The scientific reason behind this is that different levels of aluminum become available to the plant depending on the soil’s acidity level (also known as pH). Acidic soils have a high aluminum content, turning the flowers blue. An alkaline soil will produce pink blooms in hydrangeas; an acid soil will cause the same plant to bear blue flowers. The exception to this is white or green hydrangeas, which are unique varieties and don’t change color. You won’t have any success trying to change a white hydrangea to pink or blue!
Soil with a pH lower than 5. 5 will result in bright blue hydrangea flowers. When the pH is 5. 5 to 6. 5 the flowers will be an odd purple color. Soil with a pH higher than 6. 5 will cause the flowers to be pink.
If the vinegar fizzes and bubbles when it comes in content with the soil, this means that the soil is alkaline and will produce pink hydrangeas. The more intense the fizzing, the more alkaline the soil. If nothing happens when the vinegar comes in contact with the soil, this means that the soil is either neutral or acidic and is more likely to produce blue hydrangeas. [3] X Research source
Alternatively, you can take a sample of your soil to a local gardening center where they will test the pH of the soil for you.
Loam or silt loam soil will require a ¾ pound of elemental sulfur to lower the soil pH in a 25 square foot area by one unit. In other words, it will take ¾ pound to lower the pH from 6 to 5. Sandy or sandy loam soil, on the other hand, will require less than ¼ pound of elemental sulfur to lower the pH by one. Start sprinkling the elemental sulfur about 2 feet (0. 6 m) beyond the shrub drip line or outer edge of the shrub. Spread it evenly over the soil up to about 4 to 6 inches (10. 2 to 15. 2 cm) away from the stems. This is the area where most of the roots grow and absorb water and nutrients. Use a small hand rake to mix the elemental sulfur into the top 1 to 2 inches (2. 5 to 5. 1 cm) of soil, then water the area generously to help wash the sulfur into the soil. The elemental sulfur will probably need to be re-applied from time to time in order to keep the blue hydrangea blue.
You can buy acidic compost at the gardening center - it is usually labeled as “ericaceous”. Aluminum sulfate is available as a powder from garden centers or online outlets. It’s often labeled as ‘blue hydrangea powder’. Be careful not to apply the powder directly to the root of a plant, as it will scorch the tissue. Instead, add one tablespoon of aluminum sulfate to one gallon of water and use this solution to water fully-matured hydrangeas throughout the growing season. Don’t be tempted to apply a stronger concentration as this can burn the roots. [5] X Research source
This fertilizer may be labeled as an appropriate fertilizer for azaleas, camellias and rhododendrons. [6] X Research source Avoid using fertilizers such as bone meal, as this will make the soil more alkaline, undoing all of your hard work.
Used coffee grounds are particularly effective, just make sure that they have cooled completely before working them into the soil around the base of your plant. [7] X Research source You can also incorporate coffee grounds into the soil when planting a new hydrangea shrub – perhaps ask your local café if you can take some away, they are usually happy to oblige. Keep in mind that organic matter changes the acidity of your soil more slowly than chemical powders and fertilizers, so you’ll need to be patient if you decide to go down this route.
You can encourage a blue bloom even further by watering your potted hydrangea with an aluminum sulfate solution, as described above.
If you’re faced with a chalky soil, you won’t have much luck altering the soil pH so it’s best to grow your hydrangeas in containers using purchased compost rather than any dug out of the ground in your garden.
