The ground under the shrub stopped being acidic enough, so the aluminum those flowers run on quit reaching the roots. A blue hydrangea turned pink is a soil report, not a sick plant. The color comes from aluminum compounds sitting in the flower tissue, and roots can only pull that aluminum out of sour ground. Once the soil reaches pH 7.0 or higher, the supply drops off and the blooms open pink.
My first flush came out a muddy mauve, and the next one opened clean bubblegum pink on a shrub that had been deep blue the summer before. Nothing else looked off. Same leaves, same watering can. The heavy clay soil that came with the garden read 7.1 on a cheap test kit, which is over the line where aluminum gets hard to take up.
Run the chain in the right order and the fix stops being a guess. Aluminum in the flower makes blue, small amounts make violet, and none at all leaves you with pink. Hydrangea soil pH colors nothing by itself. All it does is decide whether the roots can take the aluminum up, so what you have is an aluminum problem that pH happens to govern. Feeding the plant harder will not touch it, and neither will more sun or a bigger pot.
Test the soil before you name a reason for the shift. The windows are narrow, since barely one and a half points of pH cover every color the plant can make, and a guess here costs you a full season of bloom. A cheap probe from the garden center gets you close, and a mail-in test through your extension office gets you a number you can act on.
Once you have that number, the aluminum sulfate hydrangea growers keep on the shelf is the fast way back. It is a soil drench, not a feed, and it goes on in March, April and May. Mark those three months on a calendar, because a treatment that goes on in August has already missed the window. Pour it at the base and keep it off the leaves.
Check the pH before you pour anything, so you know how far the ground has drifted from the 5.0 to 5.5 blue range.
Dissolve 1 tablespoon (15 ml) of aluminum sulfate in 1 gallon (3.8 L) of water and soak the root zone once a month for three months, spring to early summer.
Broadcast about half a cup (120 ml) of wettable sulfur per 10 square feet (0.9 sq m) and water it in. This route can take up to a year to show a change.
Pick one route and stay on it for the season. The drench asks more of you and is the faster of the two routes, while sulfur asks almost nothing and makes you wait, so the choice comes down to how much work you want to do. Running both at once leaves you with no idea which one did the work. I would take the drench on a shrub I care about and let the sulfur handle the ones out along the fence.
A hydrangea color change follows the ground, not the calendar, so treat the soil test as the actual job and the flowers as the readout. Retest every spring before the first drench goes on, and write the reading down where you will find it next year. If the number already sits in the 5.0 to 5.5 band and the blooms still come pink, stop adding sulfur. Your pH is not what is holding the color back.
Read the full article: 10 True Blue Flowers and Why Blue Is Rare
