There's lots of different cultivars of mophead hydrangeas and some will respond to color changes better than others.....for example, several pink varieties are very stable in their coloring and will never be blue, despite how acidic the soil conditions may be. A deep purply pink perhaps, but not a true blue.

Without knowing what kind of hydrangea you have, it's hard to say how blue you can get it. And we need to know how acidic your soil is. The blue of hydrangea flowers is influenced by the amount of aluminum present in the soil - soils that are not adequately acidic tie up the aluminum and make it unavailable. Even hydrangea bluing - aluminum sulphate - will not cause a major color change if the soil is not sufficiently acid as well. And IME, fertilizers for acid loving plants will only maintain existing soil acidity - they will not significantly alter soil pH and therefore have little effect on making hydrangeas blue if the soil is not already acidic.

So test for pH and if your soil is in the 6.5 range or lower, use aluminium sulphate according to package directions. The effect is not immediate. Applying now or later in fall should influence next season's flowers. If your soil tests out at a higher pH, apply agricultural sulphur to lower AND use the aluminum sulphate.

Thank you for the advice. I will test the soil. Perhaps in the next few days I can post a picture in hopes of identifying the type of hydrangeas I have.

Hi
Could someone please tell me where is the best place to plant hydrangeas. Regarding how much sun it requiers. I planted one in the afternoon sun against a wall. It seemed to be to hot. The plant wilted. I relocated the plant to another area.But I think now I have to much shade.And no flowers. Thanks

The phosphorus in an all-purpose fertilizer inhibits the uptake of aluminum. And absorbed aluminum is what the plant needs to turn florets blue.

Our local hydrangea specialist suggests compost as the only fertilizer for hydrangeas.