It is best to overwinter tropical hibiscus like yours in the pictures as a houseplant indoors (not "dormant" in a dark room). Plants are usually brought indoors in September as the temperatures start to drop into the 40's and 50's.  

Tropical hibiscus will want as much light indoors as you can provide it.  A bright window is a good start - a grow light will be appreciated even more.

It is very common for this plant to drop a lot of leaves when brought indoors.  Do not be alarmed if this happens - new leaves will grow to replace the ones lost.

Provide water when needed and do not fertilizer over the winter months.

Check for insects before you bring it indoors and diligently inspect the plant for the first two to four weeks after being indoors.  If pest issues are noticed, deal with them before they become severe.  A preventative spray with Neem oil or insecticidal soap is a nice line of defense.  Continue to monitor and deal with pests on a regular basis all winter.

The primary goal is to keep the plant alive until it can be moved back outdoors in the spring after the danger of frost has passed.  It will not likely be the prettiest plant while indoors, especially by February or March.  Once it gets back outside in the warm, humid Iowa summer, it will quickly fill out and beginning blooming again.  

It is unlikely for the plant to bloom over winter and it may form flower buds that dry and fall off.  This is also relatively common for this plant when growing indoors over the winter as humidity and light are low and plants are stressed and more likely to drop any flower buds it produces.

Further questions can be directed to your local county Extension office. This local resource can be very helpful since they are familiar with trends and issues in your part of the state. Click on this link to get the contact information for the Polk county Extension office. https://www.extension.iastate.edu/polk/

Happy gardening!

--Aaron

Wonderful to hear.  The plants look great.  Enjoy!

--Aaron