Blue hydrangeas belong primarily to the Macrophylla species of the hydrangea family and are more commonly known as 'mopheads' and 'lacecaps'. They have large, showy blossoms that bloom in the summer in shades of white, pink or blue, depending on the type of soil in which they are planted. Hydrangea Macrophylla thrives in zone eight, but can do well up to zone 5b, as long as they are properly protected in the winter.
Select an area of your property that drains well and that receives morning sun and afternoon shade. You should avoid planting hydrangeas close to a large tree as the tree might drain the nutrients and block too much of the sun. Test the soil in the area where you plan to plant your blue hydrangea. Macrophylla hydrangeas grow blue flowers when you plant and grow them in acidic soil with pH levels that range from 5.2 to 5.5.
Increase the acidity of your soil, if necessary, by mixing in aluminum sulfate or ammonium sulfate. Follow the package instructions for application instructions for the amount of reduction your soil requires. You can also apply organic materials that are naturally acidic such as oak leaves, peat moss and sawdust. You will need to retest the soil around the hydrangeas during the growing season and reapply these materials as needed to help keep the pH levels acidic.
Dig a hole to plant your hydrangea that's twice as wide as its root ball. The hole should be only as deep as the growing container. Remove the hydrangea from the container and squeeze the soil around the roots to loosen them. Place the plant in the hole. Backfill around the roots with the soil you removed from the hole.
Water the soil around the hydrangea plant after planting. Add a 2-inch layer of conifer needles around the hydrangea plant to help with moisture retention and to help maintain acidity. Hydrangeas should be kept moist in hot weather, water your plant daily as needed.
Fertilize your blue hydrangea at the beginning of May and again in July, if you live in the south. One application of organic fertilizer or 10-10-10 in July is sufficient for Northern growers. Never fertilize your hydrangeas after August, as they are heading into a dormant period. Apply the fertilizer around the drip line, not close to the trunk. A small hydrangea plant will only need between 1/8 and 1/4 cup, while a large shrub can handle between 2 and 3 cups of fertilizer during each application.