Just a little bit of rambling, but it seems, the biggest performance problem comes from your current system creating your underground from independent actors (therefore a 100x100 map apready is made out of 10000 individual actors), and not creating one actor, that contains one big mesh or landscape.
Ok, you made this, because those actors contain your values for your plants growing, but it probably is better, if those values are stored in a struct or array, or something like that.
Or… since you wrote, you would be ok with flat landscape, there might be a system, that can hold all your values for your underground, and act as a landscape, including forming your textures: A Paper2D tilemap.
Yes, usually, those are used for 2D games, but they could be used as regular flat underground too. Nobody said, that they have to be limited to 2D games ^.^ And you don´t even need to use them for rendering, just using them as a data collection, that is invisible in your game, would work.
But since you can give each tile a unique sprite/texture, and probably drive that with a blueprint too, based on your calculations/results, you could use the tilemap as landscape too.
Reason for mentioning them:
- Tilemaps are made out of one or more layers, which are made out of grids, you have access to them
- each grid is made out of tiles, and you can access each tile individual tile, and if you made your tilemap an instance, then you can even edit those tiles at runtime
- each individual tile can hold individual user data, which you also can read and write at runtime
So if you don´t want store your data in an array or whatever, you can create a tilemap, and give it some layers, like one layer for underground type (rock, sand, soil, swamp), one for water amount , one for fertilizer amount, etc.
Then you can edit the tile textures depending on those stats, and whatever your calculations say, it should look like with those values (maybe not all at the same time ^.^).