are determined by the cultivar. While some are so intensely blue (below) as to be almost neon in their effect, others (left) are very pale pastel. Just what fertilizer should you use for beautiful blue flowers? If non-organic is not a problem for you, use a granular or soluble fertilizer specifically formulated for hydrangeas. If you can't find that, use one formulated for rhododendrons and other acid-loving plants. If your hydrangea refuses to turn blue even though the soil pH is at 6 or below, you can treat it with aluminum sulfate, to ensure that aluminum is sufficiently present in your soil. Use caution, however, as too much aluminum quickly becomes toxic, even to a hydrangea. Do not exceed dosages recommended on the fertilizer package. Organic gardeners, you should use aged cow manure to feed your hydrangeas. It is the only manure that is mildly acidic. Supplying aluminum organically is trickier. In France, powdered slate (the stone used to make traditional roof shingles) is used. (I am working on sourcing this product in order to be able to offer it on this site.) It contains a slow-release, organic form of aluminum sulfate. That's why hydrangeas planted next to slate-roofed houses are always a brilliant blue; the rainwater runoff from the slate supplies plenty of aluminum in just the right, gentle doses. If your soil pH is above 6, you will need to lower it. The best way to do this (also okay for organic gardeners), is to incorporate wettable sulfur