Jim, there's lots of Yarrow around here: it seems like there must be perfect conditions for it this year, as I've seen lots of it 2' to 3' high already. I've used it to make trees, although that was many years ago, long before polyfibre and ground foam (I believe it was in the days when dinosaurs still roamed the Earth, eating Yarrow and model railroaders.) I always had problems with the seeds dropping off as the finished trees continued to dry out, although that may have been due to my poor tree-making techniques. The trees reminded me of Lombardy Poplars, with their tall, columnar shape, although mine were a bit too dense.
I've used Spirea with some success: usually multiple sprigs wrapped together with brown florist's tape. My first ones were simply dipped in some clear urethane, then sprinkled with ground foam. These again looked too dense to me. An application of polyfibre before the ground foam yielded better results, although this precluded the use of the urethane to hold the foam. Hairspray worked well in its place.
There's a bush that grows around here (no one that I've asked seems to know what it's called) that makes decent generic-looking trees when cut 12" or 13" long, but is also ideal for making smaller trees or bushes, as the tips of the twigs have ever finer "branches".
These are Spirea trees, the original urethane dip/ground foam application kind: a little too solid-looking for my tastes. These will eventually be given a makeover with polyfibre and more ground foam.
This is the same type of tree as in the previous scene, this time with some polyfibre applied after the initial construction, then a new application of ground foam. It reminds me of the lone Elms often left standing in pasture land, although still a bit too dense.
The trees to the left of the train are made with the "mystery" bush, a light application of polyfibre, and 2 or 3 different colours of ground foam. The tallest bush to the right of the train is from the same bush, made using the same methods. About the smallest I've made with this bush are 3/4" high: they look good covering the side of a railroad embankment, but pretty labour intensive.
Same place, different view
Same bush used, different scene. The tall tree to the right gives an idea of the branch structure
Again, to the left of the scene, more mystery bush
The "Junipers" to the left of the scene are made from the flowers of a Butterfly Bush. Dipped in urethane, then sprinkled with ground foam. Not too bad for mid-to-distant areas, but a bit too dense for foreground scenes.
I'll be out harvesting more of that mystery bush when the Fall rolls around, as I have about 25 or 30 square feet that needs to be covered with trees. This is layout area that's in use, although I don't post pictures of it due to the lack of finished scenery. I've also noticed some interesting looking shrubs on various properties around town, but I'm still waiting for the owners to go away on vacation.
Again Jim, my apologies for the abundance of photos.
Wayne