The famous “Queen of the Night” flower tends to bloom only two nights a year.

Tohono Chul Park is home to the world’s largest collection of the night-blooming cereus cactus, or peniocereus greggii, said Marcia Ring, director of marketing and communications for Tohono Chul Park. The park attracts many visitors hoping to witness the rare spectacle.

Biologist Mark Dimmitt said the exact nights the flowers will bloom is uncertain and unpredictable.

“Nobody knows what triggers the exact bloom,” Dimmitt said. It is hard to predict even a day or so in advanced. It is a desert mystery."

The stems look like lifeless grey sticks the rest of the year. “Then, out of it comes this two inch beautiful white flower that blooms a couple of nights a year," Dimmitt said.

The main bloom night can take place between late May and early July. On those same nights, plants in the area will bloom as well.

A few decades ago, the curator the park invited a few friends out to witness the blooming of the cereus cactus. Ring said that, since then, the park now gets between one and two thousand people to see this plant in what they call “night bloom.”

“For plant lovers, this is a summer blockbuster,” Ring said. “It’s become a special treat for Tucsonans.”

People interested in watching for the famous “Queen of the Night” can visit [www.bloomwatch.org] (www.bloomwatch.org) for more information.

Ashley Grove is an intern for Arizona Public Media and a journalism student at the University of Arizona