Many gardeners add vinegar to their watering can to change the acidity of their soil and turn their hydrangeas blue. However, you'll probably have better results using Hydrangea Blue, a liquid fertilizer that produces blue flowers.
There is one more trick up the apple cider vinegar sleeve: You can actually change the color of hydrangea flowers from pink to blue. Hydrangea flowers will be pink in alkaline soil, but change to blue in acidic soil. So, mix up some apple cider vinegar and water, and give all the acid-loving plants a treat.
It's best to start adding coffee grounds to the soil months before the blooming season begins, ideally in the late fall. You can repeat the process with your typical fertilizing schedule. With a little caffeine and a lot of patience, your hydrangeas should reward your efforts come spring with the boldest blue globes.
To turn new hydrangeas blue, use 1¼ cups of Espoma Organic Soil Acidifier. Or to transform established hydrangeas into blue beauties, apply 2½ cups of Organic Soil Acidifier. Spread evenly around the hydrangea out to its drip line, or the widest reaching branches. Then, water well.
The easiest way to acidify your soil and turn those babies blue is with aluminum sulfate, which can be found at almost any garden center. Mix ¼ oz aluminum sulfate with a gallon of water and soak the soil surrounding your hydrangeas in the spring, as soon as the plant begins to grow.
To make sepals bluer, add aluminum sulfate to soil and maintain low levels of phosphorous, moderate levels of nitrogen, and high levels of potassium. To make sepals pinker, add garden lime to soil plus maintain high levels of nitrogen and low levels of phosphorous.
So, to make sure your hydrangea stays really blue add Vitax Hydrangea Colourant, a powder containing aluminium that you can mix with the compost. You can also mix a little into the soil surface each spring and add it to the can when you water your plant.
Use a fertilizer low in phosphorous (the middle number in the content analysis) such as 12‐4‐8 to encourage production of blue flowers. For other types of hydrangeas, use a general purpose fertilizer, at recommended rates on the package. Incorporate dry fertilizer into top 2” – 3” of soil and water thoroughly.
However, many times it's easy to turn those pink imposters back to blue, should one so desire. Simply squeezing a few lemons or limes over the plant somewhat raises the acidity level of the soil, consequently turning the blooms from pink to blue.
Large hydrangea plants can take months to convert their color, so you might not see the results until next year's bloom. Sometimes you'll see some flowers change color and others not, so be patient and amend the soil as needed.
Hydrangeas planted near concrete sidewalks or concrete foundation are more likely to produce pink blossoms. Lime can leach out of the concrete and keep the soil pH too high to produce blue blooms.
You see, for most French hydrangeas (Hydrangea macrophylla), the flower color indicates the pH of the soil. In strongly acid soil (pH below 6), flowers turn blue. In alkaline soil (pH above 7), flowers turn pink or even red.
However, it takes patience to change hydrangea flower colors, as it can take a year to see a noticeable change. A quicker way to change flower color is through liquid soil drenches applied in March, April and May. To make the flowers turn blue, dissolve 1 tablespoon of aluminum sulfate in 1 gallon of water.
Coffee grounds and tea bags can be used as a mulch around hydrangeas. It changes the pH of the soil and makes the pink ones turn to blue.
Generally speaking, acidic soil, with a pH lower than 6.0, yields blue or lavender-blue hydrangea blooms. Alkaline soil, with a pH above 7.0, promotes pinks and reds. With a pH between 6 and 7, the blooms turn purple or bluish-pink. To lower your pH, add garden sulfur or aluminum sulfate to your soil.
Prune back stems to just above a fat bud — called a heading cut — in fall, late winter or spring. These plants have conical-shaped flower heads. I recommend leaving the dry, tan flower heads on the plant to provide some winter interest in your landscape, so I wait to prune these until late winter or spring.
Most common hydrangeas prefer a partial sun location - ideally receiving sun in the morning hours and shade in the afternoon. The reblooming Endless Summer® Hydrangea series prefers part shade.
Hydrangeas that bloom on new wood can be safely pruned in late fall once the plants have gone dormant or in early spring. Next year's flower buds won't be formed until late spring the same year they bloom, so there is no risk of removing the buds if you prune in fall or spring.
Hydrangeas turn green because of age. Eventually, the flower will turn from green to brown and is a normal process.