Barrel cactus (Ferocactus wislizenii) buds typically start to bloom in April with a bright yellow or orange flower. Pink and red varieties also exist but occur less frequently. The flowers only appear on the very top of the plant. As the flowers begin to wilt in early May, they may change color. A late summer desert rainstorm can produce a late bloomer as shown in this photo of the red-flowered variety in Saguaro NP, following a late September Monsoon.
EF 100 macro; f/18 @ 1/125, +0.33 EV, ISO 250