You flat-out can not open a system like you describe for a watering in March. Regretfully inform the fertilization guys that they will have to change their plans.
Only a system with every single pipe protected from cold air can be safely activated ahead of schedule. This lets out any RPZ installed to the factory specs of a foot above grade. They do make "hot boxes" to protect the above-ground backflow preventers, but they would cost thousands to install.
Now, if it happens that the RPZ valves are located below-grade (against most regional codes) in pits, their integrity will depend on the pits themselves. Very deep pits afford better protection than shallow pits. Sight unseen, mid-March is too dangerous a date to open a system in your area, even if it's all below grade. We still can't count on the weather two weeks out.