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How to make hydrangeas blue

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To make your hydrangeas turn blue, start with a type of hydrangea that is capable of producing blue flowers, such as a cultivar of bigleaf hydrangea. Then test your soil pH and nutrients. Work to bring the acidity down below about 5.5 and ensure adequate amounts of aluminum are available in the soil. The easiest way to turn hydrangeas blue is with an aluminum sulfate soil amendment product.

H macrophylla

How to make hydrangeas blue

Hydrangeas are beautiful flowers that come in a variety of colors, including blue. If you want to make sure your hydrangeas turn out blue, you can do a few things.

Apply a specialized blueing formula

The easiest way to turn hydrangeas blue is to apply a special slow-release blueing formula product to the soil around the base of the plant. These granular products generally contain aluminum and sulfur (although not always), and are easier to apply than aluminum sulfate.

Here are some popular products for turning hydrangeas blue:

  • Osmocote Blue-Max
  • Espoma Soil Acidifier

Osmocote Blue-Max is applied in the spring and every subsequent 2-3 months at a rate of 4.7 to 7.1 grams per liter (just over 4 cups). Espoma Soil Acidifier is applied in the spring and again every two months at a rate of about 1-2 cups per plant.

Aluminum sulphate

Apply aluminum sulfate

For more control over the soil chemistry, consider applying aluminum sulfate. Aluminum content is the deciding factor in whether or not hydrangea flowers are blue. Blue hydrangeas require ample amounts of aluminum (more info below).

Aluminum sulfate is most commonly applied in early-mid spring. While application rates vary, the rate is usually about 1-5 ounces per shrub (depending upon its size).

You can also apply a “soil drench” of aluminum sulfate. Use about an ounce of aluminum sulfate per gallon. Apply the drench every two weeks from early spring until after the flower buds form.

H macrophylla

Why hydrangeas turn blue

The pH level of the soil has an effect on the color of hydrangeas. Soils with higher acidity levels tend to produce more vibrant blues, while soils with lower acidity levels will create purple shades or even pinkish hues. To ensure your hydrangeas turn out blue, use a soil testing kit to determine the pH level and adjust it accordingly.

While the color of hydrangeas is a good natural indicator of soil pH, it is not an acidic pH itself that makes them blue. Acidic soil tends to have ample free aluminum, which the plants need to make the pigment. But processed, acidic soil that doesn’t contain aluminum (like a composted pine bark mix) will produce pink flowers, not blue (since there’s no aluminum to make the blue pigment).

Soils with a higher pH, typically over about 6.5-7.0, create an environment where aluminum is rendered insoluble as the free ions are converted to aluminum hydroxide. The flowers are pink since the hydrangea can’t absorb these aluminum ions.

Not all species of hydrangea will turn blue regardless of soil conditions; some varieties simply won’t change color no matter what you do. Species such as Bigleaf Hydrangea (Hortensia macrophylla) have been known to reliably produce blues when grown in acidic soils, but other varieties like Climbing Hydrangea (Hydrangea anomala petiolaris) or Oakleaf Hydrangea (Hortensia quercifolia) will not turn blue even if you treat the soil. Be sure to research which species is best for achieving the desired results before planting.

Lastly, make sure that your plants get plenty of gentle sunlight throughout their growing season; too much shade can also cause them to produce less intense colors than expected, so try positioning them near windows where they’ll receive at least 6 hours per day during peak summer months.

Hydrangea macrophylla

Aluminum availability and blue hydrangea flowers

For hydrangeas to have blue flowers, the plants must be able to absorb aluminum. The blue color occurs in the plant when aluminum reacts with the pigment that naturally exists in the flowers.

The soil the hydrangea is growing in must have ample amounts of free aluminum ions. At the roots, these aluminum cations react with citric acid to form aluminum citrate. The aluminum citrate is absorbed and moves up the shrub’s branches to all the different parts of the plant (including the flower clusters). At a cellular level, the aluminum enters organelles called vacuoles.

In the vacuole, the aluminum reacts with the aluminum ion (Al+++) complexes with the pigment molecule delphinidin-3-glucoside. The vacuoles then become the storage spaces for the resulting anthocyanin pigment. For the deepest blue, there should be more than three times as many aluminum cations as the number of pigment molecules.

Different varieties of Hydrangea macrophylla have different sepal concentrations of delphinidin-3-glucoside. Cold-hardy American cultivars and reblooming hydrangeas tend to have lower pigment concentrations than the more vivid European varieties. Changing the color of a low-pigment variety requires less time and aluminum input than changing a high-vibrancy cultivar.

“The intensity of the color depends on the concentration of pigment (delphinidin-3-glucoside) in the sepals and whether sufficient aluminum is present to complex with the pigment.”

The hydrangea book: The authoritative guide, by Michael Dirr
Hydrangea color chart - home for the harvest - gardening website

Soil acidity and hydrangea color

The color of the hydrangea is determined by the acidity level of the soil it’s planted in. Soil with an acidic pH will typically produce blue flowers, while alkaline soil produces pink blooms. To ensure your hydrangeas turn out their most vibrant shade, you’ll need to adjust your soil’s pH to a strongly acidic level.

