Yes it will. The air space will fairly effectively eliminate the benefits of conduction from the slab, but there will still be some minor benefit from heat radiated from the slab - not likely enough to make a significant difference in what will/won't overwinter for you there though.

Building a "skirt" that encloses the space below the planter (like a tight fitting box with no top or bottom) would be a big improvement and keep the soil about 10-15* warmer during periods of extreme cold. Tenting the planter would also be a good way to trap heat from the earth, but be very cautious about this method if it will receive direct sun as it will interfere with o/a cold-hardiness eventually, if plants begin to grow prematurely because of solar heat gain.

Al

Thanks Al for your reply. One question -what would you make the "skirt" out of?

Anything that would trap the heat below the planter. Wood most likely, but if the gap is low, you could pile bricks around it, or just lay a 2x4 or 2x6 against the box sides and ends to enclose the space below it. Cutting the legs off would even be better because then you take advantage of both conductive and radiative forms of heat. Turning a cardboard box (or something similar) upside down over the planter would be great.

Al

ok, thanks