Because you will be getting such a large dynamic range from the colour in the leaf's to the sky you will be getting the sky blown, there will be not much you can do about this, apart from exposing for the sky after and the merging the PP to get some detail back. If your shooting in Raw, you may be able to recover some of it, but i suspect not as there will be such a difference. Its a very hard subject to get right. Another option would be to try a fill flash, and try to use a faster shutter speed, combo to bring the range down.