Blue hydrangeas (Hydrangea macrophylla) are prized for their vibrant color, which is unique because their bloom color is not fixed but changes based on soil chemistry. These “bigleaf” hydrangeas include both Mophead and Lacecap varieties. Achieving and maintaining a true blue hue requires a specific, intentional approach that focuses on managing the growing environment and the chemical composition of the soil.
Establishing the Ideal Growing Environment
Site selection is the foundation for a healthy blue hydrangea, which needs a balance of sun exposure and protection from intense heat. The ideal location provides morning sunlight and shade during the hottest part of the afternoon. This exposure ensures the plant receives enough light to produce vigorous blooms without risking scorched leaves or wilting from midday intensity.
The soil must be rich in organic matter and must drain freely, as hydrangeas dislike waterlogged roots, which can lead to rot. While they can tolerate various soil types, good drainage is necessary to prevent the consistent moisture they require from becoming excessive. Consistent moisture is maintained through regular, deep watering, especially during dry spells and hot summer weather.
A layer of organic mulch, such as bark or pine straw, should be applied around the base of the plant. Mulch helps regulate soil temperature and retain the consistent moisture that bigleaf hydrangeas demand. It also helps maintain the required acidic conditions by slowly breaking down, supporting the plant’s overall health.
The Chemical Key to Maintaining Blue Color
The distinct blue color in Hydrangea macrophylla flowers is directly caused by the presence and availability of aluminum compounds, not simply acidic soil. The pigment molecule, delphinidin, binds with aluminum ions within the flower to produce the blue shade. Without aluminum uptake, the flowers will default to pink.
Soil pH indirectly controls the color by determining whether aluminum is available for the plant to absorb. Aluminum is chemically bound to soil particles in neutral or alkaline conditions, making it inaccessible to the roots. To ensure aluminum is readily absorbed, the soil must be highly acidic, with an optimal pH range of 5.2 to 5.5.
Gardeners must conduct a soil test to determine the starting pH before attempting to amend the soil. To lower the pH and make aluminum available, aluminum sulfate is the most reliable amendment, often applied as a liquid drench. A standard application involves mixing about one tablespoon of aluminum sulfate per gallon of water, which should be applied to the soil every three to four weeks throughout the growing season.
Pre-water the plants before applying the aluminum sulfate solution to prevent root burn. Elemental sulfur is an alternative to aluminum sulfate, but it works more slowly to acidify the soil over time. Since the color change happens as the buds develop, it may take up to a year to see the full effect of the treatments.
Fertilization practices also play a role in color maintenance because phosphorus binds to aluminum in the soil, preventing its uptake by the plant. Gardeners should use a low-phosphorus, high-potassium fertilizer, such as a 25-5-30 ratio, to minimize this binding effect. Avoiding high-phosphorus products like super-phosphates or bone meal is necessary when striving for blue blooms.
Annual Pruning and Winterizing Strategies
Pruning for blue hydrangeas (Hydrangea macrophylla) must be timed carefully because most varieties bloom on “old wood,” meaning next year’s flower buds are set on stems from the previous season. Incorrect pruning can inadvertently remove next season’s blooms entirely. Pruning should only be performed immediately after the current year’s flowers have faded, typically in mid-to-late summer.
The goal of pruning is not to reduce the overall size of the shrub, but to remove spent flower heads, dead or damaged wood, and any weak or crossing stems. For older, overgrown shrubs, up to one-third of the oldest canes can be cut back to the ground to encourage rejuvenation. However, reblooming varieties, such as those in the ‘Endless Summer’ series, are more forgiving as they produce flowers on both old and new growth.
Winterizing is necessary in colder climates to protect the delicate old wood buds from freezing temperatures. Leaving the spent flower heads on the plant during the winter can offer some natural protection for the growth buds below. Gardeners should apply a thick layer of mulch, such as shredded leaves or straw, around the base of the plant once the ground begins to freeze.
In regions with harsh winters, covering the entire shrub with burlap or an insulated wrap provides extra defense against frost damage. For container-grown hydrangeas, move the pot to an unheated garage or protected structure to prevent the roots and old wood from freezing solid. Protecting these buds ensures the plant produces the desired blue flowers the following season.