How to Choose Hydrangea Fertilizer (2024)

Hydrangeas, beloved for their large flower heads and attractive colors, need extra care when choosing hydrangea fertilizer and knowing when and how to apply it. Fertilizer and soil amendments are pretty powerful and can change the color of bigleaf or mountain hydrangea types.

Hydrangeas should only need fertilizer a few times a year, so use this guide to help you choose the best fertilizer for your hydrangeas.

Know Your Hydrangea Type

It is essential to know which variety of hydrangea you have. If you have bigleaf hydrangeas, H. macrophylla, or H. serrata (mountain hydrangea), some commercial fertilizers can change the color of your hydrangea flowers from pink to blue. This color morphing is fascinating to behold, but it can be a surprise if you're not expecting it or have a color scheme planned for your garden. Some common varieties are:

• Bigleaf: Also called lace cap or mophead, this plant grows 6 to 10 feet tall and wide with 6-inch leaves. It produces white, pink, and blue flowers, and the soil pH affects flower colors.
• Mountain: This is a lesser-known variety, but its flowers can change color from pink to blue with higher acidic content in the soil; it is a smaller variety, growing 2 to 4 feet tall.
• Oakleaf: Hydrangea quercifolia can reach around 7 feet tall and wide and has white to purplish-pink flowers. Its leaves resemble those of an oak tree, hence its common name.
  
• Panicle: Tree hydrangeas (Hydrangea paniculata) are fast-growingflowering shrubsproducing cone-shaped late-summer blossoms when few other bushes bloom.
• Smooth: These flowering shrubs are not as tall as the panicle type. They produce ball-shaped flower heads, usually white, with some cultivars producing pink flowers.

Organic vs. Inorganic Fertilizers: Which Is Best?

Organic vs. inorganic fertilizer refers to using natural ingredients or introducing chemicals to the soil. Both boost the plant's health and encourage exceptionally huge and beautiful blooms. Phosphorus is the key ingredient that promotes bigger and more flowers.

Inorganic fertilizers introduce the element directly, while phosphorus introduces it organically through compost, animal manure, and bone meal. You can choose either type; however, here are the pros and cons of each:

• Inorganic:Inorganic fertilizer combines chemical and synthetic nutrients in a lab and is usually cheaper than organic. These formulas produce larger blooms and more flowers and can increase the soil acidity, which hydrangeas prefer. Inorganic fertilizers can also be formulated as time-release to last the entire growing season, feeding the plant over weeks or months. The downside is that introducing chemicals to the soil is not an environmentally friendly option.
• Organic:While organic fertilizers are eco-friendly, they may not be as strong as inorganic fertilizers or produce the most prolific blooms. However, organic fertilizer is an excellent choice for improving overall soil health. Organic fertilizers are made with earthworm castings, poultry manure, compost, or other natural ingredients. A combination of sulfur, compost, and peat moss is a good organic mix for hydrangeas. Organic, slow-release commercial fertilizers formulated for roses, such as a 15-10-10 NPK or 10-5-5 NPK, work well for hydrangea flower production.

Some say inorganic is best because it's cheaper, works faster, and produces more and larger blooms; others say organic is best because it's all-natural and better for the environment. So the answer depends on what you prefer.

Which Inorganic Fertilizer to Choose

Typically, hydrangeas thrive when fed a balanced or all-purpose fertilizer like a 10-10-10 N-P-K or 12-4-8 N-P-K. When shopping for fertilizer, examine the labels for the amounts of nitrogen (N), phosphorus (P), and potassium (K) to determine the levels it contains. Consider a fertilizer with more phosphorus to increase the size and quantity of hydrangea blooms. Refrain from getting a fertilizer with high nitrogen levels since it could encourage more leaf growth, longer stems, and less flower growth.

Phosphorus is the middle number, so a fertilizer labeled 10-20-10 will do. If you're exploring slow-release granular fertilizer, choose one labeled "bloom boost," which may also have a higher phosphorus level.

Picking Among the Various Fertilizer Forms

Hydrangea fertilizers come in three primary forms: liquid, granule, and compressed spikes. Liquid fertilizer is fast-release, granules can be both slower or fast-release, and spikes are timed or slow-release. All three are effective but have slightly different benefits:

• Liquid:Liquid fertilizers quickly provide nutrients for the plant's roots and leaves. They come either as a concentrate that can be diluted with water or ready-to-use. The directions for use vary: Spray on the plant's foliage or pour around the base of the plant. These work fast, giving plants immediate nutrients.
• Granules:Granules are among the most popular and readily available fertilizer types. They dissolve in water and can be poured into the soil or sprinkled around the plant's base. The beauty of granules is that in solid form, they work as timed-release, or when dissolved in water, they immediately feed the plant.
• Spikes:Spikes are small, thin fertilizer sticks planted upright in the soil around the plant base. They act as a long-term fertilizer solution, gradually dissolving over two or three months. The drawback with spikes is that they only feed the plant in the exact placement spot, and if placed too close to the base or the roots, where the fertilizer is concentrated, the ingredients can burn the plant.

How and When to Fertilize

Hydrangeas benefit from fertilizer applied in mid- to late spring and at additional intervals, The interval depends on whether you choose slow-, timed-, or fast-release fertilizer. The rule of thumb is to stop applying all fertilizer after August. In late winter, just before hydrangeas return from dormancy, feed them a dose of all-purpose fertilizer. Here's how to apply the different types:

• Slow-release: If choosing a slow-release option, apply to shrubs and trees, usually only once or twice a year, and lightly cover it with soil to activate it. When the first set of flowers starts to fade, apply the same slow-release bloom-boosting fertilizer used in spring.
• Timed-release (also controlled-release): Depending on the variety and fertilizer strength (determined from the packaging), a balanced time-release fertilizer can be applied a few times a year in spring and early fall, no more than every three months. Do not give more than the amount recommended on the packaging. Applying more fertilizer does not yield more or bigger flowers; too much could burn the leaves.
• Fast-release: In March, May, and July, lightly apply a fast-release fertilizer, spreading it around the drip line of the branches—not the base. Water well. It may need more frequent application, like every one to two weeks, but read the packaging and look at your flowers to judge. If leaves begin to yellow and appear unhealthy, add a light dose of liquid iron twice a year or another fast-acting fertilizer to remedy the issue.

How to Change the Color of Bigleaf or Mountain Hydrangeas

Like other bigleaf hydrangeas, Endless Summer varieties bloom in shades of blue or pink. H. serrata, or mountain hydrangea, is another species that can also change color.

Color morphing of these species depends on the soil pH. To change the color of hydrangea flowers, add small amounts of sulfur or lime to the fertilizer. Treat hydrangeas with sulfur, a soil acidifier, to lower the pH so flowers remain or become blue. Certain commercial products are sold specifically to "turn hydrangeas blue." These products work by adding aluminum sulfate or acidifiers to the soil.

If the fertilizer is low in phosphate, purple or blue blooms may arise (though low phosphate levels may limit the plant's ability to absorb aluminum). To shift their color to pink, raise the soil pH by treating bigleaf hydrangeas with lime.