I'm making a wedding cake for the 24th decorating it with fresh flowers
my question is can I use flowers from the supermarket or should they be some special kind just for cakes.

If the flowers are going to touch the cake in any way, they need to be:
1. non-toxic
2. not sprayed with pesticides!

The type of flower will determine if it's toxic or not ...

The grocery store flowers will definately have pesticides. You need to find a florist who sells organically grown flowers which are pesticide-free.

Thank you for your quick response yes I was thinking on putting them on the cake I will try to get non toxic or re think my decoration.

My local grocery store does, in fact, carry organic edible flowers located in the produce section. Check your store--even if they don't carry them normally, they may be able to order them for you. Also, the local gourmet store carries organic edible flowers and is fantastic with special orders.