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Petunias are generally sold and grown as half hardy annuals (will tolerate slight frost and flower in the first year from seed) but in their native countries of Brazil and Argentina they are a perennial. Petunias come in all forms and colours, from short bedding types to flat, spreading ground cover types. The flowers also come in many colours from single whites, yellow and all sorts of pinks and purples through to big showy doubles. The plants can be over wintered in Great Britain by lifting in the autumn, trimmed back to about 15cm (6in), then potted up into a well drained potting compost. The plants can then be placed on a cool 13C (55F) well lit window sill and planted out in the early summer. Petunias are sticky and will attract greenfly so keep an eye on them. Don't over water in the winter and keep them well ventilated.
How to Sow for Best Results
Petunia seed is very tiny and fragile, almost as fine as dust, and can be difficult to handle. Try to avoid crushing or touching the seed as this can cause damage. The best way to sow them is to carefully cut open the foil and then pour in a teaspoon of dry silver sand on top. Carefully shake the sand to mix it in with the seed, and then sow direct from the packet, slowly tapping it to release the mixture over the surface of the compost. Do not cover the seed with compost. This will stop the seed germinating as it must have light to break the seed's natural dormancy. Sow from February to late April. Sowing earlier will not give you bigger, better or earlier flowers. The plants will fail to germinate as the light levels are to low.
The compost should be moist, but definitely not wet. (Wet compost which can even be the case straight from the bag will be cold and reduces the oxygen that the seeds need to germinate. It may even stop the seeds germinating at all!) If the compost is too dry this will also cause problems. If this is the case, sprinkle a small amount of warm, clean tap water over the compost and leave it to soak in thoroughly. When the compost has absorbed the water, take a small handful and squeeze it in your fist; if water drips out it is too wet, so leave it for a short while in a well ventilated spot to dry off. If it holds together when you open your hand, ifs just about right. You can now fill your seed tray or pot with the moist compost and you don't have to water it again before you sow the seed.
The compost surface should be firmed and level. Alternatively to sowing the seed directly on the compost, sprinkle a small covering of Sow Lite (known as vermiculite) over the surface of the compost. Sow your seed on top of the Sow Lite. Sow Lite is a natural mineral, which when exposed to very high temperatures expands to produce a honeycomb like structure. Gardeners can benefit from this structure, as it helps to absorb excess moisture, insulates the seed and allows oxygen through the compost to the germinating seed.
After sowing, place the tray or pot in a propagator in a brightly lit place, but not in direct, sunlight. If you don't have a propagator, or in a cold year, or if you live in a cold area, wait a few 'weeks before you sow and place the tray or pot inside a clean, dear plastic bag and seal with an elastic band. This will help hold in the humidity and heat required. The results may not be as good as with a propagator as it is more difficult to maintain the critical temperature required for germination.
The soil temperature is critical for good germination of Petunias. It has to be between 20C - 27C (70F - 80F). The temperature must not fluctuate as this can cause poor or failed germination. Fl hybrids have been shown to germinate better at the higher end, at around 27C (80F). Germination should take around 10 to 21 days. Check the compost for dryness regularly. If this is the case, add a little clean water from below, being careful not to over water. Too much water can kill seedlings, as it can spread "damping off fungi", and encourage other moulds and diseases.
When the seedlings are large enough to handle, (this should be after about 5 - 6 weeks), without touching the stem, just handling the leaves, transplant them into 7.5cm (3in) pots. Grow on in a cool, frost free, well lit place, but again avoid direct sun light. To ensure a really well branched plant, pinch out the growing tip after they have rooted well into their pots. Finally, pot them on into 15cm (6in) pots or baskets, (depending on the variety, or where you want them to end up) and grow them on for a few weeks in a frost free greenhouse or similar. Gradually acclimatise them to the outside conditions by placing them in a sheltered place during the day and bring them back inside at night. Watch out for cold winds as well as pets, birds, mice etc. After all risk of frost has passed they can be left out or planted into their final flowering position for you to enjoy. Petunias love the sun so plant them in a sunny spot on well light drained soil, but don't forget to water them in hot and dry weather.
Pests and Diseases
Petunias are generally pest free but still keep an eye out for greenfly/aphids and if they become a problem, use a recommended proprietary brand of insecticide. Keep the plants regularly watered and feed with a general all-purpose fertiliser. Don't forget to deadhead and remove any damaged flowers or leaves. This will help reduce the spread of any pest or disease that may arise.