Desert Senna

Senna covesii

• Content: Deborah Bird
• Photo: Brian Gersten www.briangersten.com

Desert Senna is one of the plants that uses the Pop and Drop seed dispersal method.

There’s lots of Desert Senna blooming in Sabino Canyon in October. Despite little rain, the late summer/early fall blooming plants are starting to show. Desert Senna has a bright yellow flower with 5 petals. The nice thing is that the flowers are large enough for most of us to see without the use of any magnification aids. It’s a small subshrub with four or six leaflets. The leaves are fuzzy gray-green and are shed during dry seasons.

Back when I was in training, I learned about the way plants can spread their seeds. Desert Senna is one of the plants that uses the Pop and Drop method. The fruits are slightly hardened, bean-shaped pods that split lengthwise from the tip and are usually oriented upright and can sometimes twist. The seeds are held inside the pod even after opening until the pod is bounced by wind or a passing animal and the seeds will spill out and land away from the parent plant. This tactic will give the seed a chance to grow in its own place away from the parent plant.

When I think about or study any plant, I also like to learn about its place in the environment. How does it fit into the desert ecology? How does it fit into the Web of Life? As I’ve learned in my Naturalist Journey, plants, animals, water, sun, and geology are all inextricably linked to create this habitat that we call the Sonoran Desert. Desert Senna has a place as it provides food for birds and mammals, shelter and shade for animals and smaller plants, and root systems help prevent erosion.

Desert Senna flowers provide pollen for bees and nectar for butterflies. Desert Senna is one of the plants that is pollinated by Buzz pollination. It’s one of the 330,000 (!) plants in the world that are pollinated this way. These plants all have anthers that release their pollen only through small pores at the anther tips. Certain native bees such as Carpenter bees, Anthophora Digger bees and others turn themselves into tuning forks and sonicate the pollen from these flowers. Other insects that uses Desert Senna is the Cloudless Sulphur (Phoebis sennae) butterfly as well as the Sleepy Orange (Abaeis nicippe). Desert Senna is a host plant for both caterpillars of these.

Right now, you can find Desert Senna almost everywhere especially along trails and walkways. Stop to take a closer look, you might see some sonication going on.

Definition: Sonicate: to subject (a biological sample) to ultrasonic vibration so as to fragment the cells, macromolecules, and membranes
Etymology: Senna is from Arabic Sena, meaning thorny bush, while covesii is named for Elliot Coues (1842-1899) an American naturalist who is best known for his ornithological work.