Can I prune my tall hibiscus plants before I bring them in for the winter?
Yes, if you are bringing in a potted hibiscus, you can cut it back by up to 1/3 before bringing it indoors. Otherwise, you should wait till spring. This article will help: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/tips-for-pruning-hibiscus-plants-when-to-prune-hibiscus.htm
Just as the buds begin to get ready to bloom, the stem just under the blooms weakens and the bud falls off.
This article may have some suggestions that can help: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-flowers-hibiscus-blossoms-falling-off-plant.htm
How to propagate hibiscus.
This article should help with that: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-propagation.htm
I live in Indiana where we do have freezes, but Hibiscus can be left in the ground year around. My question is, do I cut back in the fall before the first frost or the spring? (If I am to cut back, when?) I dug them up from a friend's house in the spring for transplant and they were cut back). Thanks so much.
For those left outdoors over winter, it is usually better to wait for spring pruning. Here is more information: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/tips-for-pruning-hibiscus-plants-when-to-prune-hibiscus.htm
What should I use to debug my yellow hibiscus tree before digging it up and bringing it indoors?
You can hose down the plant prior to bringing indoors and then treat with neem oil. It is safe for pets and people but kills almost all pests. It is also an effective fungicide. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
After having a couple months with lovely flowers, my hibiscus bush leaves have turned yellow. Is this normal or has my shrub a virus or disease?
There are several reasons this can happen, normally associated with stress of some kind--or it may simply be going dormant. This article explains many of them and will help you decide which is likely for your plant: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/answers-to-what-causes-hibiscus-leaves-turning-yellow.htm
How do I root a hibiscus plant?
This article will help: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-propagation.htm