This is showy milkweed, one of the larval host plants of the Monarch. The scientific name for this particular variety is Asclepias speciosa. They call it milkweed because it bleeds white when you pick a leaf off a stem. Showy milkweed can be found in agricultural fields, agricultural canals, and in isolated spots in mountain canyons.  

Because Monarch females prefer to roost in trees during the late morning and afternoons of hot summer days, when they do fly, they tend to prefer to lay their eggs on milkweeds growing near or under the trees where they roost. More.