I guess it depends on a number of things such as what type of plant, what zone you are in versus the zone of the plant, etc.

For some of my smaller conifers I have simply placed a cardboard box over them, making sure it is not 100% air tight. That works here, however, because even under the box the winter sun isn't strong enough to really warm up underneath the box (cook the plants).

I have some Japanese Maples which I take a box, cut off the flaps, and place around the tree. I then fill in around the tree with peatmoss and then leaves. Obviously my trees are very small to begin with (a foot high).

I found my magnolia was suffering winter damage, so last year I wrapped with loose plastic. Here too, this probably wouldn't work in your zone as the plant might heat up too much under the plastic, but in our zone it seems to work. (The intent being to keep the wind off of the branches and keep them from drying out.)

Anyways, it sounds like you are referring to conifers, so maybe burlap is the best solution? I read that they don't necessarily need to be wrapped tight, even making a screen on the south side to shade the conifer helps (unless the area in question is very windy, then maybe you might wrap all around?)

Glen3a-

I have been utilizing Burlap for years as wind breaks and wrapping plant material. But with the ever increasing use of different plant material, I am finding the need to winterize more plants then ever before. To be cost effective I have been look for other peoples suggestions for winterization. Spraying the plants and wrapping them is not the most cost effective measure. Burlping uses a lot of labor, spraying is a lot of time traveling. Just thought I would give it a try and see if anyone else has thought about doing it differently then the old school way.
Bryan