The Suffolk company stopped selling impatiens in 2012 after downy mildew hit. It sees the £2.49-a-packet petunia seeds as an alternative to fill the gap in its £1m annual impatiens trade.
The petunia, which is star-shaped, was developed by T&M breeders from a plant sent by a customer.
Vegetable of the year is Tomato Rainbow (£2.99) - a baby plum in four colours.
T&M has also introduced SowStrong, a naturally coated and coloured seed for improved vigour, as well as a World Garden seed range.