What type of fertilizer is best for hydrangeas? - Plant Care Guide

The best type of fertilizer for hydrangeas is one that provides a balanced blend of essential nutrients, particularly favoring phosphorus to encourage abundant blooms, without excessive nitrogen that can promote leafy growth at the expense of flowers. For Bigleaf hydrangeas (Hydrangea macrophylla), the soil's pH level also plays a crucial role in determining bloom color, so some fertilizers are specifically designed to adjust pH for blue or pink flowers. Generally, a slow-release granular fertilizer with an NPK ratio like 10-20-10 or 5-10-5 is a good starting point.

What Nutrients Do Hydrangeas Need?

Hydrangeas, like all plants, require a variety of essential nutrients for healthy growth and prolific blooming. These nutrients are primarily absorbed from the soil and can be broadly categorized into macronutrients (needed in larger quantities) and micronutrients (needed in smaller quantities). Understanding these needs helps in choosing the best fertilizer for hydrangeas.

Here's a breakdown of the key nutrients hydrangeas need:

Macronutrients (N-P-K)

These are the "big three" nutrients commonly listed on fertilizer labels as NPK numbers (Nitrogen - Phosphorus - Potassium).

1. Nitrogen (N):

   • Role: Primarily responsible for lush, green leafy growth and strong stem development. It's crucial for chlorophyll production, which gives leaves their green color and is essential for photosynthesis.
   • Too Much Nitrogen: While necessary, excessive nitrogen can be detrimental to hydrangeas because it promotes vigorous foliage growth at the expense of flower production. This can result in a beautiful, bushy plant with very few or no blooms.
   • Deficiency Symptoms: Yellowing of older leaves, stunted growth.

2. Phosphorus (P):

   • Role: Absolutely vital for flower formation, root development, and overall plant energy transfer. It's the nutrient that truly drives bloom production.
   • For Hydrangeas: A higher phosphorus content in a fertilizer is often recommended to encourage more abundant and vibrant blooms. This is why ratios like 10-20-10 or 5-10-5 are common recommendations.
   • Deficiency Symptoms: Purplish tint on leaves, poor flowering, stunted root growth.

3. Potassium (K):

   • Role: Essential for overall plant health, vigor, and resilience. It helps regulate water uptake, strengthens cell walls, improves disease resistance, and aids in flower and fruit development. It's like a general "immune system booster" for plants.
   • Deficiency Symptoms: Yellowing or browning of leaf edges (scorch), weak stems, reduced flowering.

Micronutrients (Trace Elements)

These are needed in smaller quantities but are just as crucial for specific plant functions.

1. Iron (Fe):

   • Role: Critical for chlorophyll production and is particularly important for hydrangea color, especially for blue blooms.
   • Deficiency Symptoms: Interveinal chlorosis (yellowing of leaves between the veins, while veins remain green). This is a common issue for hydrangeas in alkaline soils where iron becomes less available.

2. Manganese (Mn):

   • Role: Involved in photosynthesis and nitrogen metabolism.
   • Deficiency Symptoms: Similar to iron deficiency, causing interveinal chlorosis.

3. Boron (B):

   • Role: Important for cell wall formation, sugar transport, and flower/seed development.

4. Zinc (Zn), Copper (Cu), Molybdenum (Mo), Chlorine (Cl):

   • Also necessary in minute amounts for various enzymatic processes and plant functions.

Why Soil pH Matters for Nutrient Availability:

The soil's pH level significantly impacts how available these nutrients are to hydrangeas.

• In acidic soils (low pH), nutrients like iron and manganese are more readily available.
• In alkaline soils (high pH), these micronutrients can become "locked up" and unavailable to the plant, even if present in the soil. This is why Bigleaf hydrangeas (Hydrangea macrophylla) grown in alkaline soil often struggle to produce blue flowers, as iron availability is crucial for blue coloration.

Therefore, the best fertilizer for hydrangeas often considers not just the NPK ratio but also its potential impact on soil pH, or it may be used in conjunction with soil amendments to adjust pH for optimal nutrient uptake and bloom color.

How Does Soil pH Affect Hydrangea Bloom Color?

The soil's pH level is a fascinating and crucial factor that directly influences the bloom color of certain hydrangea species, primarily the Bigleaf Hydrangea (Hydrangea macrophylla) and, to a lesser extent, the Mountain Hydrangea (Hydrangea serrata). Other types, like Panicle (Hydrangea paniculata) and Smooth (Hydrangea arborescens) hydrangeas, are largely unaffected by soil pH and will consistently produce white or greenish-white blooms regardless of the soil.

