Every year the garden shops and centers trot out their pretty plants earlier than is wise to set the majority of them out. The last several years have seen mild weather in April which invariably causes premature bloom and is followed by a freeze or frost at the most inopportune time. Drat. I'm probably looking at another plum-free year if those blooms are blasted-and it's likely.
Bring your window box plants inside if small enough, if not they (and anything that you are worried about losing) should be covered with an air trapping woven fabric like Reemay with a rating for frost cover or old sheets/ light blankets. No plastic against leaves--they WILL freeze. Sorrel will likely be OK. Some roses are tougher than others, but it may make you feel more certain by covering. Good luck.

Pittsburgh area got a frost this morn. :( I should of covered my squash! Lucky I didnt plant the rest out. weathers been wet and cold.