Hydrangeas are a beautiful plant, native to temperate areas of Asia and the Americas, but by far the greatest diversity can be found naturally in Asia. If you live in a temperate region, they are are a must for your garden - they are a plant genus containing more than 100 species - so there will be one for you.
Hydrangeas typically are seen with pink, blue or white flowers. White flowering hydrangeas will always produce white flowers, but those that produce pink or blue flowers often change between blue and pink. But why is it that the same plant can change its flower colour from one season to the next? Some produce blue/purple flowers one season while sometimes producing pink flowers the following year.
The reason for this is most due to the hydrangea plants ability to absorb aluminium, which enables the production of blue flowers. The ability to absorb aluminium is influenced by the acidity of the soil - the more alkaline the soil, the less aluminum is absorbed and therefore the more pink the flowers will be.
So, the reason why your hydrangea flowers are pink, is beacuse your soil is either slightly acidic or alkaline - hydrangeas growing in soils that have a pH of 5.5 or higher will produce pink flowers while hydrangeas growing in soils that have a pH less than 5.5 will produce blue flowers.
Simple - lower the pH of your soil to below 5.5. This means you need to make your soil more acidic. But, be careful, the other plants in your garden might not like this. For this reason, hydrangeas are best grown on mass and not as a solitary plant amongst other species.
The easy way to adjust the pH of your soil and achieve blue hydrangeas is to add "Hydrangea Blueing Agent" which is commonly known as "Soil Acidifier". This is available from most nurseries. Simply follow the directions on the package and it is probable, that when your hydrangeas flower next season, they will start to take on a bluish colour - please remember - this will not happen quickly; your pink hydrangea flowers will not become blue. It will probably take two or more years to produce the beautiful hydrangea blue colour in your flowers.
My preferred method is to acidify the soil by using organic additives such as coffee grounds, leaf litter, compost and manures. Continue adding and digging these into the soil. You will find that the pH of your soil will gradually lower, and after a year or two, you should be enjoying beautiful blue hydrangeas.
Please remember that getting your pink flowering hydrangeas to produce blue flowers is not a quick process, but the blue flowrs are beautiful and well worth the effort and the wait.