Overwintering lavender in Iowa depends on the type of lavender. Some cultivars of lavender, especially English Lavender, can be successfully overwintered outdoors in Linn County. Other types such as French and Spanish lavender will not survive Iowa winters outdoors and should be treated as an annual or brought indoors over the winter. 

We are getting a little late in the season to ensure a zone 5 perennial plant gets well established before winter, but I think an English lavender should still be ok as long as it is planted soon, cared for, and the site is well drained. Overwintering the lavender indoors in its current container and then planting in the spring would likely be the safest option. You could also try both approaches by taking some cuttings of the lavender to grow indoors over the winter just in case the main plant does not survive the winter outdoors.

The following publication has research-based information on successfully growing lavender in Iowa: https://yardandgarden.extension.iastate.edu/how-to/growing-lavender-iowa