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The color of the flower is part of the invitation. Here, here, here, the flower hums. Come to me. ... Color is an advertisement. Red is a billboard sign.
--Sharman Apt Russell, Anatomy of a Rose, Exploring the Secret Life of Flowers
How do I know what plants to use in my wildscape?
One way to decide among the many choices of plants that will provide habitat is to think about "who" you want to provide habitat for. Take pollinators, for example. Do you want to attract hummingbirds? Native bees? Butterflies? Each of these groups of pollinators is attracted to different kinds of flowers, and particularly different colors of flowers. To decide which plants to plant, it helps to know what pollinators do and which flowers they depend on.
What's a pollinator?
A pollinator is an insect, bird or mammal that visits a flower (usually for a reward of food), and in the doing, accumulates pollen, which it carries to other flowers of the same kind, fertilizing their flowers.
The point of pollination from the plant's view is reproduction, exchanging genes with other members of its species to produce seeds, the plant's "children." Plants are rooted in place and can't just ramble about looking for others of their kind. Most flowering plants rely on mobile partners to pollinate their flowers. They put energy and effort into attracting creatures who will carry their pollen to other plants in the same species, thus cross-fertilizing the flowers and ensuring genetically diverse seeds.
The point of pollination from a pollinator's view is the reward the flower offers. Some tropical flowers offer pigments and scent-capsules which insects use in courtship. Some alpine flowers offer thermal protection, a cozy place to shelter when the weather turns nasty. Most flowers offer food in the form of nectar (sugar) or pollen (protein and fat)
What about color?
As Sharman Russell writes, color plays a big part in how flowers attract pollinators. It's an advertisement targeted to particular kinds of pollinators, because birds, insects and mammals are attuned to different parts of the light spectrum that we call color.
Hummingbirds, for instance, see the red wavelengths of light particularly well. (Perhaps more fully than we do.) So flowers that depend on hummingbirds for pollination advertise the nectar they offer, which just happens to be the optimal mix of water and sugars to power these winged dynamos' hovering flight, by coloring their flowers in shades of bright red. Here in the Rocky Mountain region, red flowers with long slender tubes like the scarlet gilia at the beginning of the post and the scarlet bugler penstemon above are generally advertising for hummingbird pollinators.
Bees and Butterflies
Bees, both native and European honeybees, see blue, ultraviolet, and yellow wavelengths particularly well. Which means flowers adapted to bee-pollinators "dress" in blue, purple, and yellow colors. They also are shaped to accommodate bees, from the fat, tubular flowers of Rocky Mountain penstemon above, designed for stout bumblebees, to the flat, disk-like flowers of sunflowers and other members of the daisy family, which give bees a wide landing pad to crawl about on.
Butterflies see a wider range of colors, including the red-orange-yellow wavelengths, and also blue and purple. Flowers in those colors will attract butterflies (as long as the flowers produce nectar and pollen.
Resources on pollinators and gardening: