Content

• To step
• Part 1 of 3: Gaining scientific insight
• Part 2 of 3: Making your soil more acidic
• Part 3 of 3: Knowing what to avoid
• Tips
• Warnings

Hydrangeas have blue (and not pink) flowers when grown in acidic soil. Sometimes hydrangeas will change color over time as the acidity of the soil changes. Therefore, you will need to monitor the pH of your soil and take precautions to keep it acidic if you wish to maintain the blue color of the flowers. This is relatively easy by gaining some insight into the science behind it and by making the necessary adjustments to the soil.

To step

Part 1 of 3: Gaining scientific insight

1. Know that blue hydrangeas grow in acidic soil and pink hydrangeas grow in alkaline soil. Hydrangeas are unique in that they can change color according to the pH of the soil. This means that the color of the hydrangeas in your garden will depend on how acidic or alkaline the soil is.
   • The scientific explanation for this is that different amounts of aluminum become available to the plant depending on the acidity (known as pH) of the soil. Acidic soils contain more aluminum, which causes the flowers to turn blue.
   • Alkaline soil will produce pink hydrangea flowers; the same plant will bear blue flowers in acidic soil. The exception to this is white or green hydrangeas, as these are unique varieties that do not change color. So you will not be able to get a white hydrangea to turn blue or pink!
2. Check the pH of your soil. To see if the soil in your garden is acidic or alkaline, you will need to test the pH level. This allows you to predict the chances you have of growing blue hydrangeas.
   • Soil with a pH below 5.5 will produce bright blue hydrangea flowers.
   • A pH between 5.5 and 6.5 will give the flowers a strange purple color.
   • The flowers will be pink when the pH is above 6.5.
3. Use white vinegar to determine if the soil is acidic or alkaline. You can get a good indication of the acidity of your soil by using distilled white vinegar. Take a handful of soil, pour the vinegar on top and wait for a reaction.
   • If the vinegar bubbles up and starts to hiss when it comes into contact with the soil, it means the soil is alkaline and will produce pink hydrangea flowers. The more intense the reaction, the more alkaline the soil is.
   • If nothing happens when the vinegar comes into contact with the soil, it means the soil is neutral or acidic and there is a greater chance that blue hydrangea flowers will appear.
4. Scientifically test the pH of your soil. If you want to know the exact pH of your soil, you can test it using a home test kit. These can be purchased at garden centers or online. All you have to do is follow the guidelines on the packaging.
   • An alternative is to take a soil sample to a garden center near you, where they can test the pH for you.

Part 2 of 3: Making your soil more acidic

1. Sprinkle pure sulfur on the soil. To keep your hydrangea flowers blue, sprinkle sulfur on the soil around the shrub to bring the pH below 5.5. The exact amount of pure sulfur you will need for this will vary depending on the type of soil and to what extent the pH needs to be corrected.
   • Loam or loamy soil requires 350 grams of pure sulfur to lower the pH value of approximately 2.5 m² of soil by one unit. So 350 grams of pure sulfur will be needed to bring the pH from 6 to 5. Sandy or sandy loam soils, in turn, will require less than 110 grams of pure sulfur to lower the pH by one unit.
   • Start by sprinkling the pure sulfur about two feet past the shrub's drip line or near the outer edges of the shrub. Spread it evenly over the soil to a distance of about 10-15 cm from the stems. This is where most roots and water and nutrients grow up.
   • Use a small rake to mix the pure sulfur with the top 3-5 cm of the soil, then water generously to help flush the sulfur into the soil. The pure sulfur will likely need to be reapplied from time to time to keep the hydrangea flowers blue.
2. Use acidic compost and aluminum sulfate powder. To grow hydrangeas in alkaline soil and still get pink flowers, you will need to add plenty of acidic compost and aluminum sulphate to the soil when planting and repeat this periodically throughout the life cycle of the plant.
   • Acidic compost can be purchased at garden centers. Aluminum sulfate is available as a powder at garden centers and online outlets. The label often mentions "blue hydrangea powder". Make sure not to apply the powder directly to the roots of the plant as this will burn the tissue.
   • Instead, add a tablespoon of aluminum sulfate to 4 liters of water and use this solution to water adult hydrangeas during the growing season. Don't be tempted to use a stronger concentration as it can burn the roots.
3. Use a fertilizer that is low in phosphorus, but contains a lot of potassium. All hydrangeas benefit from fertilization. To produce or maintain blue flowers on a hydrangea bush, you can use a fertilizer that is low in phosphorus and high in potassium.
   • Such fertilizers are often labeled as suitable for azaleas, camellias and rhododendrons.
   • Avoid fertilizers such as bone meal as this will make the soil more alkaline, making all your work pointless.
4. Use organic matter to make your soil more acidic. If you'd rather not use chemicals in your garden, know that organic materials, such as grass clippings, fruit and vegetable scraps, and coffee grounds, can help make the soil a little more acidic.
   • Coffee grounds are very effective, but make sure it is completely cool before mixing it in the soil near the base of your plant.
   • You can also add coffee grounds to the soil when you are planting a new hydrangea bush - if necessary, ask a café near you if you can pick it up, they will usually be happy to agree.
   • Remember that organic matter is less likely to change the acidity of your soil than chemical powders and fertilizers, so you will have to be patient if you decide to do it this way.
5. Water your hydrangeas with rainwater. Try to use rain water (instead of tap water) to water your hydrangeas. Using hard tap water for your blue hydrangeas will counteract the acidity of the soil and the flowers will gradually turn pink.
6. Consider growing your hydrangeas in a separate pot. Rather than trying to change the acidity of the soil in your garden, it may be easier to plant new hydrangeas in pots of acidic soil.
   • You can stimulate blue flowers even more by watering your potted hydrangea with a solution of aluminum sulfate, as described above.

Part 3 of 3: Knowing what to avoid

1. Do not plant the hydrangeas in chalky soil. Changing the pH of your soil will only work if the soil does not contain lime. You will know you have a chalky bottom when you see white clumps of lime or flint. You will see that water drains easily and does not form pools. The soil will also be very dry in the summer, because water is not retained enough.
   • If you are dealing with chalky soil then you will not have much success with your attempts to change the soil pH, so it is better to grow your hydrangeas in pots and use purchased compost instead of something that contains your garden.
2. Do not plant your hydrangeas near concrete structures. Concrete next to a flower bed (such as a concrete base for a fence or a concrete path) can affect the pH. Concrete will likely make the soil more alkaline, which will inhibit the growth of blue hydrangea flowers. So if you can, plant hydrangeas away from concrete structures to maintain the blue color.

Tips

• If you change your mind and prefer to see your blue hydrangea flowers turn pink, you will need to add dolomitic limestone to the soil or use a fertilizer that contains large amounts of phosphorus. You will need to do this regularly throughout the plant's life cycle.
• If your hydrangea has a hard time deciding what color it will go (a mixture of pink and blue) or purple, then you probably have a soil with neutral acidity. To make the color lean more towards blue, you will need to use aluminum sulfate, as described above, at a concentration of 1 tablespoon per 4 liters of water. The color will gradually become bluer.

Warnings

• Unfortunately, it is not that easy to change the color of hydrangea flowers. The strength or fading of the color depends on many factors such as temperature, humidity and the general health of the plant. Therefore, for good general health, regular fertilization is the only way to help hydrangeas develop or maintain an intense color.