I've only gotten it to grow in the Plains, and once I have a sturdy stone wall I start planting. Make a rectangular farm plot, then start at one corner. Plop it down, take three steps backwards, and then plop another. And three steps, plop, three steps, all the way to the edge. Two steps over and then repeat the process. Usually works pretty good and I always wait a minute before starting a new line, just in case it's too close. The barley turns dead looking, so you know right away if you did it wrong. Sometimes you need an extra step, or even just a half step.

It doesn't hurt to have a separate farm area with it's own workbench and walls, just to get that little bit extra space. It can be a pain to set up another stonecutter, but if you want the barley in bulk, that's the way to get it done. Just make sure to step up a nice flat area to plant in, cause hills and rocks really mess up your flow.