No, even outdoor-rated extension cords are not meant to be buried. For any wiring intended to go underground, you should use conduit and follow local electrical codes. Holiday lights, however ...
Additionally, hanging electric lights during wet ... A safer option is to use outdoor-rated heavy-duty extension cords and protect them with cord protector cable covers. What does it cost to ...
The fifty-per-foot rule is most applicable to shrubs and bushes. If you have some boxwood or yew bushes to light up, use fifty lights of string light per square foot.