My hydrangea

Add 1 tablespoon of aluminum Sulfate to a gallon of water and pour into the soil.Repeated applications may be necessary.

used coffee grounds! Hydrangea's love a acid soil and the coffee grounds give it just the right amount.

here's what I found...

They do love acid, coffee grounds, pine needles tossed around the base of the plant.

I, too, have heard it said that coffee grounds work well. You want the soil to be acidy.

What I did is I put an Iron big chain-about 2 feet in a hole for my hydrangea, then I put in the plant and filled it in with dirt. Now the hydrangea is blue. It does work.

Janet (NY) and Shannon (GA) are correct, however, sometimes the (acidic) solution runs away from the base of the hydrangea and just seeps into the surrounding garden. In other words, the solution isn't strong enough to turn the hydrangea blue. A tip my local garden club provided was: when planting your hydrangea, dig a slightly larger than necessary hole, purchase a larger pot the hydrangea is currently in, cut off the bottom of thr pot (so the roots can still grow) and then plant the hydrangea and bottomless pot into the hole. What happens is ... the acidic solution will be much more concentrated around the hydrangea and the chances of it turning blue will be much higher. I realize your hydrangeas are already in the ground but this is a good tip to remember when buying more of those gorgeous plants. I'm sure you could safely dig up your hydrangeas - whether you want to go to the trouble is your decision. Good luck!

When is the best time to trim hydrangeas. & the proper way