What is the coldest temperature that the parade rose can withstand outside in a pot in the winter?
A Parade rose is really no different than any other rosebush planted in a pot outside. It really depends on the rosebush, its hardiness and will to live. I honestly would not expect them to do well if the temps get into the -20 range. I would suggest digging a trench in perhaps a veggie garden and plant the mini roses in it pots and all. Mound the dirt up onto the canes 3 to 4 inches. Or move the potted mini roses into an unheated garage or somewhere close to your house. Mound around the pots with mulch up and onto the soils level in the pots. Having them up by the house will give them some protection from the harsh winter winds. Many rosebushes are lost over winter from the harsh drying winds of winter that sucks the moisture out of them and their soils. The other killer is not keeping the rosebushes cold. It is the warming and then quick freeze temps that will kill a rosebush. I honestly have not seen any information on exact temps that the parade rosebushes can take as far as freezing temps. In general mini rosebushes being own root tends to make them very hardy even when it comes to winters deep cold temps.