Oenothera speciosa is a species of evening primrose known by many common names, such as pink ladies and pink evening primrose, while in Colorado it is generally referred to as Mexican evening primrose. Wikipedia claims the name 'speciosa' means showy.
Being drought resistant, this plant requires very little as far as soil or water, in fact it thrives in rock gardens and extreme sun. Usually planted as a border plant along fences, walkways and rock gardens, this 12-18" tall perennial wildflower can easily become invasive if given optimum growing conditions. I keep it in check by allowing it to live in a 3' X 8' succulent bed, along a brick path.
The Mexican evening primrose produces an abundant amount of delicate pink, cup-shaped flowers from spring through autumn. Blooming both day and night, the flower tends to close during mid-day, when the full sun is directly overhead.
I think it's a lovely accent plant. Perfect in that corner of the yard where the soil is poor, the sprinklers don't quite reach and the sun hits hard.