I saw these "fall foliage" plastic flowers on sale (50% off the marked price), and I thought - bushes! These would make great bushes for the wargaming tabletop!
(No, I have no idea what they're supposed to be. All the label says is that they're made in China, by way of New Jersey. I spotted four colors - purple, lavender, orange and yellow.)
When I got them home, I realized that I'd got the scale wrong - in 28mm, they were too big for bushes, but not bad for small trees. Maybe fantasy or sci-fi trees, depending on how picky you are about your vegetation.
For my trial build, I used some 28mm figures to determine how high I wanted my tree's trunk to be (I wanted "normal" figures to be able to fit under the branches), then I bent the stem, snipped the excess off with wire clippers, and epoxied the flower to a spare CD.
(The whitish smear on the CD is from my first, failed, attempt to glue it down with superglue...)
I gave the epoxy 24 hours to cure, then applied a layer of DAS clay to the base to build up a bit of "earth" around the tree. I'm still new to working with this clay, which seems to be a combination of clay and paper mache. It leaves a clay powder crust on your fingers and everything it touches, but otherwise is easy to use - just like any other clay.
I started by making a "worm" of clay, circling the base, and working it onto the base. (Keep the CD on a flat surface, or the clay will go through the hole and make a bulge.) Then I added more clay, working outwards - trying to have some natural undulations, but sloping down to the edges of the CD.
The clay was surprisingly heavy when dry - adding to the stability of the base.
Again I waited a day for everything to dry, then I ran a rough file along the edges to remove any overflowing bits of clay. This is when I noticed that the clay really wasn't "grasping" the CD very well.
I also realized that I should have primed the CD before starting construction. Now, since I wanted the CD to be brown rather than clear, I would end up repainting the entire base. (The clay was already brown, but I couldn't come up with a good color match.)
When the paint was dry to the touch, I ran some watery-type superglue into the cracks between the clay and the CD, and clamped the base down.
For some reason, the clay acts a lot like plaster with regard to paint - you paint it, it dries, suddenly there are spots you're sure you painted before, you re-paint, more unpainted specs show up, you paint again...
I also painted the "trunk" brown, as well as some of the prominent branches. (The actual color was Regal Realms Tree Trunk.)
Now I wanted to flock the base. (So why did I paint the base brown? I'm thinking ahead to when the base has been in a few games, and the flock starts to wear off... at least there will be a "ground color" under it.) I've gotten rather fond of using Renaissance Ink's "x-fine" flocking gel as an adhesive for ground textures. So I brushed it on, then dipped the base into my tub of flock.
Then a good, heavy spray of a matt sealer to keep the flock in place.
So that's it! Cheap, quick, simple... maybe passable as a tree in the fall, more useful in a fantasy or a sci-fi environment. (Then again, my buddy says it still looks like a big flower... maybe I just have too much imagination...) 
When I do this again, I think I'll look for a smaller base (about the same size as the canopy), and I'll prime it brown before starting. And I'll forget the ground "undulations" and just go for flat - seems more playable that way. And the base turned out slightly warped, so next time I'll clamp it down to a flat surface...