I have verbal agreements for locations and scheduling all planned for my short film, but when do I need for the property owners to sign the location contract? At the time of the shoot or before?

Before you spend money. If you all turn up, they see tons of people and equipment, they might say no!

When they say OK you can use the property or as soon as possible after. Why wait until the day of the shoot? The might have forgotten or like said...they chicken out when they see a crew and gear arrive.