Hydrangeas can be grown successfully in Colorado, but the state’s climate presents three primary challenges: intensely cold winters, high-altitude sun, and naturally alkaline soil. Successful cultivation depends on selecting the right species and implementing specific care practices to mitigate these regional extremes. By addressing these local conditions, gardeners in the Front Range and mountain valleys (primarily USDA Zones 4–6) can enjoy these shrubs.
Selecting Varieties for Cold Tolerance
The most important factor for reliable flowering in Colorado is choosing a variety that can handle the cold and unpredictable spring weather. Hydrangeas bloom on either “old wood” (last year’s growth) or “new wood” (growth from the current season). Because Colorado’s harsh winters and late frosts frequently kill back old wood, new wood bloomers are the most dependable choice.
The Panicle Hydrangea (Hydrangea paniculata) is the hardiest and most recommended type, thriving down to USDA Zone 3. Varieties like ‘Limelight,’ ‘Quick Fire,’ and ‘Little Lime’ produce flowers on new growth, ensuring a consistent bloom display even after severe winter dieback. These cultivars also tolerate more sun exposure than other types, making them well-suited to the intense high-altitude light.
Smooth Hydrangeas (Hydrangea arborescens), such as ‘Annabelle’ and ‘Incrediball,’ are another excellent choice for cold climates, also blooming reliably on new wood. These typically feature large, dome-shaped white flowers and are cold-tolerant, surviving in Zone 3 areas. They provide a lush presence in the garden and require minimal winter protection.
Bigleaf Hydrangeas (Hydrangea macrophylla) are the most popular but the most challenging, as traditional varieties bloom only on old wood. Winter cold often kills the flower buds, resulting in foliage but no flowers. For gardeners determined to grow the classic mophead or lacecap types, modern reblooming cultivars like the ‘Endless Summer’ series are necessary. These rebloomers develop buds on both old and new wood, providing a backup bloom source if winter kills the older growth.
Oakleaf Hydrangeas (Hydrangea quercifolia) offer lobed foliage that turns burgundy in the fall, along with cone-shaped white flowers. While hardy to Zone 5, they bloom on old wood, meaning flowering can be inconsistent in the coldest Colorado locations. They are best used as a specimen shrub for their foliage, with the blooms being an occasional bonus.
Addressing Alkaline Soil and Flower Color
Colorado’s native soil is generally alkaline, often exceeding a pH of 7.0. This soil chemistry presents two problems for hydrangeas: nutrient availability and flower color control. The high pH can lock up essential micronutrients, particularly iron, leading to chlorosis, where the leaves turn yellow between the green veins.
To promote plant health, gardeners must amend the soil to lower the pH to an acidic range, ideally between 5.5 and 6.5. Incorporating organic matter helps buffer the alkalinity. For a more direct pH reduction, elemental sulfur can be applied to the soil, though it works slowly and must be monitored carefully.
For Bigleaf Hydrangeas (H. macrophylla), the soil pH directly dictates the flower color. In acidic soil (below 6.0), available aluminum ions result in blue flowers. In Colorado’s native alkaline soil (above 7.0), the aluminum is chemically bound and unavailable, causing the flowers to default to pink or red shades.
Achieving a true blue is difficult because the high soil pH must be consistently overcome. Gardeners aiming for blue must apply aluminum sulfate or a specialized commercial blueing agent multiple times per season. This ongoing effort is complicated by the use of alkaline municipal water, which continuously neutralizes the acidifying amendments. It is often simpler to embrace the pink and purple shades naturally produced by the alkaline soil.
Managing Placement and Winter Survival
The placement of a hydrangea is paramount, as it creates a microclimate that determines the plant’s success. In Colorado’s high-altitude environment, the sun’s intensity is greater, making afternoon shade a necessity for most hydrangeas. Planting on the eastern or northern side of a structure provides the morning sun needed to bloom while protecting them from scorching western exposure.
High winds are another significant threat, causing desiccation (the drying out of plant tissue). Winter winds are particularly damaging because they strip moisture from the stems and buds when roots cannot draw water from the frozen ground. Placing hydrangeas near a windbreak, such as a fence or an evergreen hedge, greatly reduces this winter damage.
Proper winterizing begins in late fall by providing a deep watering before the ground freezes solid. Well-hydrated roots are better equipped to survive the dry, cold conditions of a Colorado winter. Applying a thick layer of mulch (two to four inches deep) around the base of the plant helps insulate the root zone from extreme temperature swings.
For the less-hardy Bigleaf varieties, additional physical protection is warranted to prevent the loss of old wood buds. This can involve constructing a chicken wire cage around the shrub and filling it loosely with straw or shredded leaves for insulation. This covering protects the buds from harsh winds and the damaging freeze-thaw cycles that occur during warm winter days and frigid nights.