The critical elements to keeping your hydrangeas blue are soil acidity and the availability of aluminium for the plant. Aluminium is a trace element in the soil, present in tiny proportions, not usually required by plants. Aluminium is not usually associated with plant nutrition but for the hydrangea it is a key ingredient in making its flowers blue.
Soils with a high pH (alkaline) prevent the aluminium that they naturally contain from being available to the hydrangea’s root system. Without the precious traces of aluminium, previously blue-flowering hydrangeas will end up with pink flowers when planted in an alkaline soil. If you have a soil pH above 5.5 then your blue hydrangea will struggle to maintain its colour without assistance. The answer lies in modifying the soil pH and unlocking or supplementing the aluminium. One of the most straight-forward methods is to by a feed which contains Aluminium sulphate. The sulphate brings acidity and the aluminium brings the blue. We sell J Arthur Bower’s Hydrangea Colourant but there are a range of products on the market. However, excessive use of such products can be problematic so follow the manufacturer’s guidelines at all times.
For a totally organic approach things get a little more complicated. Acidifying soil can be a complicated process involving sulphur. I have read of gardeners in France using powdered slate to supplement aluminium, though sourcing this product may be tricky. If your soil is unsuitable for blue hydrangeas and you do not wish to use aluminium sulphate then your best option may be to grow them in pots, using a good ericaceous compost. Keep an eye on the watering and position the pot in morning shade / afternoon sun.
Blue hydrangeas are a treat for the eye but don’t forget there are plenty of other gorgeous hydrangeas with sumptuous flowers that will cope with a range of soil types and conditions, from coastal to shady. My own favourites tend to be the white-flowered varieties and species, such as Hydrangea quercifolia - whatever you do to the soil these will never turn blue but always be a delight!