How does a rose go dormant?
Roses are not day-length plants they are cold sensitive depending on their blood lines. If a rose has China blood in it then the bush will go dormant before say one with Gallica blood in it. The Japanese blood lines are the lest cold sensitive but will still go dormant when the temperature hits the freezing mark for too long. When the weather gets cold the rose canes harden off and pull down the sap to the roots which causes the leaves to fall off.