In full bloom: Tucson's Queen of the Night makes its annual appearance

The night-blooming cereus (variation peniocereus greggii), which normally looks like a pile of dead sticks, put its beautiful white blooms on full display for the night before wilting away with the sun on Monday morning.

The cactus blooms just once each year, usually between late May and July. Tohono Chul, 7366 N. Paseo del Norte, is home to the largest known private collection of night-blooming cereus in the world.

For weeks, gardeners at Tohono Chul kept a close eye on all of the cacti, measuring the buds at least twice a week. They considered when the buds began to grow and how fast they were growing, and then they accounted for a time of stalling — which is when the bud typically pauses its growth for a week or two. When it begins to grow again, it reaches a critical size where it’s “very unlikely to stall anymore and will probably finish their growth,” said Stephen Spikes, Tohono Chul’s interpretive programs manager.

No one knows exactly how the plants decide when to bloom, but it appears to be a regional thing. For example, the night-blooming cereus blooms on the same night at both Tohono Chul and the park’s surrounding neighborhoods. But people who have night-blooming cereus in their own gardens across Tucson might not see it bloom until an entirely different day or week.

Researchers have found that because the plant can’t self-pollinate, it relies on pollinators like the white-lined sphinx moth. The more blooms that are open at the same time, the better chances of reproduction.

“There’s some sort of mechanism that the plants have to use to coordinate their blooming,” Spikes said.