Actually, it's not that difficult to make them in 28mm.
I made this how-to almost 5 years ago for my blog:
http://thisveryblog.blogspot.com/2009/08/hedgerows-simple-fast-and-nearly.htmlSince this was a long time ago, I would do things a little differently now, and I have for my newer bocage sections that I have made since then. Much of what I would change I learned from Derek on TMP on this very topic and from some of these very pics.
Basically, if I were making these today, I would cover the whole bocage piece with sand and not add any rocks at all. Then I would cover almost all of it from the bottom up to the hedge and trees on top with static grass, small clumps of bushes, and grass tufts (maybe flowers and other vegetation as well). The trees I added by drilling a hole in the top of the hedge and adding a pin to the tree to slot in (I keep mine removable-you don't have to). You could leave a few open spaces of the dirt showing underneath, just don't use rocks! Some of the ones I made originally had rocks showing, but I know better now

I easily made over 50' of these in 12" and 24" pieces as well as a few T pieces. I still need more! The balsa and basswood is so easy to work with you can add pieces with fences and gates, broken through sections for where a tank broke through, pieces with foxholes etc. etc.
Overall, the basic process in the how to could be used to make hedgerows that look just like the pics. It's all about using the balsa piece (or wooden half round trim) to get the right shape. There are several sizes of the balsa shape you can get for model aircraft and they are literally the perfect shape for bocage (they are exactly the same shape from the side as the bocage cutaway drawing here:
http://www.lonesentry.com/normandy_lessons/combat_lessons_normandy_image1x.gif ). The same image is on Derek's blog as well. They come in 24" lengths and are anywhere from 1" to 2" tall.
When done, these suckers are indestructible. Between the wood glue and sand or fine gravel covering the thing, to the super glue holding the vegetation on, they are super tough. I toss mine in a bin and they travel to the game club almost every week and none of the clumps have come off in close to five years use.
Here's a pic of the bocage on the table with trees, roads, and other scenic bits. I still think I need to go taller with more bushes. Still, they make decent enough pieces for hedges outside bocage country such as in the rest of Europe
http://3.bp.blogspot.com/-GWrTEfWrATo/TfV3NzeA7WI/AAAAAAAAAcU/N3micYU6MKU/s1600/Normandy%2BCurve.jpg