Why is one rose of sharon healthy when another 20 feet away is wilting and turning yellow?
Please see photos. Both have in-ground watering, now turned off.
Certified GKH Gardening Expert
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