Here are some great ideas for basing figures on washers. This is exactly the technique (and the same fender washers) that I use.
modeling
Making Rocky Terrain From Wood Chips
Here’s a nice tutorial on making rocky terrain from tree bark chips.
Making a 15mm Graveyard
Here’s a tutorial on making a graveyard for 15mm scale figures. It could easily be scaled up, though.
Working With Tree Armatures
Here’s a good tutorial on working with tree armatures.
Making 15mm Hedges
Here’s a tutorial on making a 15mm hedges.