How To Make Hydrangeas Purple

If you love the look of rich, violet flowers in your garden, you might be wondering how to make hydrangeas purple. Achieving those vibrant purple blooms is a classic gardening project, and it all comes down to understanding and adjusting your soil’s chemistry.

Not all hydrangeas can change color, but for the popular Bigleaf varieties (Hydrangea macrophylla), color is a direct response to soil pH and aluminum availability. With a few simple steps, you can influence your blooms to shift from pink to purple, or from blue toward purple. It’s a rewarding process that gives you a direct hand in your garden’s palette.

Purple hydrangea blooms are the beautiful middle ground between pink and blue. They occur when the soil conditions are balanced just right—not too acidic and not too alkaline. This balance allows the plant to access some, but not all, of the aluminum in the soil, which is the key element behind blue coloring.

Why Hydrangeas Change Color

The color-changing ability of some hydrangeas is due to a natural pigment called delphinidin-3-glucoside. This pigment reacts with aluminum ions in the soil. Whether the aluminum is available to the plant depends on the soil’s pH level.

Acidic Soil (pH below 6.0): Aluminum is more soluble and available. The plant absorbs it, forming complexes with the pigment to create blue flowers.
Alkaline Soil (pH above 7.0): Aluminum is locked up and unavailable. Without aluminum, the pigment shows as pink.
Neutral to Slightly Acidic Soil (pH 6.0 to 6.8): This is the sweet spot. Aluminum is partially available, resulting in mixed pigments and beautiful purple or violet blooms.

It’s also important to note that white hydrangeas generally do not change color. Their genetics lack the necessary pigment.

Step 1: Test Your Soil pH

Never guess your soil’s pH. You need a starting point. You can use an inexpensive home test kit from a garden center or send a sample to your local cooperative extension service for a more detailed analysis. This tells you if you need to raise or lower the pH to reach that ideal purple zone.

Step 2: Adjusting Soil pH for Purple Blooms

Based on your test results, follow these steps to nudge your soil toward the neutral range.

If Your Soil is Too Acidic (for Pink Blooms Turning Purple)

To raise the pH (make it less acidic), you add garden lime. Dolomitic lime is a good choice as it also adds magnesium.

Apply lime in the fall or early spring.
Scatter it evenly around the drip line of the plant (where the branches end), not right against the stem.
Water it in thoroughly.
Start with a light application—about 1 cup for a small shrub, 2 cups for a large one. Retest in a few weeks, as it can take months to fully adjust.

If Your Soil is Too Alkaline (for Blue Blooms Turning Purple)

To lower the pH (make it more acidic), you add elemental sulfur or aluminum sulfate. Aluminum sulfate acts faster because it adds aluminum directly while acidifying.

Apply in spring as new growth begins.
Dissolve 1 tablespoon of aluminum sulfate per gallon of water.
Apply this solution to moist soil every two weeks for a couple months, avoiding foliar contact.
Alternatively, sprinkle elemental sulfur according to package directions. It works slower but is longer-lasting.

Always follow product label instructions carefully. Over-application can harm your plant’s roots.

Step 3: Maintaining the Purple Color

Once you’ve achieved purple blooms, maintenance is key. Soil naturally wants to revert to its original state.

Retest Annually: Check your soil pH each spring to see if you need a maintenance application of lime or sulfur.
Use Consistent Water: Tap water in many areas is alkaline. Using it can slowly raise your soil pH. Where possible, use rainwater to water your hydrangeas.
Choose Fertilizer Wisely: Use a balanced, slow-release fertilizer (like a 10-10-10) or one formulated for blooms. Avoid fertilizers high in phosphorus (the middle number), as it can bind with aluminum and make it unavailable, pushing flowers toward pink.

Factors Beyond Your Control

Sometimes, despite your best efforts, color can be stubborn. Here’s why:

Genetics: Some hydrangea cultivars are more strongly programmed for pink or blue. A very pink variety may only turn a mauve-purple, not a deep violet.
Native Soil: Extremely alkaline or acidic soils are a constant battle. Container gardening might be easier for total control.
Microclimates: Concrete foundations and walkways can leach lime, raising pH in nearby soil.

Best Hydrangea Varieties for Purple Blooms

Starting with a good candidate makes success easier. Look for these reliable varieties known for their purple potential.

‘Nikko Blue’: A classic that turns a true blue in acidic soil and a lovely lavender-purple in neutral soil.
‘Endless Summer’: A reblooming type that adapts well to color change efforts.
‘Blushing Bride’: Often starts white and can take on pink or purple tones as the blooms age, depending on soil.
‘Merritt’s Supreme’: Known for its deep, rich colors that can express vibrant purple.

When shopping, remember that a hydrangea’s color in the nursery pot is often a result of the grower’s soil, not a guarantee for your garden.

Common Mistakes to Avoid

Avoid these pitfalls to keep your plants healthy and on track for purple.

Over-adjusting: Making huge pH changes quickly stresses the plant. Go slow.
Wrong Amendment: Using wood ash to raise pH is unpredictable and can provide too much potassium.
Ignoring the Plant: Color change is about soil health first. Ensure your hydrangea gets morning sun, afternoon shade, and consistent moisture for overall vigor.
Forgetting About Aluminum: In very alkaline soils, even with perfect pH, aluminum may be absent. A soil test can check for this.

FAQ: Your Purple Hydrangea Questions

How long does it take to change hydrangea color?

It can take one full growing season to see a significant change. Soil amendments work gradually. Patience is essential.

Can I use vinegar to make soil acidic for purple hydrangeas?

While vinegar (acetic acid) can temporarily lower soil pH, it’s not recommended. It can harm soil microbes and its effect is very short-lived, leading to unstable conditions.

Will coffee grounds make my hydrangeas purple?

Used coffee grounds are only slightly acidic. They can help improve soil texture as a mulch, but they are not a reliable method for significantly altering pH. They’re better thought of as a mild soil conditioner.

Why are my hydrangea blooms green instead of purple?

Many hydrangea blooms start green, then turn their mature color (white, pink, blue, purple), and often fade back to green or burgundy at season’s end. This is a normal aging process, not related to soil chemistry.

Can I turn my white hydrangea purple?

Unfortunately, no. White hydrangea varieties lack the pigment that reacts to soil pH. They will remain white, though they may take on pink or green tints as they age based on genetics and light, not soil.

Learning how to make hydrangeas purple is a fun and satisfying aspect of gardening. It connects you more deeply to the science happening in your own backyard. By testing your soil, making careful adjustments, and choosing the right plant, you can enjoy those stunning violet and lavender blooms. Remember, the goal is a healthy plant first; the beautiful purple flowers are a wonderful reward for your attentive care.