To fuse a flower you will need more than one. Flowers come in varying percentages:
10%, 20%, 50% and 100%
Fusing two 10% flowers together will create 1 x 20% flower, and so on and so forth.
You can fuse flowers by heading over to your floral inventory - which can be accessed by pressing an attraction flower spot on your incubator.
Select the tab that you wish to open, and then select your flower.
If you have more than one of a flower, it will show on the corner a 'x' symbol, and a number.
Four menu buttons will pop up, one being 'fuse'. Select this button and it will automatically fuse those flowers for you to create the next level.