Petunias: characteristics, varieties and forms of cultivation

Petunias bring long-lasting colour to sunny gardens, terraces and balconies. Compact, trailing and large- or small-flowered cultivars suit different spaces, so it is worth choosing a plant for your climate and growing conditions rather than for flower colour alone.

Petunias

The genus Petunia belongs to the nightshade family (Solanaceae) and is native to South America. Most ornamental garden petunias are the result of horticultural hybridisation and selection. Their relationship to tobacco and other nightshades does not imply that they have the same uses.

Wild Petunia species originate in South America. Many familiar garden petunias belong to the hybrid complex Petunia × atkinsiana (synonym P. × hybrida). These are herbaceous plants that may be perennial in frost-free climates but are usually grown as annual bedding plants in colder regions. Depending on the cultivar, flowers may be single or double, plain or ruffled, and come in many colours and patterns. Given enough light and suitable care, many varieties flower from late spring into autumn. Their simple, often oval leaves can feel slightly sticky.

Care and propagation

For abundant flowers, grow petunias in a bright position with several hours of direct sun. In exceptionally hot climates, some shade during the hottest part of the afternoon can help. Use fertile, well-drained soil or potting mix, and make sure containers have drainage holes. Space plants according to the cultivar’s mature spread: trailing varieties need more room than compact ones. Shelter blooms from prolonged heavy rain and remove faded flowers where appropriate; deadheading is not a substitute for disease control.

Petunias are well suited to window boxes and hanging baskets. If stems become excessively long and flowering declines, trimming them back moderately may encourage new branching; trailing stems do not need cutting simply because they hang down. Water when the top layer of potting mix begins to dry, adjusting for heat, wind and container size. Avoid waterlogging and, where practical, wetting the blooms. Feed with a balanced fertiliser according to the label rather than automatically adding extra phosphorus. Petunias can be grown from seed, and some cultivars can also be propagated by cuttings. Protect young plants from frost. Most wild species are insect-pollinated, while the red-flowered Petunia exserta is adapted to hummingbird pollination.

Types of petunias

Multiflora, miniflora and grandiflora are horticultural categories based mainly on flower size and abundance, not separate botanical species.

Multiflora petunias

These produce many relatively small flowers, often around 5 cm across. Many cultivars cope better with rain than large-flowered types, but petunias remain vulnerable to frost. They are useful for colourful bedding displays and containers.

Miniflora petunias

Miniflora petunias are a commercial group with numerous small flowers, suited to containers and compact spaces. They are not a wild species from a particular region. Their growth habit, colours and ultimate size depend on the cultivar.

Grandiflora petunias

Grandiflora petunias have large flowers, often 8–10 cm across or larger depending on the cultivar. Their petals may suffer in heavy rain, so a somewhat sheltered position is helpful. Choose a container large enough for a mature plant: a 10 cm pot is generally too small for long-term growth. Flowers may be plain, veined, striped, bicoloured or ruffled; white bands are not a defining feature of the whole group.

Petunia integrifolia

Petunia integrifolia is a wild South American species, usually with violet or purple flowers, rather than a commercial flower-size category.

Hybrid garden petunias

Petunia × atkinsiana is the accepted botanical name for a garden hybrid complex; P. × hybrida is a synonym. Its breeding history is complex and species such as P. axillaris and P. inflata have been implicated. The epithet ‘hybrida’ is not a scientific label for every hybrid in the genus.

Uses

Petunias are mainly grown in flower beds, patio containers, window boxes and hanging baskets. They are generally unsuitable for dimly lit indoor positions.

Flower symbolism

In the traditional language of flowers, petunias have been given different emotional meanings in different settings. Such associations are cultural and symbolic, not botanical characteristics or universal interpretations.

Pests and diseases

Aphids, whiteflies, spider mites and slugs may damage leaves or tender shoots. Root-feeding nematodes can be a problem in some conditions but should not be assumed without evidence. Inspect new growth and leaf undersides before choosing a treatment, improve airflow and avoid unnecessary broad-spectrum pesticides. Read more about common garden pests here.

Waterlogged roots and poor airflow can contribute to rots and grey mould (Botrytis). Water according to the condition of the potting mix, remove diseased plant material and avoid keeping roots constantly saturated. If spotting, persistent wilting or distorted growth occurs, identify the cause before treating: viruses, heat stress and nutrient imbalances can also be involved. With sufficient sun and good drainage, petunias can flower for an extended season.

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Information sources: The Royal Horticultural Society (2004). Nueva Enciclopedia de Plantas y Flores. Ed. Grijalbo. Geoff Burnie, Sue Forrester, Denise Greig (2003). Botánica, Guía Ilustrada de Plantas. Ed. Könemann.