Hydrangeas are highly sought-after ornamental shrubs, but their successful cultivation in Colorado presents distinct challenges. The state’s unique climate, characterized by high altitude, intense sun, significant temperature swings, and arid conditions, requires a specific approach to planting. Understanding the exact timing and necessary environmental modifications is paramount to ensure these moisture-loving plants thrive.
Optimal Timing for Planting
The ideal time to plant a hydrangea in Colorado is during one of two specific windows: early spring or early fall. Spring planting is generally favored because it allows the root system a full growing season to establish before the intense heat of summer and the dry winter. Gardeners should aim to plant after the danger of the last hard frost has passed, typically from mid-May through early June, depending on elevation.
The secondary window is early fall, which offers cooler air temperatures that reduce stress while the soil remains warm. This encourages robust root growth before the plant enters winter dormancy. Planting must occur at least six to eight weeks before the average date of the first hard freeze to ensure the roots are well-anchored and hydrated. Planting during the peak heat of summer is not recommended, as the intense sun and low humidity place undue stress on newly transplanted shrubs.
Soil and Site Preparation in Colorado
Colorado’s native soil presents a major hurdle for hydrangeas, which prefer rich, slightly acidic, and consistently moist soil. Front Range and High Plains soils are often highly alkaline (pH 7.0 to over 8.0) and contain a high percentage of clay. This alkalinity is a significant issue because it binds aluminum, preventing its uptake by bigleaf hydrangeas (Hydrangea macrophylla) and resulting in pink or red blooms instead of blue.
The planting site requires substantial amendment before the shrub is introduced. Incorporating large amounts of organic matter, such as compost, aged manure, or peat moss, will improve the heavy clay’s drainage and aeration. To achieve blue flowers, elemental sulfur or aluminum sulfate must be mixed into the soil to lower the pH closer to the target range of 5.2 to 5.5. This pH adjustment requires regular monitoring and reapplication due to the high buffering capacity of Colorado’s alkaline soil.
Site selection is equally important due to the intensity of the high-altitude sun. Hydrangeas perform best when shielded from the harsh, direct afternoon sun, which can scorch leaves and cause rapid wilting. A location that receives morning sun and is protected by shade during the hottest hours of the day is ideal. Protection from strong, desiccating winds, common in the region, also helps prevent excessive moisture loss.
Step-by-Step Planting Process
Once the site is selected and amendments are ready, the physical planting must be executed precisely. Dig a hole that is two to three times wider than the root ball, but no deeper than the root ball itself. This width encourages new roots to spread horizontally into the surrounding amended soil rather than circling in a constricted space.
Before placing the plant, gently loosen the roots around the perimeter of the root ball, especially if they are tightly matted or circling. Position the hydrangea so the crown—where the stems meet the roots—is level with or slightly above the surrounding grade. This shallow placement prevents the crown from sitting in standing water, which can lead to stem rot.
Backfill the hole with a mixture of the native soil and the prepared amendments, tamping lightly to remove large air pockets. Immediately after planting, give the shrub a deep, thorough watering to settle the soil around the roots and eliminate any remaining air gaps. This initial saturation reduces transplant shock and connects the roots with the surrounding soil.
Ensuring Winter Survival and Establishment
The most persistent threat to newly planted hydrangeas in Colorado is the arid, freeze-thaw cycle of winter, which leads to desiccation and stem death. Establishing a consistent watering schedule is important throughout the first growing season and continues into the dormant months. Newly planted shrubs require deep watering several times a week to promote a robust root system.
Winter watering is necessary in this dry climate. When the ground is not frozen, usually once or twice a month, the shrub needs supplemental moisture to prevent its tissues from drying out. Applying a thick layer of organic mulch, ideally six to eight inches deep, around the base of the plant provides multiple benefits. The mulch insulates the roots from severe temperature fluctuations and helps the soil retain moisture.
For bigleaf varieties (Hydrangea macrophylla), which bloom on old wood, stems must be protected from winter damage to ensure the following season’s flowers. Gardeners should consider constructing a cylindrical cage or wrapping the shrub with burlap and filling the interior with dried leaves or straw. This provides a buffer against the extreme cold and harsh winter winds, safeguarding the dormant flower buds.