Flower is a game with 14 achievements in total, 13 of which can be achieved in a relatively quick (~3h) and painless single playthrough.
There are other guides on steam which describe in a neat way what needs to be done where to get them.

The 14th achievement is "Nature", and it requires you to bloom 10.000 flowers.
It's not possible to get that many in a single playthorugh, no matter how thorough one is, so either replaying entire game, or grinding one particular level is required.

Replaying the whole game is the less boring choice, but it will take more time - one should assume that 2 full playthroughs will be required to get the 10.000.

On the other hand there's grinding 1 particular level - preferably the last (City).
Depending on how thorough you were during your first playthrough, you will have to run that level 5-8 times.
Time of one run will be 8-10 minutes, so fortunatelly it's not dramatically long grind.

I recommend running the level from start to finish (end sequence) - at the end there are no flowers to bloom, but there are report from plenty of people who were restarting after highway, and not getting credit for the flowers they bloomed.

For me the achievement popped mid-level during th 6th run.

Good luck and have fun!

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