There are approximately 23 species of hydrangeas but only five are commonly sold in this area. These Japanese natives are known as big leaf, French, garden or florist quality hydrangeas. All seem to winter hardy here even though they are only rated as perennial to zone 6. Hydrangeas should be planted in an area of partial shade near woodland areas or on the north side of your home. Some deflected sun will help to produce more blooms. Too much shade is one cause for the lack of flowering.
Flowering will be reduced due to improper pruning, lack of sunlight or more likely, weather related problems. Early, hard freezing in the fall before the hydrangea goes dormant or a late hard frost/freeze in the spring are reasons for no blooms. When to prune is complicated by varieties. Some varieties need to be pruned in the late summer after blooming while others should be pruned in spring as they start to foliate. Ask your dealer.
Changing the color of your hydrangeas from pink to blue can be accomplished by watering with aluminum sulfate at a rate of 1/2 oz. (1 Tbsp) to one gallon of water but be careful...your hydrangea should not be dry when applying aluminum sulfate and, what they don't tell you, allow your aluminum solution to set in the bucket for 24 hours before applying. Cut this rate in half (1/4 oz. per gallon of water) if you are growing your hydrangea in a large container and not in the ground. Safer methods for changing your hydrangeas from pink to blue is simply adding composted material such as manure, grass clippings or coffee grounds around the surface of the soil.
When applying fertilizer to your hydrangeas, use fertilizers high in phosphorus (the middle number) for keeping you blooms pink and use fertilizer low in phosphorus but high in potassium (the last number) for keeping them blue. Also, the addition of lime monthly around your plants helps to keep them pink. Do not use lime on blue hydrangeas.
Hydrangeas are relatively pest-free however powdery mildew can become a problem, especially in late summer and early fall. Spraying most of your perennials with a fungicide is always a good idea especially during cool, damp or humid weather and before the problems arise. A light dusting of Garden Sulfur is another way of preventing mildew. Insects such as aphids attack the new, tender growth and can be controlled with most insecticides. The presents of ants are a good indication that you have an insect problem as they are there for the honeydew created by the damage from insects.