No petunias are a half hardy annual or bedding plant, you grow from seed each year or, in the UK, buy 'plug plants'.

You can take cuttings from one plant, I buy mine as plug plants then pot them up and grow them on. I pinch out the growing tips to make the plant bush out and then I take some cuttings. They need heat to germinate or at least sealing in a poly bag and leaving on a warm windowsill, and they take quite a while. But it is definately worth a try, from one plant I usually manage to get at least 3 more.