Since I'm not going for reality with my cartoon animations I want to make my own custom cartoony plants.

Do I have the ability to make my own plants that I can use the [ Start Gardening ] feature?
Is there a tutorial on how to make them?

Yes you can make your own plants (grass not trees) by changing texture (see arrow)

I was thinking exactly 3D models not the flat textures.

Ah ok. You can't make Iclone trees .It was an old request.So I use The plantfactory from e-onsoftware but it's little bit expensive and you have to
export with wind animation. The probleme is that it's a prop and take lot of ressources because Iclone don't deal with LOD for props....

Thanks for the answer, I'll have to make my own trees as props then... Seems tedious and daunting.

The foreground trees will look better as 3D models, but background trees can be "grass" It works pretty well for pine/fir tree covered mountains and such. You don't even need a mountain if the trees are really dense, just some plains at angles to plant on. This method doesn't work very well for deciduous trees though.

There are a few free tree generator programs floating around. Here's a couple.

http://3dcoat.com/forum/index.php?/topic/1003-trees-generator/
http://www.brothersoft.com/easy-tree-generator-286591.html

There was a post some years ago on how to do that effectively (we should have a catalog of such tips). In fact, I think it was a post by you, Rampa! I've used it one of my movies, and I was pleased with the results.

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So much for those, Easy Tree doesn't launch and Tree Generator doesn't save. But it gave me an idea on how to make trees.

Blender has a couple tree addons, I think.

Blender to the rescue!
I found an add-on for Blender called Sapling Tree Generator.
I'll see what it can do for me and report back.