In the Pacific Northwest, I'd vouch for being able to build walls of flat stone without a concrete base for sure, but brick might be a little more vulnerable unless you lean the walls into the dirt a bit. That'll be a rustic look. But it should work. And if it's not mortared, and you do get frost heave, you just pile them up again. Drainage through your base (sand) is the key to success in this climate.

KarinL

Since you are elevating the bed 20" above grade, if there is a potential for freezing in your area, then there is a potential for frost heave (actually, there is a risk even when building near grade, which is one of the reasons concrete paths/driveways can crack and laid pavers can heave). A contiguous slab of concrete provides a barrier between freezing/thawing water and the ascending structure that aggregate, even with good drainage and compaction, cannot entirely mimmick.

If you are set against using a concrete slab, an alternative may be the use of an insulating material. I believe that there are some products available, such as rigid foam insulation, marketed particularly as an alternative to concrete footers. You may want to take a look at the websites of the major insulation manufactures such as Owens Corning or Johns Manville to see what product are available for this application.

Frost heaving does not occur in the PNW, particularly zone 8. Our soils simply do not freeze to any significant depth - a couple of inches at most and that's only in the worst of winters. A concrete base is not necessary for your purposes and working without one will actually provide for better drainage of the beds should you decide to mortar the bricks in place. Even that is not a real necessity for raised beds, however your structures will be far more stable and long lasting if you do so. But a leveled, compacted gravel or sand footing for the bricks will suffice.

Thanks everyone. I definitely intend to mortar the bricks; I never even considered dry-stacking them.

The worst freezing I've seen here froze my clay solid down to about 4 inches (a couple of winters ago), but I never saw that before and haven't seen it since. Generally, the ground doesn't freeze at all, just gets water-logged.

My back yard (where I intend to put these beds) is a cold-sink and will get -- and stay -- as much as 10 degrees colder than the front yard year-round, so I'll have frost and ice back there when none is evident anywhere else. I would say that there is certainly a potential for freezing, but it's not a consistant thing (which, for me, makes things harder to plan for).

I might take some long walks around this area to see if I can find something similar in another yard, then figure out how they handled it. I definitely don't want to go to all this effort (it'll be just me!) and have it all be for nought.

Thanks again for all the help!

Can I suggest that you make a 9 inch wall, that is with the side of the brick at the front rather than the face and that somewhere near the bottom you scrape out the mortar on several perpendicular joints for drainage. You could also use a Flemish bond which is not easy if the raised bed is not square.

Thanks for the suggestion inkognito. The height of the wall(s) is meant to accommodate comfortable sitting. I had considered drainage, and scraping out some mortar seems like the least complicated way to achieve that. The raised beds will be VERY square (or variations on square -- i.e. all corners will be 90 degrees).