If you tinker with the acidity of the soil in which certain hydrangeas are growing, you can turn the blooms from pink to blue. Sometimes, the gardener doesn't set out to change the color but has the misfortune of having of either highly acidic or alkaline soil. Fortunately, for those who want a true, stable blue hydrangea, there is Nikko Blue (H. macrophylla 'Nikko Blue'). Hardy to USDA Zones 6a to 9B, the Nikko Blue variety blooms from late spring to mid-summer with huge, blue pom-pom-like flowers. Plant your Nikko Blue hydrangeas in the fall.
Choose a planting site in which the Nikko Blue hydrangea will receive partial sun. This is best described as morning sun and afternoon shade. Hydrangea can be grown in full sun in cooler regions but will need to be watered daily.
Remove all weeds and turf grass from within a 2-foot radius of the planting site. Weeds act as competition for water and nutrients.
Loosen the planting area with the gardening fork. Dig down to a depth of 12 inches and crush the soil. Remove any rocks or other debris that turns up.
Pour a 3-inch layer of compost over the planting site and use the gardening fork to mix it, to a depth of 8 inches, with the existing soil.
Dig a planting hole for the Nikko Blue hydrangea. The hole should be the same depth as the plant is currently growing and twice the diameter.
Remove the Nikko Blue hydrangea from the pot and gently loosen the rootball with your fingers. Place the rootball in the hole and backfill halfway with soil. Fill the hole with water, allow it to drain and finish filling the hole with soil. Tamp around the base of the plant with your feet to remove any remaining air pockets in the soil.
Add a 3-inch layer of mulch 2 inches from the Nikko Blue hydrangea's trunk.
Water the hydrangea until the water puddles.