If it is countable, such as bananas or carrots, then you should use the plural. A bunch of bananas. A bunch of carrots. A bunch of flowers (Think of it as similar to 'a field of cows' or 'a bag of mushrooms'.
I'm racking my brains trying to think of an example where they are not countable. A bunch of brocolli? (except we don't really talk of brocolli in bunches).