As I look around at flower gardens in my backyard and in my neighborhood, I see a lot of purple. Where the first flowers of spring seemed to be largely pink in color, the first flowers of summer seem to be mostly purple, a color that is not all that common in plants. Why so much purple, and why now?
Purple is for the bees, and especially the solitary bees which are the most abundant and hearty pollinators we see in cool spring and early summer weather, before honeybees and butterflies have become active. Bee eyes can detect light wavelengths in the ultraviolet part of the spectrum, and it seems that plants have adapted their floral displays to guide the bees right to the places on the flower where their search for nectar or pollen will do the flower the most good (i.e., pollination).
One study with bumblebees in Germany showed that naive bees were highly attracted to flowers in the ultra-violet to blue range of wavelengths, and those flowers tended to produce more nectar or nectar with higher sugar concentrations than flowers of other colors. Purple flowers that attract solitary bees tend to have the highest sugar concentration in their nectar (20-50% sugar), compared to yellow and red hummingbird and butterfly flowers that average around 20-22% sugar in the nectar.
The variation in “purple-ness” of early summer flowers is due to the proportion of anthocyanin pigment in the flower in response to the pH of the soil environment. In the presence of acid (or hydrogen ion), the pigment is red (or pink), whereas in the presence of nitrogen base the pigment is blue, and in the presence of aluminum, it is violet. So, you can change the colors of your hydrangea or forget-me-nots by what you put in their soil.
Purple is a coveted color in the garden, because of its rarity, and the ephemeral nature of purple flowers. Looking back in our own history, we find that limited resources and the labor-intensive process used to create the color purple made it an expensive luxury, indulged in only by the most wealthy or by royalty. Regardless of expense, its calming (blue overtones), yet energizing (red hues) make purple a favorite color.