rockm

"What is the best technique for watering a winterized tree in freezing weather? "

Chinese elm overwinter quite easily outdoors in the D.C. area. I have kept mine outdoors in wintertime here in No. Va. with mulch covering on the ground for over 12 years now with no problems.

"It gets below 30f several times in the winter and I thought water in the roots would kill the tree also"

The opposite is true. Dried out roots in the winter will kill the roots and the tree. You WANT soil around the roots and roots themselves to be moist when it freezes. Freezing water gives off heat and whenthe soil is wet, it actually provides some insulation for roots. Roots can stand to be frozen, as long as they don't get frozen below a certain temperature. this temp varies from species to species, but rule of thum is below 25 F. Placing a chinese lem beneath an overhead shelter in a shallow 6 inch deep or a little more, well-drained trench, then back filling with shredded pine bark mulch will usually protect things from the worst winter weather around here.