Plant “Types”: Where to Find
Meadow, Grove, Marsh and Alpine are types that are gained exclusively from “Common” variants grown in those four biome types.
Growing a “Mother” tree after rescuing the relevant god (Meadow Mother from Naia, Grove Mother from Gleb etc) will produce the biome type around it, converting any “Common” variant plants to the biome variant.
Likewise, when you later get “Naia’s Lifeflame” and the Lifeflames of the other gods, the area around the spring you use the Lifeflame on will become the biome type of the associated god (Note, you can only use the Lifeflame of each god in their relevant quarter of the map, you can’t use Naia’s Lifeflames in the salt flats, for example)
As for the variants of Ancient/Wild/Giant, there does seem to be some sort of pattern to them. I’ve only found Ancient Thorntails in the salt flats, for example. I’ve had to start a list of what varieties I find, and where, but I don’t have direct answers to give you yet.
The easiest way to find all the variants is to just do trips through different biomes, keep a heap of inventory space free, and just check around all the major areas of dead vegetation (particularly around springs). It’s a ton easier if you’ve used Lifeflames to revive the vegetation around a spring, since they will look visibly different based on their variants (Ancient will be pale with blue leaves, Wild is typically light brown with orange leaves/flowers).
You can tell the difference even if they’re dead based on the colour/size of the plants, but you should to do that by day since night time makes everything that weird purple.