Delphiniums were one of the first flowers I grew – so tall, so imposing, so deliciously blue – how could they not be one of my first choices when I took to gardening? Other blues have called over the years – some grown and some still to be grown: Gentiana, Lobelia, Clematis, Nigella, Ceratostigma – oh those alluring blues that beckon us so much more than white or pink or yellow. Be it rarity or be it something else, pretty much everyone seems to crave some blue flowers in their garden.
Well oddly a flower’s blue color derives from the same substance that gives leaves their red coloration in the fall – the pigment anthocyanin – how’s that for surprising?
Anthocyanin is a plant pigment whose color varies with cellular pH. In acid environments it’s red; alkaline it’s blue and neutral yields a purple hue. (I know some of you are out there going, “No, no it can’t be what about hydrangeas?” I’ll come to that momentarily.) The pH of the liquid in plant cells tends toward the acid so red dominates. Most “blue” flowers are actually sortof purple since getting the sap pH good and alkaline is hard work for the plant. There’s got to be some compelling reason for a plant to expend its resources turning its cellular sap alkaline and given the rarity of blue flowers, it is clearly rarely worth it. For example, Tweedia caerulea has blue flowers to attract bees but only once they are ready to be pollinated, before that they are pink so as not to waste energy and resources (or send out false signals to the bees).
Now to deal with the hydrangea question. Everyone knows that hydrangea flowers can be changed from blue to pink by changing the soil pH from acid to alkaline – the opposite of the above. Well, the reason is that hydrangeas use iron and aluminum for their blue pigment and both are more soluble in soils at lower (more acidic) pHs. If not enough iron and aluminum are available in the soil (due to total quantities or pH), hydrangea blooms will be pink, although of course some are engineered to be pink just to mess with our heads.