The perspective is North-facing, but the prevailing wind-direction is Southerly. The passage behind the photographic point-of-view zig-zags through a pair of ninety-degree bends, but nevertheless channels the wind blowing from the South, down the passageway and into the garden.
The petals are falling from above, and the flowers are fairly uniformly distributed on the rose-plant, so I don't think the petal distribution on the ground reflects the flower-distribution above.
If the wind were blowing from the North, one might explain the pattern by postulating that there are three regions of 'dead air', formed by the boundary layer detaching from the three visible bluff bodies: the chair, the right-hand gate-post, and the gate itself.
Given that the wind is Southerly, however, the best hypothesis I can suggest is that the gate and gate-post create stagnation points for the oncoming flow, and the third pool of petals is explained by some complex interaction between the wake of the gate, the adverse pressure gradient approaching the stagnation point of the chair, and the sudden overall deceleration as the air exits the passageway into the garden.
From this, one can see clearly why horticulturalists, just like architects, need to develop a proficiency in Star-CCM+.
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