BC,

Use a buried pvc line to the bed and a riser to go from pvc to poly with the misters. The size of your pipe depends on the demand of your system.

Do you know what your flow at the faucet is? What is the flow rate for optimum pressure for each mister? What is the pressure loss for each mister? All information needed to design your system. Also what will the spacing of the misters and the number of lines needed? Aloha