Deciduous Flowering Shrubs - Part 4

Flower color ranges from creamy or stark whites through pinks, violets, blues and reds. White forms, are always white, except that they can indicate soil pH by pink or blue central eyes on the male flowers, according to Keith Howe's 1993 article in Fine Gardening magazine.

Flower color in all other forms of hydrangeas is determined by soil pH and the amount of aluminum sulfate available in the soil. Acid soil (pH less than 7) has more aluminum available to the plants which causes them to produce blue flowers - except for clones who are actually mutants who lack the ability to use aluminum at all and who always produce pink flowers - such as 'Ami Pasquier', 'Bottstein' and 'Pia'. As soil becomes more alkaline, the amount of aluminum available to the plant decreases and flowers turn pink. You can get varying amounts of blue and pink pigment in flowers on the same plant just because the soil isn't totally uniform in pH.

Flowers will also change color as they age (something that happens on a lot of plants). All hydrangea flowers start out a pale green, maturing to their ultimate color and then fading, either back to green or to deeper wines or purples before turning parchment tan and then brown. H. 'Preziosa' has flowers that start white and mature to pinks and wines. Since the flowers open at different rates, this shrub will always have a mixture of flower colors present.

If you want blue flowers and your soil is too neutral to produce them, you can acidify it using aluminum sulfate or sulfur (follow package directions!). Results will not be instantaneous and may not occur until the next years flowering. I have also read that the best blue flowers can be obtained by using a nitrate-based rather than ammonium-based fertilizer with an NPK ratio of 25-5-30 and avoiding the use of bonemeal or superphosphate.

If you want pink or red and are getting blue, use a nitrate-based fertilizer with an NPK ratio of 25-10-10 and apply garden lime or wood ashes to the soil around the plant...again, results won't be instant. If your water source is alkaline, you need to adjust for that because watering with mains water will affect the soil pH.

Selected Species

I grow about nine or ten different hydrangeas and lust for many more. There are some nineteen species I've found in my research, with quite a number

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