Most nursery guides will tell you to plant hydrangeas in fall. The reasoning is sound enough for warmer zones: cooler air, warm soil, less transplant shock. But on Cape Cod I have watched too many fall-planted hydrangeas struggle through their first winter to keep following that advice.
The problem is timing. Our falls are short. By October the soil temperature is already dropping, and a hydrangea planted in September has perhaps six weeks to establish roots before the ground goes cold. That is not enough. The plant enters winter with a shallow root system and no reserves, and the freeze-thaw cycles of a Cape Cod January heave it half out of the ground.
I plant my mopheads and lacecaps in late April or early May, once the soil has warmed to at least fifty degrees. The plant gets a full growing season to root in — five months of warm soil, regular rain, and lengthening days. By the time November arrives, the root system is deep and established. It can hold on.
The other reason is practical. In spring you can see the nursery stock in active growth. You can tell a healthy plant from a stressed one by the color of the new leaves and the vigor of the buds. In fall, everything is winding down, and a weak plant can look the same as a strong one.
I know this goes against the books. But the books were not written for a sandy peninsula sixty miles out into the Atlantic. You garden where you are, not where the textbook was.