Here in NE Minnesota we have an excellent plant we call Yarrow. It grows about 2-3 feet tall and has an excellent tree armature on the top. Now is the time to harvest them.
I cut off the seed pods outside and stick them in florist's foam, full height. I spray them with greenish gray rattle can paint followed bt a misting of flat black to duplicate Aspen, the local dominant hardwood species. I cut the tree armature off the tops and cut the rest for pulpwood loads. The trunks scale out th about 12-24 inches diameter in HO. The armatures on top can also be broken down into smaller trees.
Very cheap and it looks good.
Varying the bark colors can vary the species.
If you can get some bottles of green paints cheap you could likely make your own ground foam foliage material yourself instead of buying it - it's foam rubber that's been ground up and colored, and foam rubber (being a common cheap cushioning substance) can be had in large quantities for very cheap.
A number of people have mentioned using twigs of appropriate shape for the trunk and branches, which does seem to be a good way of going about it. Someone also mentioned spray adhesive, which also gives good results based on what I've read (and is borne out by photos). The same source, which would be the Reverend W. Awdry's article in the December 1959 issue of Railway Modeller also suggested a means of enhancing twigs for certain kinds of trees: pressing carpet underlay against a shellaced twig while the shellac was wet and pulling it off to leave strands behind, then spraying on some more shellac afterward to thicken them.
Awdry also used cut-up rubber spunges and flock for foliage (which used depended on which sort of tree), and suggested for certain trees (such as apple trees) that tea leaves or dyed sawdust would be better. These materials are cheap and the methods simple, yet going by photos of layouts that employ them they can certainly deliver quality.
Interiors and people figures make such a difference. Especially the people.