Are all hydrangeas perennials? If not, how can you tell which are or which are not?
All hydrangeas are either woody or herbaceous perennials. The problem for northern states like Michigan is that not all hydrangeas are winter hardy, and are not able to take the Zone 5 winters that can dip to -20 degrees. We seem to crave those bigleaf hydrangeas (Hydrangea macrophylla) with lovely pink and blue, large lacecaps. They do not do well in Zone 5 unless extremely well-sited. Once you travel south into Zone 6 and beyond, the plants become more common and successful. In Zone 5, these hydrangeas should be considered container plants to be brought inside for the winter.
There is a climbing hydrangea (H. petiolaris), which is winter hardy here and can be used both as a shrub or a disguise for a large structure. It produces a white blossom in late June that is fragrant and lasts for two weeks or more. Two other hydrangeas with a large shrub or spreading tree habit are panicle hydrangea (H. paniculata) and oakleaf hydrangea (H. quercifolia). Both are winter hardy and work well in a larger landscape. In both cases, it is important to obtain a named variety from a reputable nursery to insure plant characteristics live up to your expectations.