Still careful with her newly applied polish, she hit the green send button twice and called Tohono Chul for the first time that afternoon but for the seventh time that day. She wasn’t surprised when she was put on hold. Abby, of all people, understood what Shirley Folgum was up against. Trying to ride herd on that evening’s party was a complicated proposition.

In Tohono Chul’s annual calendar, the celebration of the night-blooming cereus was an enormous undertaking. On that night alone, as many as two thousand people would show up at the park for the festivities, arriving well after dark and not leaving until early the next morning. All of that would have been complicated enough, if it could have been handled in the established way.

Most big recurring nonprofit-style events come with certain unvarying logistics. Worker bees needed to be organized. Invitations have to be issued. Potential attendees need to be given “Save the date” information. Contracts for entertainment and catering need to be arranged. All of those things held true for the night-blooming cereus party. The big difference-and the big complication-came with the reality that no one ever knew exactly when the party would take place. Not until the very last minute.

Despite years of patient analysis and study by any number of very talented botanists, despite countless computer models examining weather data-daytime temperatures, nighttime temperatures, dew points, barometric pressures, and all points in between-no one had yet been able to crack the code as to when exactly the Queen of the Night would deign to make her annual appearance. Scientific study suggested it would happen sometime between the end of May and the middle of July. As a result of this uncertainty, all preparations had to be ironed out well in advance and then put in abeyance but ready for immediate last-minute execution.

It turned out that was how Abby Tennant herself had stumbled into the event for the first time-at the last minute.

Toward the end of her first June in Tucson, Abby had been dreadfully homesick for her friends and relations back home in Ohio. For one thing, the appalling June heat was nothing short of debilitating. She had almost decided to give up and go back home when a new neighbor, Mildred Harrison, had called.

“There’s going to be a special party at Tohono Chul tomorrow night,” Mildred had said. “Would you like to come along as my guest?”

Abby’s new town house in what was billed as an “active adult community” on Tucson ’s far northwest side was just down the street from the botanical garden. She had driven past the rock wall entrance numerous times, but she hadn’t ever considered stopping in. Somehow she had never guessed that one of the world’s ten best botanical gardens would be right there, hiding out in the middle of Tucson.

W hat interesting plants could possibly grow in the desert? Abby had wondered in all her midwestern arrogance. From what she personally had observed, there seemed to be precious few plants of any kind in this desolate outpost of civilization where, even in May, the heat had been more than Abby could tolerate.

“I suppose they’re holding it at night because it’s too hot to have a garden party during the day,” Abby had groused sarcastically.

Mildred had laughed aloud at that. “It’s a party in honor of the night-blooming cereus,” she explained. “It’s the flower on the deer-horn cactus. We call it the Queen of the Night. Tohono Chul has more than eighty plants that are set to bloom this year, and they all blossom at the same time. They open up around sunset and are gone by sunrise the next morning. Someone called just now to let me know that the bloom will be tomorrow night. Are you coming or not?”

Mildred sometimes reminded Abby of her older sister, Stephanie, who was at times a bit overbearing and more than a little outspoken. On this occasion, Abby had dutifully slipped into full little-sister mode.

“I suppose,” she had agreed reluctantly.

The next day she had tried her best to back out of the engagement, but Mildred wouldn’t hear of it. Around nine o’clock that evening, Abby had ridden over to Tohono Chul’s parking lot in Mildred’s aging Pontiac. Arriving in low spirits and with even lower expectations, Abby was surprised to find the parking lot jammed with cars and parking attendants. Along with hordes of other enthusiastic attendees, Abby and Mildred had walked into the park following footpaths that were lit with candles in small paper bags.

“They’re called luminarias,” Mildred explained. “They’re traditional Mexican.”

Abby was astonished when she saw the throngs of people who were there that night. She kept wondering what all the fuss was about-but only until she saw a night-blooming cereus in the flesh. Once she caught sight of that first lush white blossom, Abby Tennant fell in love.

She couldn’t fathom how such a magnificent white flower could burst forth from what appeared to be a skimpy stick of thorny cactus. She was astonished to find that many of the gorgeous blossoms were as big across as one of Abby’s eight-inch pie plates. They reminded her of her next-door neighbor’s prizewinning dahlias back home in Ohio, but these weren’t dahlias, and the heady perfume that drifted away from each flower on the hot summer air was subtle but elegantly sweet, reminiscent of orange blossoms, but not quite the same.

Abby was dumbstruck. “They’re so beautiful!” she had exclaimed.

“Aren’t they,” Mildred said, nodding in agreement. “And now you know why it’s called the Queen of the Night. By the time the sun comes up tomorrow, the blossoms will be gone.”

Abby Tennant’s first encounter with the night-blooming cereus marked the real beginning of her new life, although her name was still Abby Southard back then. She had been so enchanted by seeing the flowers that she had insisted on taking Mildred to lunch at the Tohono Chul Tea Room the very next week. In the confines of the small cool rooms of what had once been a ranch house, Abby began to see the things about Tucson that she had been missing before-the friendliness of the people, Mildred included, for one thing, and the many subtle beauties of the desert for another.

Abby had taken out her own membership at Tohono Chul only a week or so later. Walking the park’s many manicured paths, she gradually acclimated herself to the heat of her new home. She learned to mark the changing seasons by something other than changing leaves. In spring she saw the profusion of yellow flowers on the prickly pear and the fuchsia-colored blossoms of the barrel cactus. In early summer she came to love the bright yellow blooms standing out against the green branches of the springtime paloverde and the dusky pinks and lavenders on the brooding ironwood. She loved watching the birds, especially the brightly colored hummingbirds that hovered around the equally brightly colored flowers.

Somehow, in the process of exploring this desert oasis, Abby Tennant found peace and came to terms with her new home and her new life. By the time of the first snowfall in Columbus that first year, she was no longer homesick. When Christmas rolled around and her friends were complaining about the weather, Abby took herself back to the park and volunteered her services.

At first she knew so little that all she could do was work as a stocker and a cashier in the museum shop. Later, once she was better adjusted to the climate, she went through docent training so she could lead tours and speak knowledgeably about the native plants of her newly adopted home. Because of her enduring fascination with the night-blooming cereus, it was a natural progression of her volunteerism that she went from leading daytime tours to working on the annual Queen of the Night party.

Initially she served on the Queen of the Night Committee, but when the complexity of the event outstripped the committee’s groupthink capability, Abby had finally given up and taken charge. When she came on board, there had been a complicated phone-tree system for notifying workers and guests of the impending bloom. Under her direction, phone trees had given way to a more streamlined form of e-mail notices. But after five years of running the show, it was time to pass the reins to someone else, and Shirley Folgum was her handpicked successor.