Before I begin... I want to mention that I don't recommend the following to everyone. For whatever reason I seem to be a bit warmer here than most of the northeast and what is working here may not work for you (this should be assumed at all times). This is also a work in progress as I continue to learn, but this is what I believe to be one of the best ways to plant a fig in my location with little to no winter protection and with some aesthetics in mind.
First thing I want to mention is that my trees are dying just about every year to the ground or close to it. I'm in a suburb northeast of Philly and in a straight shot I am ~5 miles from the Delaware River. We frequently reach 0-5F. This year was 2F. Last year 1F. This Hardy Chicago unk I have is the only one thus far to survive relatively unscathed two years in a row. Usually around Thanksgiving an early low comes in and damages trees that are not fully dormant and especially trees with poor lignification, so not only are we getting damage in the winter, but also frequently in the fall.
Two Key Points: An interesting experiment I've been doing is planting figs at various heights. 4 inches of the root ball above grade, 6 inches, 8 inches and lastly the entire rootball encased in a 1 foot high raised bed with wooden sides. This year and last year I discovered that I can plant figs much higher than conventional wisdom would have us believe. Reason being is that if you mulch the soil heavily, it will insulate much better and SO FAR the soil has not dropped in temperature enough even on the coldest nights to kill a single tree (even the tree in the 1 foot high raised bed planted last spring). I don't know how long that will continue, but 3 or more inches of straw or a thick layer of stones covering the root ball on every tree seems to be key.
Additionally I've been taking some soil temperature readings throughout the property. There's quite a big difference between bare soil, mulched soil (with either wood chips, straw or rice hulls) & soil covered in a layer of stones. The stones are warming the soil by at least 5 degrees more than bare soil and mulched soil being significantly cooler than both. I imagine putting down black plastic would make things even warmer than a layer of stones. My southern exposure has the warmest soil temperatures followed by southwest and then western exposures.
It makes more sense to me to treat my in ground unprotected figs in a different way. Because if they're gonna dieback all the way to the ground every year.. lets at least focus on finding a way to get earlier main crop. Of course through thinning, pinching, but most importantly my point is that we should be giving our trees as much heat as possible early in the season. The same way we treat our container figs. Planting them higher, using rocks, black plastic and thermal mass is going to net us more in the season and that's what you see below is what I've done with the first few trees. The mounds are ~1 foot high and now covered in brick and stones.