This is a bug in the minecraft 1.9/snapshot version, I couldn't select the correct version.
When you place a block at the bottom-half of double tallgrass, the top changes into a flower for a tick or so and then dissappears. It only happens if you place a block at the bottom-half, if you punch the bottom-half it doesn't happen.
Also, I've managed to get 2 different kinds of flowers on different worlds:
- The top half of a sunflower on my private worlds.
- The top half of a peony on my friend's server (has nothing to do with the sugarcane I used in the picture).
This seems to be very similar to a bug that used to happen when you punched to bottom half of the Double Tallgrass and the top changed into a flower for a moment.