Is is possible to place vegetation on a custom mesh?

The Unity terrain vegetation painting can be done on Unity terrain objects only. You'd have to write your own system to do otherwise.

bumping this because I tried looking everywhere and could not find a good result online that I was satisfied with. So my solution was to use trees.

I was able to make the main trunk really small (like 0 so its invisible) then attach leaves with my grasses texture applied to them and make it so the wind can bend the leaves but I made it so that it did not move them from the tree trunks position.

The end result for me was some decent looking grass that was easy to place where I needed to and it swayed with unities wind. Of course my grass is not too graphical so I am not sure if this would work for everyone but I am just posting this a sort of reference or idea for people like me who could not find a satisfactory result online.

One often overlooked possibility is to use a Particle (Shuriken) system to generate the grass. People might start think that it's not an optimized way because of X or Y reason, but the true is that the way the particles system works in Unity is relatively the same (if not better) than the Terrain's grass system. Particles can be GPU instantiated, meaning you save quite a bit there. Check this out : Link to the docs

/

It's possible to set the particles system so that it only generate 1 permanent batch of particle that last as long as the scene last per mesh. Just type "Infinity" in the particle Start Lifetime and the particle will last as long as the scene is active.

/

It's not a "click and it's ready" method, but it works as well as the actual Terrain's Grass system. First, ideally, you got to be able to refer the mesh data to the Particle System. If you're making a game that has Voxel-based terrain, you need to create a mesh with the relative triangles on which grass should be instantiated. Also, considering how this system work, if you modify the mesh, the grass position might also change as the position is based on the mesh's triangles (or edges) so if the number of triangles or edges changes, obviously its relevant grass will also move around. If you're using a static permanent modeled terrain, it's a lot more easier. You can either use 1 particle system per "ground" part or, if you got access to the 3d models, you can create bigger or more optimized mesh.

/

The trick is to think that the mesh only need the faces (triangles) that has grass on it. So if your terrain has rocks, sand, water, slopes, etc. that doesn't have grass, you have to remove those triangles. Then, by setting the particle system correctly, you can generate grass (as particles) on the ground object. It does have some adjustment required such as if your mesh are at -90 degrees (Z-up or Y-up) and such, but there's an option to fix it.

Here's an image that display what I'm writing about :