George:

For bushes, or dense foliage on trees, use twigs and whats called 'lichen'. Its a dense shrubby thing, but not moss. If ya buy it dried (or find it and pick it), you can prevent it crumbling by preserving it with a 50% solution of glycerin and water, and if ya want it colored, add a bit of food coloring or fabric dye to the solution. You can either let it soak for a few days, or 'cook' it over low heat for a few hours, and then suspend it over the 'pot' to drain (dollar store collander or strainer) and put the solution away for next spring to re-dose it.

I found and 'cooked' so much back in my BattleTech days, I made my $ back on buying the glycerin, and had enough to 35% populate a 6' x 8' battlefield-table.

Tarps, duffels, sandbags, etc: flour sack tea-towels (Canadian Tire, 4 for about $6) which can be painted with dollar store latex fabric paint.

Fencing and small structures: coffee stir-stix (the long thin ones like Second Cup sells (again, Dollar Store) and glue away. Also cheap BBQ skewers too.

And if you really want your wood to be strong... use thinned slow-set epoxy (MEK or Klenk's Epoxy Thinner, but add sparingly) and it'll be thin, and soak right in.

I miss anything?