Hydrangeas are popular shrubs valued for their large, striking flower heads that can transform a garden space. A common frustration for many gardeners, however, is a hydrangea that produces plenty of lush foliage but few or no flowers. While these plants are not considered heavy feeders, providing the correct nutritional support is important for encouraging bloom production. The key to abundant flowering lies in achieving a precise balance of nutrients, particularly when considering the ratio of nitrogen to phosphorus.
Essential Nutrient Balance for Bloom Production
Nitrogen (N) primarily supports the growth of leaves and stems, leading to rich green foliage. An excessive amount of nitrogen is the most frequent feeding mistake, as it promotes vigorous vegetative growth at the expense of flower development.
Phosphorus (P) is the element directly linked to robust flower initiation, bud set, and strong root development. To shift the plant’s energy toward blooming, look for fertilizers where the middle number (P) is higher than the first number (N), such as a 10-20-10 or 0-10-10 formulation. This lower nitrogen content ensures the plant is not overly focused on simply producing more leaves.
Potassium (K) contributes to the overall health of the plant, improving its resistance to drought, disease, and cold stress. While phosphorus directly impacts blooms, potassium helps fortify the plant structure that supports them. A slow-release, balanced formulation like 10-10-10 is often sufficient for maintaining established, healthy hydrangeas.
Adjusting Soil pH for Optimal Hydrangea Health
The soil’s pH level plays a major role in a hydrangea’s health and its ability to absorb nutrients. For Hydrangea macrophylla (bigleaf) and Hydrangea serrata (mountain), the soil pH directly determines the flower color, governed by the plant’s ability to absorb aluminum.
Highly acidic soil (pH 5.0 to 5.6) makes aluminum readily available, resulting in blue or purple blooms. To achieve this, gardeners can apply soil amendments like Aluminum Sulfate or elemental sulfur to lower the pH. Conversely, alkaline soil (pH 6.0 or higher) binds the aluminum, preventing its uptake and causing the flowers to be pink or red.
To encourage pink blooms, the pH should be raised by applying garden lime, typically made from ground limestone. A high-phosphorus fertilizer can also discourage the plant from utilizing aluminum, even at a lower pH. Changing the soil’s pH is a gradual process that may require several weeks or months to show noticeable results.
When and How to Fertilize Hydrangeas
Feeding should commence in early spring, typically after the last expected frost, when the plant begins its push of new growth. This initial application provides the necessary energy for the season’s development.
For re-blooming varieties, a second, lighter application may be beneficial in early summer to support continuous flowering. A slow-release granular fertilizer is preferred because it provides a steady supply of nutrients over an extended period, reducing the risk of over-fertilization compared to liquid feeds.
Stop all fertilization by mid-summer (late July or early August). Continuing to feed past this point stimulates tender new growth that will not have time to harden off before the first frost. This soft growth is susceptible to winter damage, which can kill the flower buds needed for the following year’s blooms. Always water the plant thoroughly before and after applying fertilizer to distribute nutrients and prevent fertilizer burn.
Common Non-Feeding Causes of Lackluster Blooms
While nutrition is a factor, fertilizer alone cannot correct all issues that prevent hydrangeas from blooming. One common reason for a lack of flowers is improper pruning, especially on varieties that bloom on “old wood.” These buds are formed on the previous year’s growth, and pruning types like H. macrophylla in the fall or spring can inadvertently remove them.
Insufficient sunlight is another frequent cause, as hydrangeas require sun to produce the energy necessary for flowering. Although they tolerate some shade, most varieties need at least four to six hours of sunlight per day, preferably morning sun. Receiving less than this results in a healthy, leafy plant with few flowers.
Winter damage is a significant environmental factor, particularly in colder climates. Unpredictable temperatures or late spring frosts can destroy delicate flower buds, especially on old wood. Ensuring the plant is properly sited and has adequate winter protection can be more important for bloom production than any feeding regimen.