Yeah .... You just draw the hedge in as you would a map. You're looking at it from the top. The nice thing is, you can make as many terrains as you like (each in its own layer) and then texture each one a little differently (I like to make some of them flowering plants by changing the dot color in the foliage material -- by default it's green, so I make it yellow or red or something like that). Then, I just place the first terrain, copy it, and edit the copy by putting another one of the layers in as the picture. I end up with a perfectly placed garden.

Could be worse, could be raining.