TheGardenLady received this question from Cookie:
Is it too late in the season to add aluminum sulfate to my hydrangeas to make them blue? How much would you add to each plant as I have a row of 29. The blooms are starting to come in but are very light blue at this point. Would you recommend something else to add to make them blue. I am always worried I will kill them by adding too much.
It may be a little late to get the color Hydrangea you want, but it is never to late to start.
The first thing you should do is to test your soil to find out what the soil pH is. Blue hydrangeas like a soil acidity of between 5.2 and 5.5. You can buy a soil test kit at your local hardware store or get the kit through your Master Gardener office of your local agriculture extension office. This will give you an idea of how much you want to amend your soil with things that bring up the acid level. You should have this test done every year to be sure your soil is as acid as you need it to be. Before you start amending your soil,  you want to check to see what other flowers are near the hydrangeas to know if they are also acid loving plants. Here is a list of some acid loving plants.
Now if your other plants are acid-loving, too, you can start amending. You do not have to use aluminum sulfate which can be toxic to other, even acid-loving plants like rhododendrons. This GardenLady always prefers the organic route.
TheGardenLady does not promote any organic or other product, but I will tell you that she does use Espoma products, that are organic, in her own garden. Even though it is organic, read directions on the bag for the amount to use and the frequency to use it.
There are some other suggestions to work on soil acidification throughout the year. See here.
And you can incorporate naturally acidic organic materials such as conifer needles, sawdust, peat moss and oak leaves into your soil. Just remember to have your soil tested regularly so that you know your soil’s needs.
But do get started amending your soil right away, armed with the information you have from your tested soil, and hopefully your flowers will still be a pretty blue.