Hydrangeas are beloved for their large flower heads and attractive colors. Welcome them to your garden as a glorious focal point or for showy cut flowers. They are easy to love and grow across most of the United States, but need a little extra care in the realm of fertilizing. Be sure to follow these guidelines for how to fertilize hydrangeas a few times a year. Use these tips to welcome even more lush blooms throughout summer and fall and keep foliage healthy.
Before fertilizing, it is helpful to know the type of hydrangea you're growing. The three most popular varieties are panicle, smooth, and bigleaf. This guide will stay rather general and offer a section on changing the flower color of bigleaf varieties at the end. Follow this advice to learn which fertilizers are best for hydrangeas, and when and how to fertilize these plants to ensure that they grow to their fullest potential.
Types of Fertilizer for Hydrangeas
Hydrangea fertilizers come in three main forms: liquid, granule, and compressed spikes. All three offer different benefits, albeit only slightly:
- Liquid: Quickly absorbed by the plant's roots and leaves, liquid fertilizers are good for a quick boost of nutrients. They come either as a concentrate that can be diluted with water or ready-to-use. Follow the directions on the container, which will likely either direct the user to spray it on the plant's foliage or pour it around the base of the plant.
- Granules: Able to dissolve in water and be poured on soil or sprinkled around the plant's base, granules are one of the most popular and readily found fertilizer types. Work these time-release, polymer coated granules into the soil around the base of the hydrangea plant. Let them dissolve slowly. Their nutrients are fairly long-lasting.
- Spikes: Also good for long-term fertilizer, spikes are compressed so that they can be easily pushed into the soil around the base of the plant. Spikes gradually dissolve over two or three months. Note that spikes will only feed the plant in the exact spot of placement and if placed too close to base, can burn plant.
Organic vs. Inorganic Fertilizers
Many fertilizers meant for feeding hydrangeas contain chemicals. Still, a few of them have natural ingredients. Both boost the health of the plant and encourage blooms that are exceptionally huge and beautiful.
- Inorganic: Containing chemical and synthetic nutrients, inorganic fertilizer is made in a lab with the intention of producing specific growing results. These specifications include larger blooms or an increase in their number, or an increased acidity level, which hydrangeas are fond of. Inorganic fertilizers might also have time-released ingredients, which dissolve slowly, and depending on the brand, feed the hydrangea plant over a period of weeks or months.
- Organic: The choice to go organic with fertilizer is the gardener's. While organic fertilizers have the benefit of not adding chemicals to the soil, they may not be as strong as inorganic fertilizers. Consider using organic fertilizer to increase soil health and add more nutrients. They may contain earthworm castings, poultry manure, compost, or other natural ingredients. A combination of sulfur, compost, and peat moss is proven to be a good organic mix for hydrangeas.
Which Inorganic Fertilizer to Choose
When shopping for fertilizer, examine the labels for the amounts of nitrogen (N), phosphorus (P), and potassium (K). Typically hydrangeas thrive when fed an all-purpose, balanced fertilizer like a 10-10-10 N-P-K or 12-4-8 N-P-K. To increase the size and quantity of hydrangea blooms, consider a fertilizer with more phosphorus.
Phosphorus is the middle number, so a fertilizer labeled 10-20-10 will do. If exploring slow-release granular fertilizer, choose one that is labeled "bloom boost," which may also have a higher level of phosphorus.
How to Fertilize
Learning how to fertilize hydrangea shrubs is equally as important as the type of fertilizer chosen. Apply a slow-release chemical for shrubs and trees once a year. Depending on the variety, a balanced time-release fertilizer can be applied a few times a year, in spring and early fall. Take care not to apply too much fertilizer as it could burn the leaves. Too much nitrogen could encourage more leaf growth, longer stems, and less flower growth.
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. When using a slow-release fertilizer, lightly cover it with soil to activate it. Sometimes fertilizer might not be necessary in the summer, since most hydrangeas bloom better when they are a little hungry for nutrients. 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.
When to Fertilize
Hydrangeas benefit from applying fertilizer in mid-to-late spring and additional intervals recommended by the manufacturer of the fertilizer you choose to use. While the first set of flowers start to fade, apply the same slow-release bloom-boosting fertilizer used in spring. Time-release fertilizers usually need to be reapplied every three months. Other types typically need to be applied every one or two weeks. Then, in late winter, just before hydrangeas come back from dormancy, feed them another dose of an all-purpose fertilizer.
How to Change the Color of Bigleaf Hydrangeas
Endless Summer varieties, like other bigleaf hydrangeas, bloom in shades of blue or pink. Color depends on the soil pH. To change the color of these flowers, add small amounts of sulfur or lime to the fertilizer. Treat hydrangeas with sulfur, a soil acidifier, to lower the pH so that flowers remain or become blue. Purple or blue blooms arise if the fertilizer is low in phosphate (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.
A Guide to Understanding Fertilizers. OSU Extension.
General Care for Hydrangeas. OSU Extension.