Can I extend #12 wire with #14 wire for landscape lighting?
It would depend on the transformer size existing distance of the 12 AWG and how many lights were already installed. As a great rule of thumb, never mix wire sizes. Voltage drop on landscaping lights is very important because the voltage is low to begin with. Adding 14AWG at the end of the circuit would not be good for that last light. Think of the future, you might want to add more lights. Don't limit a new installation.