Replace the wire connectors using waterproof connections
The connections at either end of your irrigation wire are as important as the wire itself. The wire inside the controller is lucky. It’s in a nice, clean dry environment, protected from the elements. The connection at the valve solenoid lives a much harsher live. If it is not protected from moisture corrosion will, not might, occur, gradually diminishing conductivity until you wonder why your zone won’t come on.
Many architects design with copper sheeting, roofing and accents because they like the changing looks copper goes through as it weathers. This is the only place that green color is good. Underground it is a big problem. You need to protect all your connections to keep your system working. The best way is to use water proof connectors. A number of types are listed HERE. Do not scrimp on connectors. Don’t use the smallest one that your wire allows. Use one that is oversized slightly. This gives better protection and allows for future changes. There are two main types but both imbed the wire in silicon grease.
The first one does not require a wire nut. The wire is inserted into the holes and the connector is snapped closed. This bends the exposed wire over a common conductor and seals out moisture. Some of these are self stripping; others require the end of the wire to be stripped first.
The second type requires a wire nut or properly crimped wires. Once the wires are connected they are imbedded in the silicon grease compound. The retaining cap is then snapped in place to hold the wires in position.
All of these will provide moisture protection. Some are made for heavier wires, some are self stripping, some come with the wire nuts and some don’t need them. All you need to do is decide which one works best in your situation.