Here's how soil pH affects Bigleaf Hydrangea bloom color:

• Blue Hydrangea Blooms = Acidic Soil:

  • To achieve blue flowers, Bigleaf Hydrangeas need a soil pH of 5.0 to 5.5 (strongly acidic).
  • The blue color is created when the plant is able to absorb aluminum from the soil. In acidic conditions, aluminum becomes soluble and available for the plant's roots to take up.
  • If your soil is naturally alkaline or neutral and you want blue blooms, you need to lower the soil pH and/or add an aluminum source.
  • Amendments to Promote Blue:
    • Aluminum Sulfate: The most effective and fastest-acting way to provide aluminum and significantly lower pH. Apply according to package directions.
    • ** elemental Sulfur:** Lowers pH more gradually.
    • Coffee Grounds, Peat Moss, Pine Needles/Bark Mulch: These organic materials decompose and contribute to soil acidity over time, providing a more gradual, long-term effect.

• Pink Hydrangea Blooms = Alkaline to Neutral Soil:

  • To achieve pink flowers, Bigleaf Hydrangeas need a soil pH of 6.0 to 7.0 (slightly acidic to neutral) or higher (alkaline).
  • In these higher pH ranges, aluminum becomes insoluble or "locked up" in the soil and is unavailable for the plant to absorb, even if it's present. Without available aluminum, the blue pigment cannot form, and the flowers will develop a pink hue.
  • If your soil is naturally acidic and you want pink blooms, you need to raise the soil pH.
  • Amendments to Promote Pink:
    • Garden Lime (Calcium Carbonate): This is the most common amendment used to raise soil pH. Apply granular lime according to package directions, typically in fall or early spring.
    • Dolomitic Lime: Contains magnesium in addition to calcium, also raises pH.
    • Bone Meal: A natural source of phosphorus, which can also slightly increase pH.

• Purple/Mauve/Mixed Blooms = Slightly Acidic to Neutral Soil (pH 5.5 to 6.0):

  • In the transition zone where the pH is neither strongly acidic nor strongly alkaline, or if there's a moderate amount of available aluminum, hydrangea blooms can appear purple, mauve, or even display a mix of pink and blue on the same plant or even within the same bloom cluster. This often happens if the soil pH is somewhere around 5.5 to 6.0.

Important Considerations:

• Soil Testing: Before attempting to change your hydrangea's bloom color, it's crucial to test your soil's current pH level. This will tell you what adjustments, if any, are needed. A simple soil pH test kit is inexpensive and easy to use.
• Aluminum Availability: For blue blooms, simply lowering pH might not be enough if your soil naturally lacks aluminum. In such cases, adding aluminum sulfate becomes necessary.
• Potted Hydrangeas: Changing the color of potted hydrangeas is often easier because you have more control over the potting mix pH and can use specific amendments more effectively.
• White Hydrangeas: White varieties of Bigleaf Hydrangeas (e.g., 'Madame Emile Mouillere', 'Snowball') will always remain white regardless of soil pH. Their genetics do not allow for blue or pink pigmentation.

Understanding the relationship between soil pH and aluminum availability is key to successfully influencing the bloom color of your Bigleaf Hydrangeas. It's a precise science, but a rewarding one for those seeking specific hues.

What are the Different Types of Fertilizers for Hydrangeas?

Choosing the best fertilizer for hydrangeas means understanding the different forms and their applications. Fertilizers vary not just in their NPK ratios, but also in how quickly they release nutrients and how they are applied.

Here are the different types of fertilizers commonly used for hydrangeas:

1. Granular Fertilizers (Slow-Release vs. Quick-Release):

   • Description: These are solid pellets or granules that are spread around the base of the plant and then watered in.
   • Slow-Release (Recommended for Hydrangeas):
     • How it works: Nutrients are coated or formulated to release gradually over weeks or months, often triggered by temperature or moisture.
     • Benefits: Provides a steady supply of nutrients, reducing the risk of over-fertilization and "burning" the roots. Requires fewer applications. Promotes consistent, healthy growth.
     • Examples: Osmocote, Espoma Holly-tone (for acid-loving plants), specific hydrangea fertilizers like Miracle-Gro Shake 'n Feed for Hydrangeas.
   • Quick-Release (Use with Caution):
     • How it works: Nutrients are immediately available to the plant once watered in.
     • Benefits: Can provide a rapid boost if a plant is showing severe deficiency.
     • Limitations: High risk of fertilizer burn if over-applied. Nutrients can leach out quickly. Requires more frequent applications. Not ideal for sustained feeding.

