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.
What is phenology?
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. 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.
See graphs from each individual year
We have also observed that the buds and flowers are not evenly distributed over the crown. Recent research by park staff showed they typically appeared first on the eastern side of saguaro crowns, then spread radially in a counterclockwise direction. Saguaro flowers were consistently more abundant on the northern part of the crown than on the eastern part. 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. Read the article
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.
Last updated: March 10, 2022