Hi, Sandra! Thank you for your excellent question to AskExtension/Ask a Master Gardener. I believe I can help you with your Hibiscus.

There are hundreds of species of Hibiscus plants. Many are in the tropical Hibiscus family, Hibiscus rosa-sinesis while others are in the family Hibiscus moscheutos and are hardy outdoors in colder climates, such as Ohio. There is a tropical variety called Sunrise Sunscape (sold at many of the Big Box stores), so I am going to assume that you have a tropical Hibiscus plant that you would like to overwinter in your home. Please let me know if my assumption is incorrect, as that would change my answer.

It is not uncommon for Hibiscus leaves to yellow and drop. It happens when they are underwatered, overwatered or when their environment is changed. With the light and temperature changes caused by a move to indoors, it is not surprising that your Hibiscus is losing leaves. It will likely drop leaves throughout the winter. By spring it may have many fewer and much smaller leaves. It will bounce back once it lives outdoors again.

As to the yellow mold on the soil, it is almost certain that this is a harmless mold. Usually when the soil dries out it disappears. Otherwise, you can scrape it off if you wish. Or you can spray it with a 1:4 solution of hydrogen peroxide to water. Repeat as needed but store the spray bottle in a dark place or mix a fresh solution each time; hydrogen peroxide is not stable when exposed to light.

I have overwintered tropical Hibiscus many times with great success. They tend to get smaller and lose many leaves throughout the winter. Pruning them back helps conserve their limited energy, as does removing blossoms. (Plants need to expend a lot of energy to bloom.)  You can fertilize lightly and infrequently or not at all. Recognize that the plant will be in a mild hibernation, so it will need less water, less food and will produce fewer and smaller leaves. Be very careful not to overwater in order to avoid root rot. Your plant will do best with the brightest sunlight light you can give it, southern exposure if possible.

The biggest problem I had when overwintering hibiscus was pests. Aphids, thrip and spider mites are the most common culprits. Monitor your plant frequently and closely. Treat immediately if you see signs of pests. Neem oil is my go-to for pests, but there are many options on the market. Check the labels and follow directions carefully.

One last helpful hint: when spring arrives, and you are ready to return your hibiscus to the outdoors it will be best to do it in increments. At first, leave it out for about four hours in dappled sun or cloudy conditions. Gradually increase the amount of direct sunlight and time your plant spends outdoors. This will decrease the amount of “shock” to the plant and increase its chances of survival.

If you need clarification, or if you have further questions you can reply to this email and it will come directly to me. I would love to hear from you!

Here is an article that you might enjoy reading. It has a section on overwintering.

Care of Hibiscus rosa-sinesis Smithsonian Gardens https://gardens.si.edu/learn/educational-resources/plant-care-sheets/care-of-hibiscus-rosa-sinensis/

Good luck with your Hibiscus! They are beautiful plants … one of my very favorites!