Question
I have Hydrangeas in my garden which are normally pink but this year one or two are growing some blue flowers. Why?. Can you please tell me how to make others turn blue?
Thanks Jack

Answer
Your soil is turning acid.  That's a natural trend, caused by metabolism of good soil bacteria.  Note it is a slow trend -- it does not happen overnight, although ads for fertilizers and general advice seems to give people the impression that last week's pink flowers will turn blue if you just change the pH of the soil or add Miracid. Nope.

If you prefer the blue, get started now.  Don't go crazy.  Blue flowers come to those who wait.

Acidify the soil with top dressings of peat moss, humus, aged compost and coffee grounds.  Avoid alkaline exposure -- wood ashes, grass-pleasing lime you may have added to your lawn -- and don't expect overnight results.  A watched pot never boils.

If you can't stand it, run out and get a simple soil testing kit to use weekly.  This is like losing weight.  We don't go on a diet and lose 10 lbs a week without surgery.  Think of it the same way.  Test once a month in the growing season and write it down so you can see progress.  Pouring vinegar in the soil and then testing the pH is like taking off your shoes and weighing yourself to see how many pounds you've lost.  Remember: Slow and steady wins the race.