Colorado encompasses nine U.S. Department of Agriculture plant hardiness zones, from 3a, with minimum annual winter temperatures as low as negative 40 degrees Fahrenheit, to 7a, with minimum annual winter temperatures as low as 0 degrees Fahrenheit. By choosing climate-suitable species and varieties of flowering hydrangea and cultivating them to enhance the plants' strengths, it is possible to grow an array of healthy and hardy hydrangeas in all of Colorado's plant hardiness zones.
Not all hydrangea species and varieties are suitable for Colorado's coldest zones. Good choices for all of Colorado include smooth hydrangea (Hydrangea arborescens), hardy in USDA zones 3 through 9 and growing 3 to 5 feet tall and wide. The plants feature cream-colored flowerheads composed of clusters of tiny flowers. Planted in consistently moist, protected locations, the hydrangeas can even survive being frozen to the ground in winter, regrowing from the roots and blooming the next season. This habit of growing on "new" wood means that pruning can be done at the end of winter.
In Colorado, the bigleaf hydrangeas (Hydrangea macrophylla), generally hardy in USDA zones 5 or 6 through 9, depending on variety, will grow in the state's warmest zones. But many traditional bigleaf hydrangeas are susceptible to damage from late spring frosts that can destroy flower buds for the year. To optimize the chances for flowers, purchase modern hybrids like "Bailmer" Endless Summer (Hydrangea macrophylla "Bailmer" Endless Summer," hardy in USDA zones 4 through 9. Endless Summer blooms on both old and new wood. If buds are frozen in spring, flowers will still be produced later in summer.
Another hydrangea suitable for all of Colorado's climate zones is the peegee hydrangea (Hydrangea paniculata "Grandiflora"), hardy in USDA zones 3 through 8. Because it blooms on new wood, oakleaf hydrangea is not susceptible to bud loss through exposure to freezing spring temperatures. The plants can be grown as multistemmed shrubs or trained as small trees, growing up to 25 feet tall and 16 feet wide. Plant in soil that is consistently moist. The colder the hardiness zone, the more sunlight the plants tolerate. The plants are so vigorous that they have been reported as invasive in a few places.
Colorado Hydrangea Considerations
Hydrangeas appreciate protection from cold winds and climate extremes. Because the genus loves moisture overall, group hydrangeas of any species together with other water-loving plants. When planting, allow enough space between individual shrubs to accommodate each plant's mature size. Irrigation with drip systems provides the most consistent moisture with the least evaporation. Fertilize with a balanced product, like 24-8-16, using 1 tablespoon of concentrated dry product per gallon of water, applied every seven to 14 days during the growing season. Gardeners with bigleaf hydrangeas should protect them with agricultural fleece, plastic sheeting or even old blankets, if late frosts threaten in spring.