What type of begonia did you buy ? foliage begonias or flowering begonias ?

I grow tuberous begonias, and I like to use a diluted mix of miracle grow and/or peters, such as 1/2 tsp miracle grow or 1/2 tsp peters with every 2nd watering.

Another option, you can use a light application of OSMOCOTE 14-14-14 (multicote 14-14-16, etc). If the normal application for outdoor plants is 1 tbs, then use just 1 tsp instead. With flowering begonias, make sure the nitrogen is not higher than the two other numbers.