1, 2, 3, 2, 2, 1

look to the trees for the solution

Look at the distances between the trees, and use the flowers as "steps" of distance from one to the next. Almost like looking at how many flowers you can fit in the perspective area between each tree and it's neighbors, if you were to make a path with them.

Each tree is in a 'layer', back, middle, front (they aren't exact). If you treat the distance between each layer as the number of flowers between the two it become pretty easy.

Take the first two trees: they are both in the middle, one a bit further back than the other, so they get one flower.
Whereas between tree three and four there is a larger 'layer' gap since tree three is way front and tree four is way in the back. This gap gets three flowers.
The one previous, between two and three, gets two flowers because it is between a middle layer and the far front.
And so on.