I'm rather confused about this issue. Summer soil temps are not a concern. Winter, however, I don't understand at all. The raised stone bed below faces directly south and is nearly 100% protected per the diagram. The landform and vegetation to the north protect us from harsh northern winds, and southern winds that can howl off of the water and completely broken by the house. This past winter was harsh due to ice, warm-freeze cycles, lack of snow, and many things were lost. I have lavender growing in this raised bed that is barely hardy for my zone. (I'm technically 6a, but if I was a better spitter I could hit 5b just up the road.) Per the Shaker Community in Canterbury, NH, when it was planted two or three years ago I covered the soil with gravel after planting to both help with draining and warm the soil in summer. But, I assumed gravel on top would not protect roots from the cold seeping in to the raised soil from the vertical wall, as well as the brutal thaw-freeze cycles we had. I thought for sure it was a goner.

We did have a sunny winter, and I know the sun hits that rock full on every winter day. I have always wondered if that makes it safer for plants or more dangerous. The soil could warm when the sun is shining, but when it is not the soil temp should plummet to match surrounding air temps, which would make it colder than ground soil. I assumed this raised bed would be a harsher growing area because it would be subject to larger temperature fluctuations in winter.

This lavender is ecstatic this year. That's the last thing I would have expected. What actually happens behind raised stone walls in winter?