The FAQ for this forum has abbreviated instructions for altering flower color. You should also be aware that not all macrophylla cultivars are as easily altered as others - some are very stable in their coloring and resist change. And your existing soil conditions will play a large role - it is extremely difficult to significantly alter soil pH and for any length of time. Most pH changes will be slight and very temporary. If you have very alkaline soil, growing blue flowered hydrangeas will best be accomplished by growing them in containers, where the soil conditions can be most easily manipulated.

Here is a link that might be useful: changing flower color

Hello, jetlag98. Like gardegal48 said, changing the colors is complicated because you have the monitor acidity levels forever and you have to monitor levels of other nutrients as well. Maintaining the desired blue hue stable year after year can be a pain so many persons do it only with plants in containers rather than with plants in the ground. But many are successful so try it out and see if this works for you.

To get blue, you need to acidic soil with adequate levels of aluminum. The opposite of acid soil is alkaline soil. These are measured using a logarithmic scale called the pH Scale. It ranges from zero to 14. Soils whose pH is 7.0 (the mid point in the scale) are considered neutral, soils above 7.0 are considered alkaline and soils below 7.0 are considered acidic. When reviewing readings below 7, a reading of 6.0 is considered more acidic than an acid reading of 6.5. On the extreme sides of the scale, plants cannot absorb many other nutrients so do not try getting your soil much below -say- 5.

Also, not all colored hydrangeas trigger blue blooms at the same pH Level; H. Pia for example, resists color change from pink to blue. Other hydrangeas like H. Glowing Embers are heavily pigmented towards pinks so they produce purples. But somewhere between 6.0 and 6.5 is the point where you generally start to change from pink with most colored hydrangeas. Then it becomes a case of whether you like the current color hue.

To acidify the soil, you have to amend it and keep amending it yearly (or more often) as the changes are only temporary. The second time though, you do not have to apply as many amendments as the first time. Amendments that can be added to acidify the soil: elemental Sulphur (also known as soil Sulphur, flowers of Sulphur, powdered Sulphur), aluminum sulphate, Espoma's Garden Sulphur, etc

1. Determine the pH Level of your soil: Local nurseries sell pH kits that can approximate what your soil pH Level is. These kits are cheap, can be used often but they are not accurate; look for one that gives results in the pH Scale or some type of numeric scale. A formal soil test is more accurate but it takes a while to get back the results, costs more and normally is done every 3-5 years.

2. Determine the type of soil that you have: sandy, clay, normal. A nearby local nursery could help you with that if you do not know.

3. Determine the pH Level that you want your soil to go down to. A reading near 6.0 is a good starting point.

With these three pieces of information, you can then go to Table 2 in the link below. The table is located near the bottom of the webpage and titled "Pounds of elemental sulfur needed to lower soil pH of a silt loam soil to a depth of 6 inches." The first column, titled "Present pH," refers to Item 1. The next five columns, titled "Desired Soil pH" all refer to item 3, the soil pH that you want to attain. The place where the two numbers intersect is the amount of amendments needed to reach your goal; the amount is given in pounds per 100 square feet. Then you tweak the number of pounds like this:

* if your soil is sandy, multiply the number of pounds given by the table times two thirds or 2/3.
* if your soil is clay, multiply the number of pounds by 1.5

For example: Assume that your soil is clay; that your current pH is 7.0 and that you want to lower the pH down to 6.0. Procedure: (a) under the "Present PH" Column, look up 7.0 and read what the table says to the right under a column whose title is "6.0". (b) The table that says you will need 2 pounds per 100 sq ft to bring the pH down from 7.0 to 6.0. (c) But because you have clay soil in this example, the amt needs to be tweaked upwards to: 2 x 1.5 = 3 pounds per 100 sq feet. If the amendment that you are using says on the product label to apply it at rates of 1.5 pounds per 100 sq feet every two weeks, apply 1.5 pounds per 100 sq feet today and then amend the soil again in two weeks.

If you use aluminum sulphate instead of Sulphur, make one last tweak: (d) multiply the result in "c" times 6.9 in order to get 3 * 6.9 = 20.7 pounds of aluminum sulphate per 100 sq ft. That is "the bottom line," the amt someone would need to mix to his/her soil in order to bring the Ph of clay soil from 7 to 6.0 using aluminum sulphate.

The reaction that lowers the pH using elemental Sulphur is a biological reaction that produces sulphuric acid from the Sulphur, oxygen, water and soil bacteria. As such, do not expect instant/immediate results. Water the soil the night before and after amending it. Aluminum sulphate reacts chemically to produce sulphuric acid so the soil acidifies somewhat faster and watering before and after applying a/s is very important.

Other notes:

* Aluminum is a key ingredient in turning the hydrangeas blue. If your soil lacks high enough levels, you will not get blue by amending with elemental Sulphur; you have to use aluminum sulphate instead of plain Sulphur. Only a soil test can tell you if your soil has adequate levels of aluminum. But you can try adding Sulphur by itself and see what happens. My shrubs in alkaline soil produce blues with Sulphur only so that tells me that aluminum in my soil is in good supply. Hint: recycle the aluminum in your dried blooms back into the ground as mulch.

* Soil pH Levels above 6 to 6.2 begin to inhibit absorption of aluminum.

* Use fertilizer amendments with little phosphorus. In large amounts, phosphorus also inhibits aluminum absorption in acidic soil, which is why people who really want to get pink hydrangeas also apply triple superphosphate.

* To facilitate the absorption of aluminum, you can also maximize potassium levels since high levels of potassium help the plant absorb more aluminum.

* The next time you check your soil acidity, get a new reading and re-use the table again to see how much you need to maintain the pH level you want from whatever newer lower level you have. This second time around, you will need a smaller amount per 100 sq ft to maintain your pH Level.

Does that help?
Luis

Here is a link that might be useful: Clemson University's Table

I have taken the necessary steps to change soil conditions. My question is, will anticipated color change occur in existing buds and blooms, or must I wait for new buds before seeing any blue? Also, about how long does this take, and does exposure to sunlight make any difference? Kind thanks, Nancy

It takes time to adjust pH and make the nutrients available to the plants. One should generally figure on about 6 months so add pH adjusting supplements in fall for the next season's flower color.

And it is important to understand that whatever you do to adjust pH will not be a permanent fix. IOW, you will have to repeat the process routinely.