How long does the temp have to be below freezing to freeze the roots of large plants?

Freezing temperature is usually referencing the point at which water freezes. For most purposes this is 32° F or 0° C. While some hardy plants may survive these temperatures, herbacous garden plants may be killed by temperatures even above of this. Unfortunately, the lowest temperatures plants can tolerate and still live will vary from plant to plant, so you may need to find the specific temperatures that the plants growing in your garden can tolerate.