Landscaping is my primary interest in housing, but I haven't found any information or recipes that would lead me to believe that "crafting" plants, shrubs, trees, rocks, etc. is supported. I see there are plants available from vendors (haven't found rocks), with full size trees being fairly expensive (20,000 g), and of course everything is available for crowns. Just wanted to check before spending coin/crowns.
I think these types of items are good additions to the crown store so long as the prices are kept reasonable; however, I find the crown store item preview to be only partially helpful for landscaping because it's hard to grasp the scale of an item. It would be much nicer if we could preview them in-place, or at least use a default location in your house/yard to show the preview such that you can understand it's relative size and/or how well it blends.