I'm a beginner decorator. I made up my buttercream in advance of the cake so I can practice making roses. The buttercream was a bit too soft and the roses were "melting" so I added confectioners sugar to stiffen. It did help. But now, my question is, how do I know if the icing is stiff enough to withstand the roses. It seems stiff enough now, but how will I know that my roses won't wilt on the cake after a couple of hours? I'm going to be keeping the cake in the refrigerator until it's ready to present. (12 x 18 sheet cake).

You'll know while you're making them. If it isn't stiff enough, the petals will droop as you pipe (not later). I make a rose, and if I'm not happy with it, the pastry bag gets squeezed out back into the mixer for more powdered sugar to be added. You'll know if it's stiff enough when the roses look good.

If you make your roses in advance to let them air dry, they'll hold their shape. If they're slightly droopy when you put them in the refrigerator, they'll return to the same droopy roses that they were when you take them out. Air drying several days before you need them makes them so much easier to place where you'd like without messing them up with scissors and such.

Thank you Unlimited. That really does help. I'm not baking the cake til Monday and I'll be freezing it until I decorate it on Friday or Saturday. So that gives me time to keep making roses until I'm satisfied with them.

This is a recipe I was given by a cake decorator for making roses. These roses will air dry in two to three days or you can use them right away.
2 pounds sifted powdered sugar
1 1/2 cups shortening
1/2 cup cake flour, sifted into the sugar
1/3 cup boiling water
flavoring if desired
Cream the shortening well, add the sugar all at once and the 1/3 cup water. Mix on low scraping the bowl until smooth. Of icing is to stiff, add additional water 1 teaspoon at a time. Let the icing sit for a few hours and remix before piping flowers.