The DC 2150 Blog has an entry on making wire trees.
Armored Ink Blog has an article about making cheap trees from nails.
Be careful. A wargaming pal of mine did this and at one point, lost his balance, tried to catch himself and put his hand right down on one of his trees. Drove the nail right through his palm.
The Charge! blog has an article on making cheap trees for your gaming table out of roofing nails and old sponges or foam.
A guy in our gaming group did this many years ago. Tragically, at one point, he leaned over the table, lost his balance and put his hand down to catch himself. His hand landed on one of his own trees, and drove the nail through his palm.
That avenue for cheap trees was a dead end, as far as the rest of us were concerned.
I’m not one to spend a lot of time making trees for my wargames. I tend to buy the big bags of pre-made model railroad trees, preferring to spend my effort on painting miniatures or making buildings. But one thing that’s always been missing from the lines of plastic trees are birches. Youc an get them, but they’re typically very expensive compared to the plastic connifers and deciduous trees.
Terragenesis has the solution with a tutorial on making cheap (and what looks like quick) birch trees from fabric wrapped floral wire and floral tape.
The author didn’t put any leaves on them, but it wouldn’t be too much trouble with a can of spray adhesive and some appropriately colored foam flocking.
Here’s a tutorial on making trees from scouring pads.. They’re very nice looking, and it looks like a fun project, but frankly you can get nice pre-made trees from a variety of sources. Still, they’re bound to be a lot cheaper, and if you’re a do-it-yourselfer ….