i have built a few trees myself, so has John, if you dont like the store boughten ones, take some extra copper wire you got, twist the bottom end around really tight, then take the top end and branch them out like tree branches... take your branches and paint them what ever wood color you prefer, then take some CA glue and coat the branches real thick, then sprinkle a whole lot of foliage stuff on the branches, you can get some nice looking trees.
for a winterized look, kinda go with the tree being bare, take some spackle, light weight spackle prefered, and mix it with some future so its a little thicker than paint, and brush it on top of the branches to give it a snowed on look, thats what i did for my Dogfight Diorama. but thats 1/48th scale, but still go check it out if ya want to see what i mean.