For orchards there are two issues: the trees, and the orchard itself.
I shall address myself mostly to the question of the trees. I can offer a few suggestions in addition to what has already been suggested.
First … clump foliage is a GOOD thing. Get some. Any hobby shop that caters to the model RR market will have it. I suggest two shades of green. Hardly matters what two shades. I find that mixing the shades on any given tree makes the tree MUCH more interesting / realistic to look at.
The comment about wrapping/twisting wire is a how-to, but if you don't want to do that how-to yourself, you can buy it pre-made as picture-handing wire. The twisting of the wire provides some texture to the "trunk" of your trees. But unfortunately that texture is often visible as a spiral, which I find less than fully desirable. But the roots and branches is an advantage to this approach.
Sometimes you can find a picture-hanging wire that is a braided mesh rather than a twist. This makes very good looking textured tree trunks. Particularly if you want to do winter or burnt out terrain, with bare trees all around, this is some useful stuff.
I find flat-head nails easier and just as useful. They are easy to glue down, and then it's just a matter of getting the clump foliage to stick in a ball shape (for orchard trees).
Wall panel nails (with ribbed texturing) can give you a textured trunk, if you really want one. I find the ribbed/rings a bit more pleasing on the eye than the spiral of twisted wire.
The problem with nails is the branches. Unless you are doing blown up / burnt / winter orchards, the foliage will hide any branches or lack of branches. But you need to get the foliage to stick to your trunk, and having some "branches" helps a lot on this. I find I can wrap two thin wires around a nail (I get two ends of each wire, so 4 "branches" total) pretty easily. A drop or two of white glue holds the wires in place on the nail, and then I have support for my foliage. Painting, as suggested above, should be done prior to applying foliage.
Another approach, easier to build but harder to find and buy, is to use metal nail-gun nails. If you can find them, these can be bought in a "string", with two wires connecting each nail to the next. Cut the connecting wires mid-way and you have 4 wires (branches) sticking out from each nail. Easy and fast. Problem is that most nail-gun nails are now in plastic carriers rather than in wire strings. Sigh …
One more approach for those who prefer reuse and an organic approach, is to use grape stems. Now is the season to collect them. After you eat your grapes (nom nom nom), clip off the stems in various shapes, and clip the ends off of the stems (the connecting point for the grape). Leave them to dry (or put them in the oven at 200f for half an hour), and they darken to brown, shrink a bit, and get stiff. Perfect tree trunks and branches, whether for bare-wood burnt / winter forest scenes or for adding foliage. But somewhat more fragile than nails.
My first effort at an orchard was grape stems, lichen, and a hot-glue gun to stick them to a painted cardboard base. I don't recommend the lichen, the hot glue gun, or the cardboard base. But the grape stems worked out OK.
I have seen folks put red or yellow "fruit" on their orchard trees. This can be done just with paint dots (less effective), or by gluing colored objects to the tree foliage (more effective). To get your objects you can apply some colored paint to some talus. Then put some white glue all over the foliage of your trees and dip them in to the painted talus. Shake off (so you don't get too many on any given tree) and you're good to go. I've never done this myself, but I've seen it and it looks kind of nice.
After you are done making your trees, spraying them with a water/white glue wash is a good idea. They'll get a fair bit of handling, and they don't actually need to be fresh and springy -- stiff and crusted works just fine and lasts longer.
For the orchards you might want free-standing trees or to base an entire orchard. I use pennies as bases for most of my trees. For orchard trees it's one tree per base. For forest trees sometimes it is two or even three trees per base. To give some "grip" to the base of a tree trunk I often put 1/2 to 1mm depth of self-drying clay on the penny, with a hole / socket for the trunk to fit in. Once the clay is dry the tree trunk is glued in, and the base is painted and flocked.
For an orchard I put the orchard trees, on their pennies, in a regular patter of rows on a felt cloth. This might get a wall around it, or dirt roads around it, to outline it's perimeter. I have also seen folks model their orchards onto bases, but I find that approach difficult on the storage, and a bit less flexible than I like. I prefer to be able to use my trees a variety of ways.
Just some ideas. There are many ways you might try. These are ones I have used or seen in use.
-Mark
(aka: Mk 1)