Testing your soil

The first step is to test your soil’s pH levels before planting any hydrangeas. You can purchase a simple home testing kit at any garden center or online store for around $10-20 USD. Once you have your results, you’ll know if you need to make adjustments before planting or not.

Alternatively, if you’ve been growing this plant for a few years, check your photos to see what color it has been blooming in. Pink flowers may indicate that the soil pH is 6.5-7.0 or higher, or it may indicate that there is some chemical reason that the plant is not able to absorb aluminum to make the blue color. This is usually because there is no aluminum present or there is a competing ion such as excess phosphorus.

Adjusting soil pH

If your soil isn’t acidic enough for blue blooms (below about 5), then adding sulfur can help lower its pH levels and make it more acidic over time. Depending upon the starting condition of your soil, this process takes several weeks to several years, so plan ahead.

Blue lacecap hydrangea

Species that will turn blue (and those that won’t)

When it comes to hydrangeas, the two species that will turn blue in acidic soil are bigleaf and mountain hydrangea. Bigleaf hydrangeas (Hydrangea macrophylla) have large mophead or lacecap blooms with a wide range of colors from pink to purple to blue. Mountain hydrangeas (Hydrangea serrata) produce small flowers in shades of white, pink, and blue. Most hydrangeas that will turn blue are bigleaf varieties, while only a few mountain varieties will turn blue.

Blue cultivars of hydrangea

While all bigleaf hydrangeas will turn a bit blue in the right soil conditions, certain cultivars are certainly known to turn blue more reliably.

Here are some blue hydrangea varieties to consider:

Choose a variety known for its blue tone if you’d really like blue flowers.

Blue hydrangea flower

Watering your hydrangea shrubs

Watering is an important part of keeping your hydrangeas blue. Too little water can cause the flowers to become dry and wilted, while too much water can lead to root rot and other issues. To keep your hydrangeas looking their best, finding a balance between watering enough but not too much is important. Additionally, it’s good to know the pH of your irrigation water, as water acidity varies.

When it comes to how often you should be watering your hydrangeas, the answer will depend on where you live and what kind of soil you have in your garden. Generally speaking, however, most varieties of hydrangea need about 1-2 inches of water per week during the growing season (spring through fall). This amount may vary depending on weather conditions, such as temperature or rainfall amounts, so make sure to adjust accordingly.

In addition to regular weekly watering sessions, it’s also important that you give your plants a deep soak every once in a while – especially during periods when there isn’t any rain or if temperatures are particularly high. A deep soak means soaking the soil until it is completely saturated with moisture all the way down, at least 8-10 inches below ground level; this helps ensure that all parts of the plant are getting adequate hydration throughout its entire root system.

Finally, remember that even though overwatering can be bad for hydrangeas (and other plants), underwatering them is just as detrimental. Therefore, it is important to check regularly for signs of dehydration, such as wilting leaves or brown spots on petals, before giving them another drink; this will help ensure they stay healthy and vibrant throughout the growing season.

Blue lavender hydrangeas

Sunlight for optimal blooming

Sunlight is an important factor in keeping hydrangeas blue. Hydrangeas need at least four to six hours of direct sunlight each day for optimal coloration. If the plants are not getting enough light, they may become pale or even white instead of their desired blue hue.

When choosing a spot for your hydrangea bush, make sure it’s in a sunny location with plenty of access to natural light. Avoid areas that are shaded by trees or buildings, as this can prevent the plant from receiving adequate sunlight and will reduce its ability to turn blue.

It’s also important to remember that too much sun can damage hydrangeas, so you should avoid placing them directly under intense midday or harsh afternoon sun during summer months when temperatures tend to be higher than usual. Instead, try finding a spot where the plant will receive morning and late afternoon sun but won’t be exposed to hot midday rays. This will help ensure your hydrangea stays healthy while still getting enough light exposure throughout the day for vibrant blooms come springtime.

Light blue hydrangea shrub

FAQs about making hydrangeas turn blue

Do coffee grounds make hydrangeas blue?

Coffee grounds don’t really make hydrangeas blue. The acids in coffee tend to be water-soluble and pass into the coffee, leaving the grounds only a slightly acidic pH. Coffee grounds are high in nitrogen and may contain some aluminum, depending on the beans and the brewing method.

Does watering with vinegar make hydrangeas blue?

Vinegar is not a particularly effective way to make hydrangeas blue. While distilled household white vinegar is quite acidic (about 2.5), it needs to be diluted by at least about 1:10 before applying. It would need to be applied on a regular basis and would likely lead to decreased soil life and potentially harm the ecosystem.

Mary Jane Duford
Mary Jane Duford

Mary Jane Duford is a quintessential Canadian gardener. An engineer by trade, she tends to an ever-expanding collection of plants. In her world, laughter blooms as freely as her flowers, and every plant is raised with a dash of Canadian grit.

Mary Jane is a certified Master Gardener and also holds a Permaculture Design Certificate. She's also a proud mom of three, teaching her little sprouts the crucial difference between a garden friend and foe.

When she's not playing in the dirt, Mary Jane revels in her love for Taylor Swift, Gilmore Girls, ice hockey, and the surprisingly soothing sounds of bluegrass covers of classic hip-hop songs. She invites you to join her garden party, a place where you can share in the joy of growing and where every day is a new opportunity to find the perfect spot for yet another plant.

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