In warm, sunny areas petunias are amongst the most showy and reliable summer bedding plants. Graham Rice explains how to grow them from seed and chooses the best varieties for ideal and less than ideal conditions.
The petunia is one of the most popular summer bedding plants producing stunning displays in the sunny situations which suit it so well. Like most popular bedding plants a great deal of hybridisation has been undertaken over the years. Like the geranium it has been transformed from a perennial plant raised from cuttings and used mainly as a pot plant into a seed raised bedding plant which although not the easiest plant to raise is within the scope of most home gardeners.
Although there are about forty species growing in the wild the showy varieties that we grow now are all derived from just two perennial species which grow naturally in Argentina. Petunia axilliaris is a relatively tall, upright plant reaching about 2ft (60cm) with rather tubular, sweetly scented, white flowers with the sticky foliage which still characterises modern varieties. P. violacea, on the other hand, reaches only 12in (30cm) and has a much more prostrate habit; the flowers are broader and more open in shape and come in dark rose or violet. These have been united in P. x hybrida which amalgamates the main characteristics.
In the UK the first varieties were treated as perennials and grown as much in greenhouses as outside. This was partly because the first varieties were less weather resistant than the varieties grown today and partly because gardeners did not realise quite how happy they were outside. Cuttings were taken in late summer and over-wintered in a frost free greenhouse to provide plants for the following year. This is not a technique which should be totally ignored today. In the southern United States where the climate is rather milder they were always grown outside.
Petunias, have particular requirements which have led them to be such popular plants in the southern United States. They are primarily plants for sunshine and to ensure that they flower well sunshine really is essential. It's true that they will put up a reasonable show in half shade but give them plenty of sun and the difference is dramatic. Even though there has been a great improvement in the weather resistance in modern varieties, the flowers are still badly damaged in wet and windy weather. Weather resistance has been improved in two ways. Some varieties have petals which are inherently more resistant to rain while others solve the problem by opening new flowers very quickly after open flowers are spoilt. Even so, when the weather is wet there is still a tendency to produce foliage at the expense of flowers.
As well as a sunny site and a little shelter from the rain and wind they prefer a well drained soil but it is not always realised that petunias prefer an acid soil. This characteristic has been minimised in modern varieties through a simple selection process in which plants which do not thrive under a variety of soil conditions are excluded from breeding programmes but it's still true to a certain extent.
For flowering outside in summer, petunias are sown in the warmth in spring. To produce bushy plants to set out in late spring or early summer, sow in early spring. Choose a peat based seed sowing compost and if it is one with no or very little drainage material add some extra sharp sand, perlite or vermiculite.
The temperature for germination should be between 65 and 75F (18-24C) at seed level and this can usually be provided in a heated propagator or, if this is not available, in a warm room in the house. A room which becomes cool at night should not be chosen.
It is important to sow thinly and not to cover the seed. Some varieties need light to germinate and even a thin covering of compost can severely reduce germination. The lack of a compost covering necessitates very careful monitoring of the moisture in the compost for if the surface of the compost dries out the young seedlings will quickly die. This is best achieved by covering the seed pots with polythene or glass and a sheet of newspaper to reflect strong light, so the surface of the compost does not heat up too much, but some light still penetrates.
If sown thinly the seedlings can be left in the seed pot until two true leaves can be seen and the seedlings will suffer less of a check if moved at this stage. To ensure that seedlings have enough space in the early stages it may pay to use a seed tray when you might otherwise use a pot.
After pricking out the temperature is important. Temperatures below 50F (10C) discourage growth of the main central shoot and encourage the development of side shoots from low down on the plant. Unfortunately this also delays the appearance of the first flowers. At temperatures above 65F (15C) basal branching is restricted, the main stem grows more quickly and flowering is hastened. Although garden centres like to sell plants in flower, this does not matter to the home gardener so by sowing in early spring and keeping the temperature after pricking out cool, well branched plants should be produced which will flower more effectively when planted out. The length of daylight also influences flowering time and basal branching but the control of day length using artificial lighting and blackouts is beyond the scope of the amateur.
