Add Aluminum sulfate to increase acidity.
No Blooms? The most common cause is winter damage. A frost will kill the buds. Too much shade and poor soil conditions can also result in no blooms.
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For small areas, aluminum sulfate or sulfur can be tried. You will need to consult with your County Extension Service or other garden expert to find out how much to use on your soil. This method will be quite expensive.
The flower colors may be controlled by adding aluminum sulfate to the soil prior to budding to produce or keep blue flowers; or by liming or adding quantities of superphosphate to the soil to produce the pink ones.
Aluminum sulfate or sulfur to lower pH
pH (potential Hydrogen) is the measure of the general acidity or alkalinity of a soil. If the soil is too acidic or too alkaline, needed nutrients can be 'tied up' in the soil and not available to the plants.
To lower the pH level, aluminum sulfate and sulfur are typically used. The aluminum sulfate will quickly change the pH of the soils for roses where sulfur will take longer as it requires the aid of the soils bacteria to make the change.
This is done by adding aluminum sulfate to the soil. Aluminum sulfate is sold at most garden centers and the rate of application is given on the package. Follow label directions carefully and don't overdo it.
Authorities recommend that a solution of 1/2 oz (1 Tbsp) aluminum sulfate per gallon of water be applied to plants (which are at least 2-3 years old) throughout the growing season.
The key is aluminum ions in the soil, so aluminum sulfate is a good quick fix, but prolonged use year after year may cause a fatal build up in the soil. Iron sulfate is a safer fix, or try alternating.
The lower the pH, the deeper the blue. For blue flowers, apply aluminum sulfate or a commercial "bluing" compound. For pink flowers, add a high-phosphorus fertilizer or garden lime. Use any amendment according to package directions.
The University of Nebraska Horticulture Department recommends adding elemental sulfur or aluminum sulfate to lower pH levels but says that large amounts of sulfur are required to make even a small change.
Depending on the aluminum sulfate that is used or the acidity of the soil, pink hydrangeas can be turned purple or blue. However, that also depends on the variety.
Bigleaf hydrangeas change color based on the pH of the soil. Pink plants can be kept pink with lime, while blue needs aluminum sulfate.
Oakleaf hydrangeas have large panicles of white flowers which turn dusty rose as they age.
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You can add acid peat moss to try, or use acid-forming fertilizers such as ammonium sulfate or aluminum sulfate. Some advocate the use of sulfur for decreasing the pH value.
Acidic soil (pH below 5.5) brings out the flower's blue coloring, while less acid to alkaline soil (pH of 6.0 or higher) enhances its vibrant pink color. If you'd like the blue flowers, add aluminum sulfate (alum) to the surrounding soil.
Generally speaking, lawns prefer a pH range of 6.2-6.8 (6.5 is optimum). Lime is used to raise the soil pH; aluminum sulfate is used to lower the soil pH. The only way to tell a soil's pH is to have a soil test done.
Lime or wood ash can be used to raise pH; sulfur or aluminum sulfate can lower pH. Keep in mind that it's best to raise or lower soil pH slowly over the course of a year or two.
It's difficult for plants to get the nutrients they need if the level is too high or too low. To adjust a pH level which is too high, meaning the soil is alkaline, add aluminum sulfate.
Additional organic matter also helps to improve soil drainage around your rhododendrons, and mulching will protect the roots from temperature extremes. Dick is emphatic that growers not use aluminum sulfate, ...
 See also: Soil, Plant, Flower, Fertilize, Water
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