Petunia Juss., 1803 nom. cons.
Petunia is a widely-cultivated genus of flowering plants of South American origin, closely related with tobacco, cape gooseberries, tomatoes, deadly nightshades, potatoes and chili peppers; in the family Solanaceae. The popular flower derived its name from French, which took the word petun, meaning "tobacco," from a Tupi-Guarani language. Most of the varieties seen in gardens are hybrids (Petunia × hybrida).
The origin of P. × hybrida is thought to be by hybridisation between P. axillaris (the large white or night-scented petunia) and P. integrifolia (the violet-flowered petunia). P. axillaris bears night-fragrant, buff-white blossoms with long, thin tubes and somewhat flattened openings. The species was first sent from South America to Paris in 1823. P. integrifolia has a somewhat weedy habit, spreading stems with upright tips, and small lavender to purple flowers. It was discovered in South America by the explorer James Tweedie, after whom the genus Tweedia is named, who sent specimens to the Glasgow Botanical Garden in 1831. Many open-pollinated species are also gaining popularity in the home garden. A wide range of flower colours, sizes, and plant architectures are available in both the hybrid and open-pollinated species.
Some botanists place the plants of the genus Calibrachoa in the genus Petunia. Botanically speaking, tobacco, tomato, potato, and petunia are all in the family Solanaceae.
Petunias are generally insect pollinated with the exception of P. exserta, which is a rare, red-flowered, hummingbird pollinated species. Most petunias are diploid with 14 chromosomes and are infertile with other petunia species.
The foliage of Petunias is sometimes eaten by the larvae of some Lepidoptera species including Dot Moth and Hummingbird hawk moth.
This type of petunias has the largest flowers, up to 4 inches in diameter. Of all the petunias these have the widest variety of forms and colors but are the most likely to be damaged by heavy rain. There are four types of grandiflora and they are classified by their colours, namely, ‘Daddy Series’ (shades of pink and purple), ‘Merlin Blue Morn’ (blue and white), ‘Supercascade Series’ (many colours) and ‘Ultra Series’ (many colours including bi-colour).
Hedgiflora or spreading petunias (sometimes called ground-cover) are characterised by their low height (usually about six inches), but a large spread (about three to four feet). They will cover a large area provided they have adequate water and fertilisation. ‘Purple Wave’ was the first introduced cultivar of spreading petunias and grows to a height of 4 inches. ‘Tidal Wave’ is another spreading type of petunia, but is much taller (between sixteen and twenty two inches). ‘Opera Supreme’ is a cultivar with large flowers.
Multiflora compared with grandiflora are half the size of 2 inches in diameter, are not easily damaged in heavy rain and can tolerate more sun. Multiflora petunias cultivars include: 'Carpet Series "(many colors),' Surfinia Series' (pink, blue, purple and white), and 'Wave Series' (pink, coral and purple). They spread quickly and are ideal for baskets. 
Milliflora are the smallest of the petunias and about 1 inch across. These are prettiest when mixed with other plants in containers, along garden beds, and edges. Milliflora are available in 'Fantasy Series' (red, purple, pink) and are the easiest to find. 'Supertunia Mini Series' (blue, pink, lilac, purple and white) are also available in the milliflora category. They tolerate harsh weather better when compared with grandifloras and multifloras.
1. ^ Winterrowd, Wayne. Annuals and Tender Plants for North American Gardens. New York. Random House. 2004. Print.
Source: Wikispecies, Wikipedia: All text is available under the terms of the GNU Free Documentation License