Why Roses are Red

Rosa @DBG LAH 016Color. It’s probably the primary reason we grow flowers. Yellow daffodils and red roses, irises in every color of the rainbow. Without flowers, our yards would be much more subdued. Gardeners rejoice in the options available, but why do flowers come in so many colors in the first place? What do they get out of it?

For one thing, not all flower colors arise in nature. Plant breeders have spent centuries coaxing new combinations from the available genes—even inducing mutations to increase the possibilities. The wild roses that grow in our foothills range in color from pale to deep pink, but at the garden center I can buy rose bushes that produce blooms of yellow, peach, salmon, orange, lavender, burgundy, pure red, and white—or even combinations of these. Petunias were originally a pinkish lavender. Now they come in every hue except green.

But what about the colors courtesy of Mother Nature? Why are some flowers so drab we hardly notice them, while others are ostentatious and flamboyant? It all has to do with sex.

Papaver_rhoeas_ShirleyPoppies_TaylorU-IN_20090613_LAH_2878

More specifically, it has to do with how plants are pollinated. All flower-producing plants need some way to transfer pollen from the anthers to the pistil, so that fertilization can occur. (See “Where (plant) Babies Come From.”)

Some flowers rely on wind to accomplish this transfer. They produce copious amount of microscopic pollen, so small it’s easily carried by the slightest breeze. This is typically the pollen that triggers allergies in hayfever sufferers. Many trees are wind pollinated. When is the last time you noticed the flowers of an oak, maple, or juniper? Grasses fall into this category as well. 

Other flowers have heavier pollen. Some of these, such as peas and tomatoes, are self-fertile. Others require help to move their pollen around. That’s where the birds and the bees—and bats, and moths and other insects—come into play. Flowers are colorful to entice these pollinators to visit. When they do, they’re rewarded with sweet fruit or nectar.

Bee over Dahlia 'Tropic Sun'_HudsonGardens-LittletonCO_LAH_9312

It’s all really a giant advertising campaign.

Different colors come from different kinds of chemicals. These pigments absorb some wavelengths of light while reflecting others. We see the wavelengths reflected.

For example, anthocyanins reflect reds, blues, and purples. (The color depends on pH, which is why hydrangeas can be pink or blue, depending on the soil they’re grown in). Betalains reflect red or yellow light. They give beets their red color. Carotenoids reflect reds, oranges, and yellows. Chlorophylls mainly reflect green. (While green flowers could help with photosynthesis, we rarely see them because pollinators wouldn’t see them against green foliage.) White flowers are white because they don’t have any pigments.

From a metabolic standpoint, these chemicals are not cheap. To create them, the plant must use energy—energy that could be used for other essentials such as growth and reproduction. In order for a plant to thrive, the return on this investment has to outweigh the cost. Therefore, a plant will produce the least amount of pigment that will do the job.

This is why wildflowers are often not as large and intensely colored as cultivated flowers. This also explains why the undersides of low-growing flowers are usually not as colorful as the tops—pollinators usually approach from above.

Different pigments attract different pollinators. Hummingbirds are attracted to red, but also pink and purple. Bees are partial to yellow, plus ultraviolet colors our human eyes can’t see. Nocturnal pollinators can’t see much color, so the flowers moths and bats pollinate are largely white.

Broad-tailed Hummingbird_XG-COS-CO_LAH_9386

There’s another benefit to having colorful flowers, one that is often overlooked. From an evolutionary perspective, the most successful species is the one that produces the most offspring. Dandelions are hugely successful. They’re everywhere! So are pigeons, found in nearly every city on six continents. One reason these species are so prolific is because they’ve had some help from us humans. We carried dandelion seeds (perhaps unintentionally) from the Old World to the New. We’ve built cities that provide pigeons with an ideal habitat.

Taraxacum officinal_Dandelions_LeadvilleFishHatchery-CO_LAH_2029

And so it is with flowering plants. Those that we prefer are propagated and pampered in numbers far beyond what would otherwise be possible. From the plants’ point of view, having big, showy flowers is well worth the investment. As a result, we all benefit.

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