What are some techniques you use for wintering trees in cold climates. I have been using a cold frame with mulch around the pots in order to protect roots from the cold which can reach negatives easily.

I do nothing much. Untill temps drop below -5 pretty much all temperate species are fine, except for mediteranean climate species. Below that I start getting worried by sustained -10, -15. I just take them out of sun and wind.

My full take: www.growingbonsai.net/what-kills-bonsai-in-winter/

I will have to take a look and read through the whole take. I like in a climate that can easily get to -20 to -30 Fahrenheit so i am trying to take extra precaution with the roots.