When the rosettes of foliage cover the compost the trays can be moved from the greenhouse to frames and grown cool. As long as the plants are frost free they are happy. Although they are not as hardy as their relatives the nicotianas, they are tougher than many people think. They can be planted out as soon as the last frost has passed.
The main problem with petunias during this stage is damping off.
In the garden petunias can be used in beds and borders, tubs, window boxes and hanging baskets and every year more new varieties appear. These are usually divided into two groups although the distinction is becoming more and more blurred. The grandifloras tend to have the larger flowers and need ideal conditions to make the best show - weather resistance is not their strong point. In the grandifloras the leafy segments behind the flower that make up the bud are large and rounded. The multifloras have rather smaller flowers in larger quantities, are a little more compact in growth and tend to put up with bad weather rather better. The bud segments are smaller and pointed.
This is a difficult business because there are so many varieties and series of varieties and each seed company has its own favourite which it recommends above all others.
'Resisto' series The 'Resisto' multifloras are varieties on which most people agree. The series contains at least eight colours but the blue and the rose are especially good. All are weather resistant but the rose is exceptional and is probably one of the best of all bedding plants.
'Pearl' series The aim here is for smaller plants with smaller flowers but far more of them. They do very well in wet weather and the soft blue is especially impressive and they are good when grown as pot plants.
'Prio' series A weather resistant multiflora with large flowers bred in Holland for northern European conditions.
'Plum Pudding' Known as 'Plum Crazy' in America this is a mixture of colours - mainly purples, pinks, lilacs and yellow all with darker veins.
'Summer Sun' Yellow petunias are never quite as successful as the other colours unless they have ideal conditions but this is the best of the few yellow multifloras.
'Mirage Velvet' Large flowers in an unusual deep wine shade which last summer proved amongst the most weather resistant of all.
'Orange Bells' A very soft and subdued orange with a white throat - it may not sound exciting but it is a unique shade.
'Cloud' series Wonderful in sunny conditions with large flowers up to 4½in (11.5cm) across in many colours including two starred types.
'Express' series One of the best grandifloras for weather resistance there are 13 colours including starred types and picotees.
'Picotee' series The most brilliant and most stable of the picotee types with four colours - red, rose, blue and purple ('velvet') each with a clear white rim.
'Daddy' series The grandiflora equivalent of 'Plum Pudding' in various pinks and purples with dark veins. Above average weather resistance.
'Razzle Dazzle' A mixture of colours, each flower has a white star marking for extra brilliance.
'Recoverer White' Quite simply the best white petunia, with large flowers and plenty of them.
'Super cascade Lilac' A relatively new variety with big soft lilac flowers and weather resistance up with the best.
'Chiffon Magic' A delightful soft, slightly lilac pink which is so delicate that if planted where it gets the sun all day it will fade almost to white. The one variety for which a little shade is helpful.
'Yellow Magic' A second colour from the 'Magic' series and the best yellow grandiflora. The 'Magic' series is a good one and overall is almost on a par with the 'Cloud' and 'Express' series.
The picotee types can be rather unstable and although the 'Picotee' series is better than most, this too can occasionally be unstable in difficult conditions. In wet conditions with low temperatures and when the plants are in especially rich conditions the white picotee edge will be very narrow or fade away altogether. In very dry conditions when the plants are underfed the white picotee edge becomes very broad indeed. Sometimes reducing the main colour to a starry eye.
I mentioned at the beginning that petunias are naturally perennial plants and this means they can be raised from cuttings. So if you fancy a few winter pot plants, you can take cuttings of your favourite colours from the garden in early autumn. The cuttings should be 3-5in (7.5-12.5cm) long and they root easily straight into potting compost. Pot them into 3in (7.5cm) pots and then 5in (12.5cm) pots of potting compost with extra grit or perlite and keep a minimum temperature of about 50F (10C). By using this method you will get substantial plants for the winter far more quickly than by sowing seed in the winter.
Please note: Some varieties mentioned may no longer be available.
Graham Rice is Editor of Growing from Seed and author of A Handbook of Annuals and Bedding Plants (Croom/Helm/Timber Press).
Source of article
Growing From Seed - Spring 1987 Vol. 1 Number 2
© The Seed Raising Journal from Thompson & Morgan