2. Liquid Fertilizers (Concentrates and Ready-to-Use):

   • Description: Soluble fertilizers that are mixed with water and applied directly to the soil (or sometimes as a foliar spray).
   • How it works: Nutrients are in a liquid form, making them immediately available for plant uptake.
   • Benefits: Fast-acting, good for a quick boost or precise nutrient delivery. Easy to control dosage by diluting.
   • Limitations: Requires more frequent application than slow-release granular fertilizers. Higher risk of over-fertilization if not diluted correctly.
   • Examples: Liquid plant food for acid-loving plants, common all-purpose liquid fertilizers (ensure appropriate NPK).

3. Organic Fertilizers:

   • Description: Derived from natural sources like compost, manure, bone meal, blood meal, fish emulsion, worm castings, etc.
   • How it works: Nutrients are released slowly as microorganisms in the soil break down the organic matter. They also improve soil structure and microbial life.
   • Benefits: Gentle on plants, low risk of burn, improves soil health over time, environmentally friendly. Can provide a broader spectrum of micronutrients.
   • Limitations: Slower acting, nutrient ratios can be less precise, may have an odor. May not provide enough immediate impact for severe deficiencies.
   • Examples: Espoma Holly-tone (mentioned above, also considered organic), compost, bone meal.

4. Specialty Hydrangea Fertilizers (Color-Changing):

   • Description: These fertilizers are specifically formulated for hydrangeas, often with an NPK ratio favoring phosphorus (e.g., 10-30-10 or 10-15-10). Some also contain aluminum sulfate to promote blue blooms, or a higher calcium content (like lime) to promote pink.
   • Benefits: Tailored to hydrangea needs, simplifies the choice for gardeners, some can directly influence bloom color.
   • Limitations: Can be more expensive than general fertilizers. Still requires attention to soil pH for consistent color change.
   • Example: Many brands offer fertilizers explicitly labeled "for Hydrangeas."

When choosing a fertilizer, always consider your soil's current pH, the desired bloom color (for Bigleaf hydrangeas), and the current health of your plant. A soil test is highly recommended to determine precise nutrient deficiencies and pH levels, guiding your fertilizer selection for the best results for your hydrangeas.

When Should I Fertilize Hydrangeas?

Timing your hydrangea fertilization is just as important as choosing the right type of fertilizer. Applying fertilizer at the wrong time can be ineffective, promote unwanted growth, or even harm your plant. The goal is to provide nutrients when the plant is actively growing and preparing to bloom, and to stop when it's entering dormancy.

Here’s a general guide for when to fertilize hydrangeas:

1. Early Spring (After the Last Frost / When New Growth Appears):

   • This is the most crucial time for the first application.
   • As new green shoots begin to emerge from the base of the plant or along the stems, and all danger of hard frost has passed, it's time to feed.
   • This initial feeding provides the necessary nutrients to support vigorous new growth, develop strong stems, and build up energy reserves for the upcoming blooming season.
   • For slow-release granular fertilizers, one application in early spring might be sufficient for the entire growing season.

2. Late Spring / Early Summer (Optional, for Mid-Season Boost):

   • Depending on the type of fertilizer (quick-release liquid or if your slow-release is a shorter duration) and your plant's performance, you might consider a second, lighter application.
   • This is often around late May to early June, before blooming is in full swing. This ensures the plant has ample nutrients to support the development of its large flower heads.
   • Be cautious with this second application, especially with nitrogen, to avoid promoting too much leafy growth over flowers.

3. Stop Fertilizing by Mid-to-Late Summer:

   • Critical Timing: It is crucial to stop fertilizing your hydrangeas by mid-July to early August at the absolute latest (earlier in colder climates).
   • Why Stop?: Fertilizing too late in the season, particularly with nitrogen, encourages tender new growth that will not have time to harden off before the first frost. This new, soft growth is highly susceptible to cold damage and can lead to dieback.
   • The plant also needs to begin its natural process of slowing down and preparing for dormancy in fall and winter. Fertilizing disrupts this process.

Summary Table for Fertilization Timing:

Additional Considerations:

• Soil Test: Always conduct a soil test before fertilizing. This helps determine if your soil has specific deficiencies or pH issues that need addressing before applying a general fertilizer. A soil testing kit can provide valuable insights.
• Established Plants vs. New Plants: Newly planted hydrangeas often don't need fertilizer in their first year if planted in well-amended soil, as their focus is on root establishment.
• Bloom Color Changes: If you're using amendments to change bloom color (like aluminum sulfate for blue), apply these earlier in spring, typically around the same time as your first fertilizer application, or even in late fall. These are not fertilizers but soil amendments.
• Follow Product Instructions: Always read and follow the specific application rates and timing recommendations on your chosen fertilizer product, as concentrations vary.

