How can I achieve a gradual cross-fade of terrain and speedtree textures/materials for season variation?

Hello, I am trying to change the environment according to weather conditions. So far I've been able to change terrain textures by switching the textures of the SplatPrototype class of the terrainData. Also I've been able to switch the textures of the materials included in the unity speedtrees. But as they change immediately through script, the transition is not natural and I couldn't figure out so far how to slowly crossfade from one texture to another.

Anyone knows how can I achieve this effect?