Even cacti are stressed by Phoenix’s extended extreme heat

Phoenix is poised this week to climb back up to dangerously high temperatures

After recording the warmest monthly average temperature for any U.S. city ever in July, Phoenix is poised to climb back up to dangerously high temperatures this week. That could mean trouble not just for people but for some plants, too.

Residents across the sprawling metro are finding the extended extreme heat has led to fried flora, and have shared photos and video of their damaged cacti with the Desert Botanical Garden. Nurseries and landscapers are inundated with requests for help with saguaros or fruit trees that are losing leaves.

Phones have been “ringing nonstop” about everything from a cactus to a citrus tree or ficus, said Sophia Booth, a landscape designer at Moon Valley Nursery, which has nearly a dozen locations across the Phoenix suburbs.

“A lot of people are calling and saying their cactus is yellowing really hard, fell over or like broken arms, that sort of thing,” Booth said. “Twenty-year-old trees are losing all their leaves, or they’re turning a crisp brown.”

She advises people to give water and specialty fertilizer to a distressed tree or plant every other day and not to trim them.

At the Desert Botanical Garden, three of the treasured institution's more than 1,000 saguaro cacti have toppled over or lost an arm in the last week, a rate that officials there say is highly unusual.

These saguaros, a towering trademark of the Sonoran Desert landscape, were already stressed from record-breaking heat three years ago, and this summer's historic heat — the average temperature in Phoenix last month was 102.7 degrees Fahrenheit (39.3 degrees Celsius) — turned out to be the cactus needle that broke the camel's back.

“Since 2020, we have had elevated mortality in our population of saguaros compared to mortality rates pre-2020,” said Kimberlie McCue, the garden's chief science officer. “So part of our thinking is that there are still saguaros today that were compromised from what they went through in 2020. And that this could be sending them over the edge.”

Saguaros can live up to 200 years and grow as tall as 40 feet (12 meters). Some in the Desert Botanical Garden date beyond its opening 85 years ago, and the largest there measure almost 30 feet (9 meters), according to McCue.

People commonly assume that cacti are made to endure scorching heat, but even they can have their limits, McCue said. It wasn't just this summer's 31-day streak of highs at or above 110 degrees Fahrenheit (43.3 Celsius), but also the multiple nights when the low never dipped below 90 degrees (32.2 Celsius). Nighttime is when cacti open their pores to get rid of retained water and take in carbon dioxide, she explained.

“With water loss, if they become dehydrated, that can compromise the structural integrity that they have in their tissues," McCue said.

A cactus' size can also influence its susceptibility, said Kevin Hultine, the garden's director of research, and bigger plants with more mass are more prone to the effects of heat and drought.

“Larger (and older) plants have more arms and thus, they tend to be the first to start to lose structural integrity,” Hultine said via email. “The first sign of heat-related stress in a population are arms falling from large plants. Eventually, the entire plant might fall over from the stress.”

There is hope that the arrival of thunderstorms during the delayed monsoon season, which traditionally starts June 15, could bring moisture that will help struggling flora.

In the southern Arizona city of Tucson, which has already seen some monsoon activity, the outdoor living Sonoran Desert Museum isn't running into the same problems with its succulents, McCue said.

“We have the double whammy of this heat dome that seems to have decided to sit over Phoenix. And we’re also this massively spread out space with highways and parking lots,” McCue said. However, “the story isn’t complete yet.”

Booth, of Moon Valley Nurseries, agreed that rain could still keep some plants and trees from reaching the point of no return. In the meantime, staffers at the nursery are preparing for temperatures to soar again this week.

“We do take a lot of precautions, especially to our planters and people that don't just work in the office,” Booth said. “Our yard crew, they're in long sleeves. They have their straw hats on. We make sure we have bottled water in the fridge at all times. We haven't had any heat exhaustion yet out of this (location).”

As of Wednesday, there was no rain in the forecast anytime soon according to the National Weather Service. After two days of a slight drop, high temperatures are expected to be 110 degrees (43.3 Celsius) or more for the next 10 days.

There has been some monsoonal activity in southern and northern Arizona, but Phoenix is “stuck in the middle,” meteorologist Matt Salerno said.

“There’s still hope maybe the middle of this month the monsoon will become more active again,” Salerno said.

There will likely be some record-breaking before then, however. The Weather Service plans to issue an extreme heat warning Friday through Monday, when the highs will be between 111 (43.9 Celsius) and 117 (47.2 Celsius).

In the meantime, the Desert Botanical Garden has been working to propagate cacti that seem better able to endure searing conditions after staffers noticed the 2020 heat was more difficult for some plants than others. Some just seemed to have a genetic makeup that allowed them to thrive.

“We want to try and capture that and grow more saguaros from seed here to add into our population at the garden with the idea that over time, that is going to bring more resiliency into into our population here," McCue said.