Blood-on-the-Snow

Mountain hydrangea is the cold-bud-hardy alternative to bigleaf hydrangea, and it is the right species pick for any Puget Sound microsite where late frosts routinely kill macrophylla flower buds. Those microsites are real and named: the Deschutes River valley around Olympia, the Snoqualmie Valley floor, cold air pooling pockets in the Carnation and Duvall flats, the Skagit delta, and any north-facing slope that holds cold longer than its neighbors. A gardener in one of those locations will lose bigleaf hydrangea blooms most years to late frost on the old-wood buds, and the standard recommendation to "prune in March, not August" does not rescue the crop once the buds are dead. Serrata in general, and the Tuff Stuff line in particular, was bred to solve exactly that problem: flower buds that tolerate more cold, plus remontant genetics that set a backup crop on new wood when the old-wood buds do fail. Outside the cold pockets, serrata performs well everywhere macrophylla does but on a smaller plant (most selections 2-3 ft rather than 4-6 ft), which makes it a useful substitute in tight spaces. Flower color follows the same aluminum-availability chemistry as macrophylla, so Puget Sound clay soils typically produce blue flowers by default. NC State's Extension Gardener Plant Toolbox positions serrata as the "smarter" macrophylla, which is a reasonable framing for this region.

— Chris Welch, ISA Certified Arborist