I use a couple of different techniques to create hedges. As others have noted, sometimes hedges are grown as purely functional boundaries between agricultural plots, and other times they are well manicured decorative boundaries.
For the less well kept, more ad hoc hedges I use model RR "clump foliage". I bought a couple small bags for something like $5 USD each about 15 years ago, and have worked my way through almost 1/4 of each bag, so this approach does have a certain cost to it. But not a very big cost…
This clump foliage can then be applied around the edges of other terrain bases I am creating, or can be glued directly to small wooden stirring sticks that I save from my visits to local coffee houses. I usually paint these sticks an dirt-tone color and flock them a bit before gluing down the clump foliage.
My other approach is similar to what is described above, except that instead of slicing sponges I use scouring pads. These are sold in my local grocery store in 8 x 6 inch x 1/8 inch thick sheets (200 x 150mm x 3mm), typically 4 or 6 to a pack. The ones I find are already a deep forest green, which is quite convenient. Most go to the kitchen, but occasionally one will "disappear" from the cleaning supplies only to reappear on my hobby table. I slice off strips and these too can then be glued down around the edges of terrain bases, or onto flocked stirring sticks.
Here is a view of both techniques being used on a game table. In this case I was trying to create the small town of Loboikivka in the Ukraine, near the somewhat larger town of Petrikovka, where the Italian CSIR closed the net on several Soviet rifle divisions during the summer of 1941.
In the picture the agricultural fields in the foreground have clump foliage hedges, while the churchyard in the background has the more manicured scouring pad hedges.
This pic, from that same game, shows a bit more detail of one of the clump-foliage-on-stirring-stick hedges. Note that with 2 bags of clump foliage (ooh, big spender!) I get to put a bit of variety of color into the hedge, making it look that much more "unkept".
This pic, set up last year just to showcase some of my newer models (not an actual game), shows a bit more detail of a scouring-pad-strip-on-stirring-stick hedge. I do a bit of dark wash and lighter dry-brushing on the hedge just to give it some imperfections and make it a bit less synthetic in appearance. But this technique is quite quick and easy, and makes a nice "well maintained" hedge for use in towns and cities.
After figuring out this approach I also started making stone walls using all the same techniques except with cork sheeting instead of scouring pad sheets.
Here is a pic of one of the cork stonewalls. I used the rounded tip, clipped off of one of the stirring sticks, to serve as a closed gate.
-Mark
(aka: Mk 1)