The old tree idea is great! I may have to hoard some clearance Christmas items this coming year after Christmas and steal that idea. They always have some small greenery bits and ends that would work great and not take up that much space.
The broom bits are also a clever idea. Old crap is a great source for dio parts, I'm getting a bit tired of spending money on craft items. Old foam cushions pinched off into tiny bits, and painted green, would work great for moss. That's essentially what you buy bagged at craft stores (although it comes pre-shredded in fine chunks, which saves that effort).
Barrels would be a good idea too. Whatever "junk" can be in the yard, I'd say try and work it in. The Dreadnoks are not tidy people. You can probably also find some good old rusted sign images online that would work, too. All you'd need is to print them out onto cardboard or styrene and glue them on, or even laying around. They probably steal roadsigns and use them for target practice, too.