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Pretty, Easy Petunias

Petunias are easy to grow, bloom reliably all summer, and are available in a wide range of colours, flower forms, and growth habits. 

Petunias are amazingly easy to grow, both in the garden and in containers.

Petunias are amazingly easy to grow, both in the garden and in containers.

Native to South America, the first petunia specimen (Petunia multiflora) was collected by an explorer at the mouth of the Rio de la Plata and was white in colour. The original varieties were strong-growing plants that had trailing 5cm to 8cm stems and incredible scents. These scents have been lost in many of the modern-day varieties. Fortunately many of the old-fashioned varieties are still available to gardeners interested in growing petunias. The trailing types are suitable for growing in hanging baskets.



Petunia varieties can be divided into four main groups or categories based on flower and growth habits: grandiflora, multiflora, milliflora, and groundcover or spreading. 

Grandifloras have large single or double flowers. Single grandifloras produce large blooms up to 12cm across. Some single varieties have ruffled or fringed petals. Others possess a trailing habit that make them ideal for window boxes and hanging baskets.  Double grandifloras produce double, fringed flowers and are best suited to containers. Generally, grandifloras (single and double) don’t flower as heavily as multifloras. Also, the flowers don’t hold up as well during rainy weather. Plants often become unkempt and straggly by late summer. Popular grandiflora petunias include varieties in the Supercascade, Dreams, Ultra, and Storm Series. 

Multiflora petunias produce smaller flowers than the grandifloras, but in greater quantity. Both single and double varieties are available. Multifloras are generally more compact and resistant to wet weather than the grandifloras. Single multifloras are excellent in mass plantings in flower beds. Double multifloras are most suitable for containers. Excellent multiflora petunias include varieties in the Celebrity, Primetime, and Carpet Series. 

Milliflora petunias are compact, miniature plants that produce large numbers of 1- to 11⁄2- inch-diameter flowers. Their compact size makes them an excellent choice for containers and edging beds and borders. Varieties in the Fantasy Series are widely grown milliflora petunias. Spreading or groundcover petunias are vigorous, low-growing plants that spread like groundcovers. By the end of summer, some varieties may cover an area 3 to 4 feet in diameter. 

Groundcover or “spreading” petunias are only about six inches tall, but spread so rapidly that they cover a huge area over one growing season, provided they’re watered and fertilized frequently. This makes them ideal for scrambling down a hillside garden, or planting atop a retaining wall. They’re also amazing in hanging baskets and window boxes, where they trail two to three feet or more over the summer. When grown in full sunlight, they are so covered by flowers that you hardly see any foliage.


Site Preparation

Petunias require full sunlight to grow well, but will tolerate some shade. The more shade they receive, the fewer flowers they’ll produce. Soil should be average to rich and well-drained. Prior to planting work a shovelful or two of organic matter, such as compost or well-aged manure, into the soil. This helps condition the soil, which improves drainage, and will also increase the ability of lighter soils to hold water and nutrients.

How to Plant

Petunias may be grown from seeds or seedlings. If growing from seeds, sow indoors on the surface of the soil, 8 to 10 weeks prior to last frost. Containers used for starting seeds should be clean and have drainage holes in the bottom. Previously used containers should be washed in soapy water and then disinfected by dipping in a solution containing one part chlorine bleach and nine parts water. Fill the container with the germination medium to within 15mm to 25mm of the top and press lightly to firm. Moisten by partially submerging the container in water until the surface becomes wet. Allow the container to drain for 30 to 60 minutes before sowing seeds. Petunia seeds are very small and thus hard to sow. Pelleted petunia seeds are coated with a material to make them larger and easier to handle. Carefully sow the seeds on the soil surface, then gently press the seeds into the germination medium with a pencil or a small block of wood. Petunia seeds require light for germination. They should not be covered with additional material. 

Petunia seeds should germinate in 10 to 12 days. When seedlings have 3 true leaves, transplant into plastic cell packs, peat pots, or other containers. To produce stocky plants, keep the seedlings in a cool location under fluorescent lights or in a sunny window, allow the potting soil to dry between waterings, and fertilize every 2 weeks with a dilute fertilizer solution. Harden the seedlings outdoors a few days before planting into the garden.

Pinch off the top 2cm before planting to encourage good branching and transplant when the danger of frost has past. For good ground cover, space 30cm to 45cm apart. Fertilise monthly with a balanced organic fertiliser to promote healthy growth and remove spent flowers on a regular basis to extend the blooming period.

Selecting Nursery Grown Seedlings

When buying petunias for spring planting, select compact, stocky plants. Tall, spindly plants take considerably longer to recover from transplanting. Seedlings can be transplanted into the garden after the danger of frost has passed.

Care in the Garden

Too much fertiliser can cause excessive vegetative growth and sparse flowers. In high pH soils (alkaline soils) additional iron or iron sulfate may be beneficial to reduce yellowing of the foliage.

Water to a depth of 15cm when the soil becomes dry. The frequency of watering will depend on the type of soil, weather conditions and the amount of mulch. A mulch will not only reduce soil water evaporation but will also reduce splashing of water onto the lower leaves, moderate soil temperatures and reduce weed competition. Do not allow the soil around the plants to remain excessively wet for several days, as this can lead to stunted growth or disease.

Whenever feasible, it’s a good idea to remove faded flowers, including the portion below each flower where seeds will develop. This practice, called “deadheading,” encourages blooming by preventing seed maturation. Although it may not be practical to deadhead masses of petunias in the garden, its a must for flowering annuals in containers. Deadheading not only helps prolong blooming, it also keeps plants looking fresh, healthy and well-groomed.