As the frost kills plants, remove them. Annuals should be dug up or pulled out by their roots. Perennials, except for roses, should be cut off an inch or two about ground. Put undiseased plant material in a compost pile, chopping up large pieces. If plant material is diseased, put it on a special, separate pile to prevent disease from spreading.
Excerpt from Gardening in Iowa and Surrounding Areas by author Veronica Lorson Fowler