Saguaro cactus

Flowers are white and reach diameter of 8 cm (3 in).

An excerpt from the article 19 facts about Saguaro cactus
They bloom since April to late June, opening after sunset till mid afternoon next day. Flowers form on the top of a plant as well as on tips of its branches. They are being pollinated by bats and moths at night and during the day by bees and birds.