Answer: Your zip code places you in zone 3. This very cold for growing Hydrangea macrophylla successfully, even this one which is considered root hardy into zone 4. So I am not sure you will be able to overwinter it successfully unless you bring it into a sheltered place such as a garage where the plant can be kept cold but the soil does not freeze solid. In order to bring it inside, though, it should be growing in a container. So that is not really an option at this point.
Most varieties of H. macrophylla bloom on the old wood from the year before. If winter kills off the branches or if spring frosts damage the early buds, then the plant will not bloom. (And if you prune it in fall, winter, or spring or early summer, it will not bloom for the same reason.) The Endless Summer variety however is able to bloom on both old growth and on new growth of the season. If the winter kills off the old growth, it can still bloom later in the summer and into the fall on the new growth. In climates where the plant does not die back at all in the winter, this can make its bloom season seem "endless".
In climates where it is normally too cold in winter to bloom these hydrangeas, the late blooming on new growth allows the plant to bloom nonetheless. To try to maximize the bloom potential of your hydrangea, it should be grown in a spot that is a warm microclimate, sheltered and protected from the winter wind. You might consider a wind break if it is in the open.
In your exceptionally cold climate I would also apply a very heavy mulch over the root area in late fall once the soil has frozen. I would also erect a wire mesh cylinder around it, attach burlap to that, and fill around the plant with a dry, nonpacking insulating material such as straw or oak leaves. Then put a tarp over top, but leave some holes for air circulation to prevent heat buildup and condensation on sunny days. Put this on in very late fall and remove it in the spring when the weather moderates so it can wake up naturally with the season.
If your perennial hibiscus is a Rose of Sharon shrub or Hibiscus syriacus, you could try to protect it the same way, covering the base of the plant. In spring, trim it back to remove any winter damage and hope it is root hardy for you. Normally this plant is only hardy to zone 5, too.
If your hibiscus is one of the perennial flower types that die back to the ground each winter, cut off the frost killed stems, then mulch very heavily over the root area and mark the location of the plant. In spring, remove the mulch so the soil can warm up sooner. Leave the marker in place to remind you where the plant is. This plant begins to grow very late in the season so be very patient waiting to see it come up again. Once it emerges it grows very fast in rich, moist soil.
Good luck with your plants this winter!
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