I don't think a 3/8" pad, z-lite, Ridge Rest, or Ridge Rest Solite will be warm enough, but they might be worth a test if time and temps allow.
As Brad said, look at the R value and get something around R 5 or greater. I use a full-length Exped Downmat 7 (R 5.9, 32 oz) once temps get in the 20's (F). A full-length pad is nice for colder temps, although a 3/4 length one might due depending on your height and typical sleeping position.
If you don't mind carrying a bulky pad, consider the Ridge Rest Solar, which has an R value of 3.5. It might not be warm enough, but it worked for Andrew Skurka in Alaska in temps below 0F. That was on snow, which provides some insulation itself.
Another pad to consider is a Thermarest Prolite Plus Women's at R 4.5.
You could of course always use evergreen boughs or piled up pine needles, leaves, or dead grass if you have the time and these are available without impacting live trees. :)