Hardy hibiscuses are cold-tolerant and usually grow well in USDA zones 5 to 8. Wisconsin consists of zones 3 to 5, but according to Bachman’s Floral Gifts and Gardens, hardy hibiscuses usually grow well in zone 4, and they also grow marginally well in zone 3 if adequate mulch is applied during the winter. When choosing your hardy hibiscuses, consult with the master gardener or nursery personnel on site to discuss what varieties would do best in the area of Wisconsin in which you live.
Obtain and plant you hardy hibiscuses as soon as they become available in the spring, especially in the colder zones of Wisconsin. In general, they can be planted as late as Labor Day, but if you have a harsh winter, they may not survive.
Select and prepare a planting site in full sun. Amend the soil so that it is loose, rich in nutrients and well-draining. To do this, till the top 12 to 18 inches of the planting site and mix in several inches organic matter (such as sand, peat moss and compost).
Dig holes that are twice as wide, but just as deep as the hardy hibiscus’ container. Space multiple hibiscuses according to their mature size. They usually are planted the same distance apart as they are in mature spread, but if you are planting a hedge, they can be placed up to half that distance. For example, Hibiscus syriacus grow to be 4 feet wide, so plant 2 to 4 feet apart from one another.
Take the hibiscuses out of their containers and set them in the holes upright. Backfill the soil and pack it down to remove voids in the soil. Slowly water the planting site.
Cover the area with mulch in the fall before the ground freezes. In zone 3 and 4, use at least 6 inches of mulch. In zone 5, a few inches will probably suffice. Straw, pine needles, wood chips and bark make excellent mulches.