As we weren't fully exposed, to rain, snow, etc, we had what you might call normal 'indoor' scenery, that wouldn't survive fully exposed. Some of the 'grass' went to hay very quickly and cardboard buildings also changed colour over time, but this wasn't always for the worse. The Peco backscenes also faded very quickly, but this just made them look a bit more 'distant' really.
Expansion was our biggest problem, but it can happen in the real world, too.
We also had some problems with lack of connection through some of the track joiners after a long period of time, you might like to make provision to directly connect each piece of track. It could be worth the extra trouble at the start in the long run.
Miniature gardening is a subject in itself. This book is very good, though there are many others, too - http://www.amazon.com/Gardening-Miniature-MARTIN-BAXENDALE/dp/0706367626/ref=la_B001HPSB26_1_2?s=books&ie=UTF8&qid=1403516965&sr=1-2 .