A while back there was a post about plants that are poisoness to yorkies. I bookmarked it and if you'd like to look at that, here it is:
http://members.cox.net/jonanyorkies/aapoison.htm#C
This will be the first summer for my yorkie and I was concerned about one side of our yard that has a cactus, a rose garden and a pond. I bought some of those little landscape fence pieces and put all along that side of the yard. My yorkie could easily jump that little fence, but seems to know that it's there for a reason and so far, she hasn't tried. Before that fence, she was always running into that area. I love gardening and from the list of poisoness plants, I figure I've got lots of that out there and don't want to rip out all my plants, so I plan to teach Maddie to stay out of the garden areas.
Hope this helped some and good luck with your landscaping!