The bigleaf hydrangeas (Hydrangea macrophylla or H. serrata) are the alchemists of the garden. Regardless of what colour they are supposed to be -- pink or blue -- according to the nursery tag, these plants flower in whatever colour the soil's chemistry dictates. Your Forever Pink hydrangea may turn blue if there is too much aluminium in the soil. To keep the hydrangea pink requires a bit of chemistry.
Conduct a soil analysis. The county cooperative extension office supplies these tests for a nominal fee and the results go a long way in helping you determine what to add to the soil to help the hydrangea keep its rosy colour. Aim for a pH of between 6.0 and 6.2. While some experts claim that the soil pH for pink hydrangeas should be between 7.0 and 7.5, others say that a pH above 6.4 makes the shrub chlorotic due to a lack of iron.
Sprinkle dolomitic lime on the soil around the hydrangea two to three times a year, with the first application in the fall. Read the back of the lime package to determine how much lime to add. As a rule of thumb, start with 0.227kg. of lime for each 100 square feet. Use a rake or hand cultivator to scratch the lime into the top 2 inches of soil. Water the soil slowly and deeply, to a depth of 10 inches, after applying. The water helps the lime spread throughout the soil.
Fertilise the pink hydrangea in early spring with a 25-10-10 formula. Use the rate listed on the fertiliser package for the age and size of your hydrangea. Scatter the fertiliser on the soil, six inches away from the base of the hydrangea, and spread it out to the dripline. Water as you normally do.
Grow the pink hydrangea in a pot if all else fails. Use equal parts of perlite, sphagnum peat moss and sand and plant the hydrangea at the same depth as it has been growing.