By timing your hydrangea fertilization correctly, you ensure your plants receive the right nutrients when they can best utilize them, leading to healthier growth and more abundant, vibrant blooms.

How Do I Fertilize Hydrangeas to Change Bloom Color?

Fertilizing hydrangeas to change their bloom color is a specific process that applies primarily to Bigleaf Hydrangeas (Hydrangea macrophylla) and involves manipulating the soil pH and the availability of aluminum. It's not about the fertilizer's NPK ratio, but rather adding specific amendments.

Here's how to fertilize (or amend the soil) to influence hydrangea bloom color:

Step 1: Know Your Current Soil pH (Crucial!)

• Before doing anything, perform a soil pH test. This is absolutely essential to know where you're starting from. You can use a simple soil pH test kit or send a sample to your local extension office.
• Understanding your current pH tells you whether you need to acidify (for blue) or make it more alkaline (for pink).

Step 2: Understand the Mechanism - Aluminum Availability

• Blue Hydrangeas: Need acidic soil (pH 5.0-5.5) to make aluminum soluble and available for the plant.
• Pink Hydrangeas: Need alkaline or neutral soil (pH 6.0-7.0+) to "lock up" aluminum, making it unavailable.

Step 3: Choose Your Amendment and Apply Correctly

A. For Blue Hydrangeas (Lower pH & Add Aluminum):

• Amendments to Use:
  1. Aluminum Sulfate: This is the most effective and fastest-acting product for both lowering pH and providing available aluminum.
  2. Elemental Sulfur: Lowers pH more gradually.
  3. Organic Matter (Long-Term): Peat moss, pine needles, or coffee grounds can gradually acidify the soil.
• Application Method:
  • Timing: Start applying in early spring, as soon as new growth appears. Continue light applications every 2-4 weeks through summer if necessary, but stop by late summer.
  • Dilution/Rate: Always follow the product's package directions precisely. Aluminum sulfate can burn roots if over-applied. A common recommendation is 1 tablespoon per gallon of water for liquid application, or sprinkling granular around the drip line.
  • Application: Water the plant thoroughly before applying, then apply the amendment evenly around the drip line of the plant. Water in again after application.
  • Repeat: It often takes several applications over weeks or months to see a significant change, and you may need to reapply annually.
• Important Notes:
  • Some soils naturally lack aluminum, so even if you lower the pH, you might still need to add aluminum sulfate.
  • Avoid phosphorus-heavy fertilizers when trying for blue, as phosphorus can bind with aluminum, making it unavailable. Choose a balanced fertilizer or one with a low phosphorus ratio for blue hydrangeas.

B. For Pink Hydrangeas (Raise pH):

• Amendments to Use:
  1. Garden Lime (Calcium Carbonate): This is the most common and effective product for raising soil pH.
  2. Dolomitic Lime: Contains both calcium and magnesium, also raises pH.
  3. Bone Meal: Can slightly raise pH over time while providing phosphorus for blooms.
• Application Method:
  • Timing: Best applied in late fall or early spring, allowing time for it to work into the soil.
  • Rate: Follow package directions carefully. The amount needed depends on your current pH and desired pH change. Typically sprinkled around the drip line.
  • Application: Spread the lime evenly around the base of the plant, avoiding direct contact with the stems. Water thoroughly after application to help it leach into the soil.
  • Repeat: It can take several months to see an effect, and annual reapplication may be necessary.
• Important Notes:
  • Be patient; raising pH is a slower process than lowering it.
  • Avoid high-aluminum fertilizers or amendments if you desire pink blooms.

General Considerations for Color Change:

• White Hydrangeas: Remember, white hydrangea varieties will always remain white, regardless of soil pH. Their genetics prevent color change.
• Consistency: Consistent application of amendments over several months is often required to see a noticeable change in bloom color.
• Patience: Don't expect immediate results. It often takes a full growing season, or even into the next year, to achieve the desired color, especially with established plants.
• Potted Hydrangeas: Changing color is generally easier for potted hydrangeas because you have more control over the entire soil volume. You can simply use a specific hydrangea colorant designed for this purpose, which often contains aluminum sulfate or garden lime.

By carefully testing your soil and applying the appropriate amendments at the right time, you can successfully influence the stunning bloom color of your Bigleaf Hydrangeas.