It's general modding/meshing knowledge that TS3 meshes do not render backfaces.
...usually.

When cloning the base game generic harvestables (GardenPlantBush and PlantSmallVine), I can see in Blender that many pieces do not have undersides.
Red in the image below represents a backface. This mesh looks fine in-game, it isn't invisible from underneath.


The vine uses the Foliage shader, which has an entry called "BackFaceDiffuseContribution" which seems to be the most obvious source, with one exception.
The bush doesn't use that shader, yet still has some (albiet fewer) one-sided pieces. Replacing it with my own mesh gave invisible sides.
TS4 does the same thing for plants so it doesn't surprise me the '2D-3D' plants work like this, never seeing it referenced anywhere is the surprising part. Is anyone familiar with how it works?

More importantly, is there a performance gain to using the ingame function over doubling the polycount?