Blue is a calming color and a good choice for a relaxing garden. With the exception of hydrangeas, flowers in vivid shades of blue aren't grown any differently than flowers of other hues. Cool season flowers prefer early spring temperatures and start to fade when summer comes. Warm season flowers love the warm days of June through August. A blue and white flower garden is cooling to view during hot weather.
Select a spot that receives dappled shade, or morning sun and afternoon shade, for blue violas, hydrangeas, delphiniums and pansies. These blue flowers prefer cooler weather and shade.
Select a sunny area for petunias, larkspur and lobelia. These are warm season blue flowers.
Dig the garden bed and add soil amendments if required. Blue flower beds benefit from additional compost or organic matter. Spread a 4-inch layer over the bed. Add peat moss and sand to clay soils. Add extra organic matter to sandy soils---exactly how much add depends on the specific condition of your soil. Spread slow release fertilizer over the bed. Work into the soil and rake smooth.
Keep hydrangeas blue by adding aluminum to an acid soil. According to hydrangeashydrangeas.com, a mixture of one tablespoon of aluminum sulfate in a gallon of water will transform pink or purple hydrangea blooms to vivid shades of blue. Water the plant with plain water before using the aluminum sulfate solution. Keep the soil on the acid side by adding vegetable peelings, tea bags and coffee grounds.
Plant blue hydrangeas as deep into the soil as they were in the pot. For example, a 4-inch transplant would be planted in a 4-inch deep hole. Cover the soil of the pot with a 1/2-inch layer of garden soil.
Fertilize blue hydrangeas with a fertilizer low in phosphorous and high in potassium. When you look on the package you'll see a set of three numbers, such as 10-10-10. The first number is nitrogen, the second for phosphorous and the last is for potassium. A good fertilizer for hydrangeas would be 25-5-30. Fertilize other blue flowers with a balanced fertilizer.