Posieh, i grow vegetables in beds 4'x4'. The only perennial i have is asparagus. 2014 will be its 3rd year. i planted them in 2012 and mulched that fall. i did not in fall of 2013. i am not much help as i don't grow flowers in my beds. those i grow directly in the ground. it would seem that yes the boxes are freezing and you likely need to protect with mulch over and around your boxes, prehaps with straw bales.

jodi

It is not the winter cold, frost, or depth of frost that is killing your plants (assuming they are hardy for your zone). What is killing them are the freeze-thaw cycles that occur mostly in late winter/early spring after the snow cover has melted away. Raised beds can be especially touchy regarding this issue. The increasing sun angle heats those boxes up during the day, thawing and warming the soil, then the below freezing nighttime temps refreeze the soil. The process repeats. The plants are being "tricked" into coming out of dormancy prematurely, and are being killed off by refreezing and late cold snaps. It is very likely your plants are making it through the winter just fine and are being killed off in late winter/early spring mere weeks away from the start of the growing season.

Do not think of mulch as winter protection for the plants (Ma Nature usually does that just fine by providing snow cover), think of mulch as an insulation layer to protect from freeze-thaw cycles that can occur in late fall and possibly even mid-winter during thaw periods if minimal snow cover, but mainly late winter/early spring. You want your plants to stay as frozen as possible until the spring rains and warmer temperatures wake them up, and mulch works well for this purpose. It is also very important to ring the beds with straw bales or something similar to keep those retaining walls cold as well, at least on the sides that get direct sun on them.

In early spring (usually early April for my area) when all my snow is melted away and the lawn is thawing out and mushy/spongy to walk on, I can reach into a flower bed, raise up a patch of mulched leaves, and still see frost crystals on the surface of the frozen soil. This is what you want to see until the spring rains come and pull out that frost, and that is when you remove the mulch so the plants can shoot out and grow properly.

Wishing success,
-Tom