According to respected plants-man Michael A. Dirr and others, these are the best of the best hydrangeas in the blue range:

• Hydrangea macrophylla 'Dooley': mophead, pink to rich blue, cold-hardy to -11°F; mildew-susceptible; robust, 6 feet high; shade garden-worthy after 2 years; some dieback, but regrowth with abundant flower buds
• 'Endless Summer'®, 'Decatur blue', David Ramsey', 'Oak Hill', and 'Penny Mac': remontant (repeat-flowering) types; quite similar; flowers on new growth; foliage flat medium green; moderate mildew susceptibility; 3-5 feet high and wide; have suffered cold damage in trials, but new shoots develop flower buds and fully expressed inflorescences; laden with flowers
• 'Générale Vicomtesse de Vibraye': mophead, opening soft cream-yellow, finally pink to blue; entire to serrated sepals;  8 feet high; early French hybrid; stems spotted (speckled) wine-red; medium green leaves; mildew susceptible; shade garden-tested after two years, strong growth with no die back; abundant flowers
• 'Nikko Blue': popular true remontant; cold-hardy (revived in trials after freezing to the ground); mophead, not very drought-tolerant, prefers sun but needs half-shade in very hot places, 5-6 feet tall and 4-6 feet wide, emerald green leaves
• 'Mini Penny': mildew-resistant, powder-blue mophead with dark green leaves, 3-4 feet high and wide, full shade to part shade; unlike most hydrangeas, blooms on both new and old growth

Most blue hydrangeas hold their color best in slightly acidic soil--ideal pH is 5.2-5.5--and become pink in more alkaline soil. In the eastern U.S., most soil is naturally acidic. However, if soil pH is too alkaline (pH 6.0 or higher), aluminum sulfate can be added to the soil. Be careful not to add too much, as aluminum is toxic to plants at high levels. Adding organic matter such as coffee grounds, fruit and vegetable peels, or lawn clippings to soil will also help lower the pH.  Fertilizer that is low in phosphorous and high in potassium is also helpful in producing a good blue color. A good fertilizer ratio is 25/5/30 (potassium is the third number).

For tips on a variety of gardening topics, see our Plant Information Guides.
- Courtesy of NYBG Plant Information Service