting

A rich, free-draining potting mix works well, whether the plant is in open ground, a window box or a hanging basket. A good compost for flowering plants, lightened with a little perlite for baskets, gives the best results.

Petunias are heavy feeders. In a pot, the nutrients in the compost run out within a few weeks, the plant grows and flowers that fast. A liquid feed for flowering plants every one to two weeks through the whole growing season prevents yellowing foliage and keeps the flowers coming generously right into autumn.

Temperature and humidity

Petunias cannot take frost and will not forgive a single night below zero. Anywhere winters drop below freezing they are frost-tender through and through: grown as annuals, planted out in the garden or in containers once the frost risk has passed, and gone again with the first cold of the cold season.

Never plant petunias outside before your average last frost date has gone by. A few mild weeks at the end of winter guarantee nothing: a late frost can still strike and wipe out overnight plants bought too early from the garden centre. If you do buy them ahead, keep them under cover and bring them in on risky nights.

Pruning and upkeep

Classic petunias, with their large single or double flowers, need their spent blooms removed regularly and their trailing stems pinched back. This simple, repeated task encourages branching and clearly extends the flowering right through summer.

Many surfinias and modern varieties sold today are described as self-cleaning: the spent flowers drop off on their own without setting seed or rotting. They need less upkeep, but any petunia, classic or surfinia, that gets leggy and worn out mid-summer can be cut back by a good third without hesitation: it comes back within a few weeks with flowering denser than before.

Pests and diseases

Under a humid summer, grey mould (botrytis) and various leaf spots go after petunias, especially when the foliage stays wet for long stretches or plants are crowded together with poor air circulation. Space out the containers, avoid watering over the foliage, and remove damaged flowers and leaves without delay.

Aphids happily settle on young shoots and buds, especially early in the season. Chlorosis, recognisable by yellowing between the leaf veins, is almost always a nutrient deficiency rather than a true disease: a proper feed sorts it out within a few days.

Propagation

Sow indoors, in the warmth, ten to twelve weeks before your last frost date, so as to have plants already in flower at planting time. Petunia seeds are tiny and need light to germinate: sow them on the surface, uncovered, on fine, moist compost. Germination takes one to two weeks at a temperature of 20 to 25 degrees.

Cuttings also work very well from an established plant: take a ten-centimetre, flower-free tip cutting, remove the lower leaves and pot it straight into a small pot of light, moist compost under a propagator lid. One thing to note: many shop-bought surfinias are protected varieties, and propagating them is reserved for the licensed grower.

Month-by-month care calendar

Green Hub tips

• Never plant petunias outside before your last frosts have gone by: a few mild weeks at the end of winter are a trap, a late frost often comes back right afterwards.
• In a window box or hanging basket, water and feed generously all summer: a hungry or thirsty petunia turns yellow and stops flowering outright.
• Deadhead the classic varieties regularly, or pick a self-cleaning surfinia from the start if you want less upkeep.
• Give containers a little shelter during storms: heavy rain damages petunia flowers within hours.