Although the tests are hurried, I am not entirely sure why it is "easiest" for Cynthia to manifest her powers as... flowers. Technically she should be able to change any matter into any other state of matter, or energy even! And not just change things, but create things and destroy things ABSOLUTELY.
And, I believe she can, with time. But for right now there are re-occurring flower petals emerging nearly constantly. She says it is because she has always loved flowers, and nature, and is deprived of them in the snowy wastes. This is likely true.
But the energy itself, separate from her, seems to almost have a memory of a flower in it.
-Notes from Kay's Lab Journal