It is hoped the venture will help raise up to £250,000 for the organisation, which runs clubs around the country to help work on community projects.
Members of its 910 clubs will sell tubers in packs of five for £10, with £3 from each sale going to the charity.T&M will dispatch the tubers direct to the customer together with growing instructions from the National Begonia Society.
T&M Young Plants managing director Paul Hansord said: "This Begonia really is something special and it's wonderful that it will be raising money for good causes."
'Sweet Charity' was named by the Lions Organisation and features large double flower heads in a mix of pastel tones. The scent incorporates hints of rose, lemon, orange, cinnamon, honey and spices. Each bloom can reach more than 15cm in diameter and lasts four to six weeks.
The new variety is a follow-up to the hugely successful 'Aromantics', which was launched in 2005.