Answer: Last winter's oscillating temperatures were very stressful on many plants and caused winter damage on many plants. Both of these are winter hardy into zone 5, and you are located at the edge of their hardiness, so a difficult winter could set them back badly. The hydrangea blooms on old wood, so excessive winter damage could prevent them from blooming. Spring frosts/freezes can also damage tender new growth and prevent blooming as a result.
Depending on your microclimate where they are planted, you may find that these hydrangeas do not bloom well (or at all) most years. A spot with protection from sweeping winter wind as well as avoiding a low spot (frost pocket) might be an important factor in how well they perform.
Hydrangeas also need a rich, organic soil that stays evenly moist yet is well drained. This means it should be damp like a wrung out sponge, not sopping wet and not dried out. Overly wet soil or dry soil would both cause poor growth.
This type of hydrangea is pruned in spring only to remove dead wood, then in summer if needed it can be thinned or lightly shaped after bloom.
The Hibiscus syriacus shrubs tend to suffer winter damage in cold winter climates such as yours and usually need to be pruned in the spring to cut off the winter killed branches. If necessary, they can be pruned a bit more to restore a symmetrical look. Early spring is the only time these should be pruned. They come into growth fairly late in the spring, but you should see vigorous growth by now. These shrubs bloom on the new growth of the season, so they go on to bloom later in the summer.
Rose of Sharon needs full sun all day long to bloom its best, it likes heat and sun. Average soil is fine, but it must have a well drained location. In a poorly drained spot it will be less winter hardy and will also sometimes suffer root problems.
Another issue that could be causing poor performance would be related to whether or not they are rooting into the soil as they should. If planted with encircling roots left intact rather than cut or untwined and directed outward, they could be suffering from constricted roots that have stayed in the original planting hole.
As far as fertilizing, an annual spring top dressing of compost along with a slow release granular fertilizer per the label directions should be adequate. Using an organic mulch year round will also help feed the soil slowly as it breaks down over time.
I hope this helps you trouble shoot.
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