Saguaros, which usually burst into brilliant bloom in late spring and early summer, don’t always stick to that schedule. Hikers and saguaro researchers have found a remarkable number of the big cacti blooming in mid-November this year.

“I just observed a patch of saguaros last week that are presently blooming out in the Ironwood Forest National Monument,” said Bill Peachey, a Tucson-area geologist and longtime saguaro researcher. “Several of them — five or six cacti — had single to multiple stems with both buds and flowers.”

Don Swann, a biologist at Saguaro National Park, said he’s seen an unusual number of saguaros blooming this month at the park’s unit west of Tucson.

“I didn’t count, but just a rough guess was that maybe one out of every 20 mature saguaros had flowers or evidence that there had been a flower,” Swann said. “While we sometimes see them flowering at non-traditional times, this seems like more than normal.”

Other saguaros with blooms were spotted by an Arizona Daily Star reporter in the Catalina Foothills on Sunday.

What’s causing this unusual bloom?

“It’s probably some kind of weather event that contributed to this,” Swann said. “But we haven’t had time to look at it systematically for more information.”

Peachey, who has monitored a saguaro observation plot southeast of Tucson for nearly two decades, said his observations and record-keeping have led him to consider a somewhat complex explanation for late-season saguaro blooms.

“I found a characteristic that I’ve seen multiple times following the normal bloom time during years when there have been damaging freeze events earlier in that same year,” Peachey said, noting that such freezes could inhibit the saguaro’s ability to bloom.

Then, later in the year, Peachey said, warmer-than-normal nights and days might have told the controlling cells at the stem tips to stimulate the very small portion of the population that had not bloomed during the regular season to bloom now.

Mark Dimmitt, a retired director of natural history at the Arizona-Sonora Desert Museum and a saguaro expert, noted that the giant cacti have been known to bloom, sporadically, in practically every month of the year.

“The last time I heard, they’ve been seen in every month except February,” Dimmitt said, but saguaros might have bloomed in that month as well in recent years.

Contact reporter Doug Kreutz at dkreutz@tucson.com or at 573-4192. On Twitter: @DouglasKreutz