When planting iris bulbs in Minnesota, it is important to water them thoroughly after planting. However, continuous watering is not necessary in the Minnesota climate. To protect the rhizomes from freezing and thawing, it is best to spread a two to three inch thick layer of wood mulch over the beds. If you have used Christmas tree branches, straw, or any other material to keep the iris from shaking, make sure to remove it as soon as possible.
Any type of mulch around rhizomes during spring rains can cause rotting.Loose leaves and debris can also be a breeding ground for leaf-chewing slugs. Some gardeners prefer to trim the iris foliage after the first hard frost of the fall. Others leave the old green leaves in place to provide some protection during the winter and continue to feed the rhizome on mild days. Cleaning is also important for drill control.Iris borers are present in many states in the U.
S., but not all of them, especially in the Northeast, Mid-Atlantic, and Midwest regions. Check with your agricultural extension service or the local chapter of the American Iris Society to see if they are in your area. Don Scoop, of Irises at Winterberry Gardens, has suggested that pine bark mulch can be used on iris beds (although not too thick).