Answer: These shrubs normally leaf out very late in the spring, well after many other plants have begun to grow. So what you are seeing may be normal. Hibiscus syriacus or Rose of Sharon is considered winter hardy into zone 5. Your zip code places you in zone 5A or the coldest part of that zone. Depending on your microclimate, it might actually be as cold as zone 4, especially if you garden in an exposed and windy location. I mention this because it could affect the expected winter hardiness of your shrubs. In a sheltered location with well drained soil and with a generous layer of mulch over the root area, I would expect them to be just fine. Each spring you will probably need to trim them back to remove winter damaged stems and then perhaps trim a bit more to give them a somewhat symmetrical look. This does not limit flowering because they bloom on new growth of the season.
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