An age old question!
We were asked this at a market a few weeks ago. I wasn't quite sure how to answer, because it doesn't really have an answer. Anything can technically be pressed.
You can press a hydrangea by preserving the petals individually and then piecing it back together in a flat form shape.
You can press a rose by removing some of the far outer, and tiny inner petals.
You can press bluebells if you've patience.
We don't have the patience (well, Emma doesn't) , or the time to press the traditional way. We use a combination of pressing and silica gel to press our flowers. Our exact method varies bloom by bloom but has been tested over hundreds, if not thousands of flowers by now, and very very rarely lets us down. If it does let us down, it's because it's a flower we've not tried before.
Flowers that don't traditionally press well in a book would include calla lillies, orchids, and chrysanthemums. However, we can press and preserve the colour using our method.
One of the things that's important to note is that flowers bruise easily - particularly white flowers. Bruises generally wont show up until you put them in resin. We will always tell brides with white flowers that they may see some bruising on the flowers. Rather than detracting from it, to us, it's a sign that the bride has held the flowers whilst she's hugged someone, or put them down to dance - these aren't bad things! A bruise on a white wedding rose should hold a memory - not be seen as a blemish. Bruising and oils can also be caused by mis-handling of the flowers by your botanical artist - we take all precautions to ensure this doesn't happen, your flowers are treated with the upmost care.