For centuries, people have searched for a true blue flower. Elusive and rare, it is seldom found in nature. Or, to put it another way, it is rarely perceived in nature. It all has to do with what each of us sees as blue.

To find out why, I signed up for an on-line lecture given by Brandon George, a grad student working at Cornell Botanic Gardens. His research not only produced a great list of blue flowers, but also shed some (hint) light on the issue.

WHAT CONSTITUTES BLUE

So why is blue so rare in the plant world? For starters, I’ll ask you to refer to the color wheel below.

Blue is a primary color. On the visible color spectrum, it is located between green and violet. But that’s where things get murky. Take a look at the wheel. Some blues tend towards green, while others tend towards violet. Do all of these colors merit the name blue?

At the moment, the Royal Horticultural Society (RHS) is the go-to authority. The RHS Colour Chart, created in 1966,  contains 920 pigments that can be matched precisely to flowers, fruits and other plants. Over the decades, it has become the primary means by which growers and horticulturalists communicate about colors around the world.  

Still, the color chart doesn’t explain why a true, pure blue remains so elusive. And here’s why. Though blue is a prominent color on earth, it is rarely produced in nature. In fact, of all the 280,000 known species of flowering plants, only 10 percent are blue.

HOW PLANTS PRODUCE BLUE

It turns out that plants aren’t born blue. Instead, like artists, they must mix naturally-occurring pigments to achieve their blue hue. The most common of these pigments are called anthocyanins. However, anthocyanins change their color depending on soil pH.

Hydrangea macrophylla ‘Nikko Blue’

In fact, just the tiniest tweak of metal ions in the soil can result in the same plant producing entirely different shades of blue. (Think blue hydrangeas, which are produced by adding acid to the soil.) Finding a true blue flower is really hard indeed.

WHEN PURPLE LOOKS BLUE

Even then, some of us may still see purple as blue.  Deutan Color Blindness (do-tan) is an anomaly of the ‘M’ cone (Medium Wavelength Light) in which spectral sensitivity is shifted toward longer wavelengths. If you have it, you may experience confusion between colors such as purple and blue. Take a look at the photo below. Do you see purple and blue or just blue?

Photo credit/enchroma.com

TRUE BLUE PRETENDERS

Nowadays, blue flowers are highly prized. As a result, growers are introducing more and more plants that are labelled as blue. But beware – many are not truly blue! To differentiate among cultivars, horticulturalists now use the term ‘true blue’ to indicate a more true blue pigment.

Take, for instance, the hardy geranium ‘Rozanne’, a popular choice for the perennial garden. Some growers list it as violet, others lists it as blue. How do you perceive it? To my eye, it tends towards purple, while my client sees it as blue.

Hardy geranium ‘Rozanne’/Photo: anniesannuals.com

And have you ever wondered why the same blue plant can look entirely different across catalogs? Just because a plant has blue in its name doesn’t necessarily mean it’s blue. Some growers manipulate photos to make plants appear more blue. Still others use tricks of light. If you can’t see the plant in person, George recommends consulting user uploads rather than seller photos to get a better perspective on a plant’s true color.

TRUE BLUE FLOWERS

There is one plant family that typically produces the truest blue flowers in nature. Boraginaceae, also known as the forget-me-not family, includes more than 27,000 species. The plants of this family are frequently hairy and include such garden ornamentals as Virginia bluebell (Mertensia virginica), Lungwort (Pulmonaria), Forget-Me-Not (Myosotis)and Siberian Bugloss (Brunnera macrophylla). Horticulturalists agree that these are indisputable blues.

Virginia bluebells

Other indisputable blue flowers include Grape hyacinth (Muscari), Siberian squill (Scilla siberica) and Blue Drumstick allium (Azure allium), in addition to pH-sensitive Hydrangea macrophylla ‘Nikko Blue‘, which in acidic soil (a pH below 6) will turn blue. 

Blue Drumstick, Allium caeruleum

And don’t overlook the sky blue flowers of Siberian larkspur (Delphinium grandiflorum), and Hardy plumbago (Plumbago cerastostigma), which are all great additions to the spring/summer border.

TRUE BLUE ANNUALS

Perennials not your thing? There are some great almost-blue annuals. Evolvulus ‘Blue My Mind’ is a dwarf morning glory with fuzzy, silver-green foliage. It looks great in containers or windowboxes, where it will happily trail over the edge.

Other great true-blue annuals include Cape Plumbago (Plumbago auriculata), a very light blue, Love-In-A-Mist (Nigella), and Gentian sage (Salvia patens), a tender perennial that has the deepest blue flowers you’ll find.

Gentian sage, Salvia patens

DESIGNING WITH BLUE

When working with blue, remember it is considered a cool color, so it will recede into the landscape. Consider bringing it forward to enjoy it and plant cultivars in mass for a stronger effect.

But a word of caution. Placing a lot of different ‘blues’ together will often cause some to look bluer than others (see below). To prevent this from happening, separate them out and plant them instead next to contrasting colors (such as orange or yellow), which will give the illusion of a brighter blue.

Blue or purple? Delphiniums growing in Dalat, Vietnam