Answer: Oleanders remain evergreen unless the temperatures dip down below freezing. If that happens, the tops will die down but the roots should remain healthy. Don't put plastic over plants - the plastic will suffocate them. If your area is predicted to get a short overnight freeze you can drape old sheets over the tops of the plants to protect them; be sure to remove the coverings immediately after temperatures warm to above freezing.
Since you've purchased soil amendments, use the material by sprinkling it over the entire planting area (don't just amend the planting holes), then dig it in to a depth of about 8", level the soil, then plant your shrubs.
Best wishes with your new shrubs!
Q&A Library Searching Tips