Admittedly I neglected this thread for a while, my apologies.
It's the first time I've heard of the issue as you describe it,
Natuurzoon. The timing of seasons is controlled by the Flora Tuning Parameters. If you have a modern terrain mod (Lowkee or Gobias) this should be in order, otherwise you'll need the
Seasonal Flora Patch for the appropriate terrain mod.
If you already have this in place, then another possibility would be that some random plugin inadvertently includes a set of Flora Tuning Parameters, thus overriding the proper settings. For instance, Gizmo's underwater flora comes with a few terrain related files iirc.
I'm also curious: how long have the cities been running for? Or rather how many years after the initial planting does it take for seasons to get out of sync?
xtsigs, your thinking is correct for it's indeed possible to create a set of flora brushes and use them in a iterative fashion, thus arriving at highly customized flora combinations. It's actually how I go about in my own region, and in the past I have given some thought to the idea of releasing a set of brushes. However, I decided against it because it would a nightmare to provide support. See, in order for such an approach to work, you'd need two versions of each brush (enabled/disabled) and constantly swap between the two depending what you're working on. This alone will cause plenty of confusion among average users who just like to 'plug-n-play', so to speak. But here's the real kicker: if you ever, at any time, fail to load a brush already in use (which is a considerable risk given the file swapping) and save the city, you will inevitably end up with CTDs when terraforming the terrain where the brush used to be. And there're no brown boxes or other signs a brush is missing from the plugins. And yes, I had my fair share of CTDs.

So in short: yes a set of individual brushes would allow superior flexibility and customization far beyond what a single, 'monolithic' tree controller can provide. However, such an approach would need an impeccable tracking system of what brushes are in use, and ideally handle the swapping as well. If you are still interested in the idea, I'm all ears.
