Saguaro Flower Power Project

close up of white flowers against a blue sky
Saguaro Blossoms

T. Foley

 

Introduction

 
Photographing saguaro flowers
Sometimes science staff have to get creative and design their own tools.  In this case, a custom saguaro selfie stick
Each spring the giant saguaro cactus produces one of the most beautiful flowers of the Sonoran Desert. The large white flowers with yellow centers bloom at night and close the following day. They are pollinated by birds, insects, and even bats. We acknowledge that Saguaro National Park is within the traditional lands of the Tohono O’odham People, who continue a long traditional relationship with the saguaro through an annual fruit harvest.

Temperatures in the park have increased about 2o F in the past century. Many people wonder whether the increasing temperatures are causing flowers to bloom earlier than in the past, which could cause the saguaros to become out of sync with some pollinators.

In 2017, Saguaro National Park started a Citizen Science project to study saguaro flowering phenology. Each day our interns and volunteers go into the desert and photograph the buds, flowers, and fruit of more than 50 saguaros, using a digital camera mounted on top of a 30 foot long "selfie stick". Our goal is to tie the flowering dates with meteorological measurements and past data to learn more about changes over time. We are proud of our diverse volunteers and interns from the Latin Heritage Internship Program and Next Gen rangers supported by the Friends of Saguaro National Park and Western National Parks Association.
 
crown of saguaro flowers viewed from the pole-camera perspective above, showing bio-tech down below
Aerial photo of saguaro flowers taken with custom selfie stick

NPS/Lupe S.

 

What is phenology?

 
Saguaro fruit opens in July showing bright red flesh

Phenology is the study of periodic plant and animal life cycle events, such bird migration in the spring, or when saguaro cacti bloom and produce fruit. Saguaros reach reproductive age and put forth their first flowers when they reach a height of about 2.2 meters (7 feet), or when they are 30-65 years old. Saguaro flowering begins around the last two weeks of April, and peak flowering occurs during the last week of May through the first week in June.

Saguaro flowering is triggered by winter rain as well as the increased day length, and warmer temperatures of spring. The fruit typically ripens into a deep red color in late June and early July. They drop thousands of tiny seeds in anticipation of the summer rains during July and August.

 

What have we learned so far?

 
We have collected photo data on thousands of buds, flowers, and fruit over the past four years. We see that the saguaros produce more flowers some years than others, with slight changes in timing that is probably related to spring temperatures and rainfall.

We have also observed that the buds and flowers are not evenly distributed over the crown. They typically appear first on the eastern side, then gradually appear on the north and west sides. Why saguaros do this is not understood but may have to do with early buds taking advantage of the morning sun on the east side. Buds later in the season may be located to avoid the sun as the desert temperatures climb.
 
Graph showing summary of data collected over four years (2017-2020) of saguaro phenology monitoring. Most flowers exhibit during May across all years. Highest total in 2018 at 6,411 flowers.
Saguaro Phenology Monitoring data shows that from 2017 to 2020 the greatest number of flowers presented during the month of May. In 2018, the number of flowers simultaneously open hit a maximum at around 350. The most flowers observed in a single year during this study occurred in 2018 at 6,411!

NPS

See the rest of the data! Graphs from each individual year: 2017, 2018, 2019, 2020
 

Become a Citizen Scientist

 
Volunteers collect the photo data from April 1 until mid-July in the western district of Saguaro National Park (in the Tucson Mountains, a few miles north of the Arizona Sonora Desert Museum). Although some citizen scientists return each year, we may have openings during the 2021 field season (depending on Covid) for volunteers who can commit to most or all of the season. We typically start early in morning and work for 1-2 hours. It is a hot time of year and the terrain is very rugged, so you must be heat-tolerant and sure-footed! If you are interested in participating, please email Don Swann at Don_Swann@nps.gov.
 
park staff taking photos of saguaro
Park science staff use a looooooong selfie stick to snap pix of the saguaro blossoms

Last updated: February 25, 2021

Contact the Park

Mailing Address:

3693 S Old Spanish Trail
Tucson , AZ 85730

Phone:

(520) 733-5153
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