Butterfly House

A male monarch butterfly rests on an penstemon Wednesday in the new Southwest Butterfly House at the Arboretum at Flagstaff.

Jake Bacon/Arizona Daily Sun

If you live in northern Arizona, you know that there are many beautiful variations of penstemon flowers, but have you ever wondered why that is?

With 280 species, penstemons are the largest genus of plants native to North America, and 90 percent are found only in the American West. In northern Arizona, we are lucky enough to have 59 of those species and one is even endemic to Flagstaff, growing exclusively in the area around Sunset Crater Volcano.

These unassuming flowers are easily recognizable when you know what to look for. Their tubular shapes are varied — but always adorned by two petal-like lobes on top and three on the bottom. Around 80 percent are blue/purple colored, with another 15 percent red and the remaining 5 percent coming in a range of colors, including white, yellow, orange and pink. And the fuzzy stamen protruding from their centers explains why they are commonly called “beard-tongues.”

So why are there so many distinct shapes and colors to penstemons? The reason can be linked to their pollinators. To ensure the continuation of their species, flowering plants like penstemons have adapted distinct sets of specialized traits to attract specific pollinators. Flowers pollinated by hummingbirds are often red-hued, with downward hanging flowers and narrow tubes. Conversely, bees prefer the blue/purple flowers that have large lower petals to act as “landing platforms,” and wider tubes.

After seeing the matching shapes of the long narrow beaks of hummingbirds with the long narrow flowers they use for food, Charles Darwin hypothesized that through time these pairs formed mutualistic partnerships. The pollinator receives food and by feeding, moves pollen from flower to flower, fertilizing them in the process.

Research has shown though, that while flowers evolved with bees, some later adapted traits which encouraged hummingbird pollination — and that also served as deterrence against pollination by bees.

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Bees, it turns out, are sub-par pollinators when compared to hummingbirds. The larger bodies, faster speed and greater distances traveled by hummingbirds make them superior pollinators to bees. Hummingbirds search for flowers by vision in order to collect nectar; their unique physiology requires three times their body weight in nectar every day. Over time, the flowers they visited became brightly colored, with little scent and lots of sweet nectar. (Fun fact: hummingbirds are the only birds that can detect sweet tastes.)

Bees on the other hand, find flowers by smell, and their specialized eyesight makes it difficult for them to differentiate between red and green. Both species have the ability to locate blue/purple flowers, but red flowers act as a signal for hummingbirds while being inconspicuous to bees. In addition, the longer narrower shape of red flowers allows hummingbirds access to nectar while simultaneously blocking the the smaller bees.

An excellent place to see penstemons and their pollinators in action is The Arboretum of Flagstaff. On May 28th, the Arboretum is holding their annual Spring Plant Sale & Celebration, free of admission. At this event, you can purchase penstemons for your own garden, or view the 97 species in their nationally-accredited Penstemon Collection throughout the grounds.

Dana Howard is a Master’s Degree of Biology candidate and a docent at The Arboretum at Flagstaff. Lynne Nemeth, executive director of The Arboretum at Flagstaff, is the editor of Gardening Etcetera. To reach her with articles or ideas, please email Lynne.Nemeth@thearb.org.

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