If you add corn starch in you moisture before the liquids.
Although I live in an area which is very hot and humid in summer, we have air conditioning, so it is not a great issue. Over the years, I have noticed that some brands hold up to heat and humidity better than others. The one I’ve found to be the strongest, longest lasting, is Nicholas Lodge’s homemade gum paste. It is easy and inexpensive to make and lasts a really long time in the refrigerator. Though not as realistic, in summer, I like to leave the petals of my flowers a little thicker, which helps to resist the heat and humidity.
Sugar Sugar by SSmiley
I make my own florist paste using Maggie Austin’s recipe (not sticky like Squires and doesn’t flop as easily). I make and arrange my flowers around a month ahead of the order date so they are dry when decorating the cake. I store them in cardboard cake boxes filled with biodegradable maize packaging peanuts. A disposable nappy also helps to absorb any moisture. For my display cakes I store them in a small ground floor alcove cupboard which happens to have a central heating pipe in which keeps the air dry. UK summer heat is mostly humid rather than dry. I made the mistake of storing a display cake on the first floor once and the flowers all flopped and drooped within a day. x