You'll need to bloom all three secret flowers in any area, the first level is the easiest.
1. After blooming all the flowers in the first area.
2. Bloom all the pink flowers in the second area.
3. Hidden behind the dead tree at the end of the level.
Please note that if you've already bloomed one of the secret flower bunches, they will not re-appear, so if you've already gotten a one (or two) in a level but can't seem to find it again, you've (likely) already bloomed them.
For all secret flower locations, see the trophy below.
The Dream trophy in Flower (PS3) worth 166 pointsBeyond all flowers lies another room.