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The best times are in late fall or winter or in spring before buds form. But they bloom on new wood so that gives you other opportunities, too.

https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/pruning-rose-of-sharon.htm

This article should help:

https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/rose-of-sharon-not-blooming.htm

Normally, an established Rose of Sharon will not need any extra water. They are semi drought tolerant, and can be sensitive to overwatering. Water only when the soil is bone dry down to about 4 inches or so.

With that being said... It seems that time has caught up to the unlucky one with less of an "immune system". From here, I would treat with a fungicide, and treat again in the spring. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

Make sure to let the soil dry out quite a bit between waterings. You will likely not need the watering system, unless you just want it handy for times of severe drought.

Of course, that is just the most common reasoning for this to happen. Depending on the location, and your feeding habits, the tree can be lacking in nutrients from being higher in elevation, or can be overfed do to sitting in a low spot.

If you do not feed at all, then this can cause yellowing leaves as well.

This article will help you to care for the shrub: https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/rose-of-sharon-bush.htm

Rose of Sharon can suffer from a number of pests and diseases. You can move your shrubs to the garage or anywhere it is above freezing to get them out of your living area. Neem oil may help with the bugs.

Does it get below -20 F (-29 C) in Richmondhill? That is the minimum winter temperature for Rose of Sharon. If it gets colder than that, when you plant it next summer, you will have to take precautions for the next winter.

This article may help you treat the diseased parts of the plant:
https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/rose-of-sharon-problems.htm

The second link has some information about other ways to protect your plant in the winter time.

https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/rose-of-sharon-winter-care.htm

The second link has some information about other ways to protect your plant in the winter time

Unfortunately, your photo did not come through. Unless you see other signs of the pest, like insect bore holes or shavings from damage, it can be very hard to pin down damage.

Bark coming off around the base is only a problem if it is excessive, and makes a complete ring around the shrub.

Other things that can cause this issue can be fungal or bacterial infections, and sometimes simply overwatering too often.

Soft rot can be common, and this article will address the issue:

https://www.gardeningknowhow.com/plant-problems/disease/bacterial-soft-rot.htm

This article will help you with Rose of Sharon care: https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/rose-of-sharon-bush.htm

Here are two articles that will help

https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/rose-of-sharon-plant-cuttings.htm

https://www.gardeningknowhow.com/ornamental/shrubs/rose-of-sharon/rose-of-sharon-seeds.htm