You can put a stake in the ground right next to the trunk and then drape a sheet down over the tree. Or fill some 5 gallon buckets with hot water and leave them there, they will help keep it warm. Mid to Upper 30s aren't really much of a concern though. As long as it doesn't get to around 28 or lower you should be fine. I'm leaving mine as they are, uncovered, in Boston -- our low will be 38 F.