The solar powered part is going to complicate things. If that is indeed a must, then you will likely have to go the DIY route if you also want it to support ZigBee or Z-Wave. You would need something like what @rcjordan suggested, plus a solar generator, and a compatible solar panel. The generator would need enough battery life to prevent the relay from losing connection.
You might be able to convert an out of the box solution as well by splicing in the relay, but it will likely lose power and connection frequently due to either the battery or if there is a light sensor control. Also, you will need to make sure the voltage of the string matches the relay.
For mine, I just bought a cheap AC powered warm white set from Walmart and am using a ZigBee outlet to control it. Cheap and simple.
There is a zigbee 'switch' (surface-mounted actuator) made my Third Reality that is battery-driven, so it would not be dependent upon the solar panel for power. It'd be a DIY, and require wiring a paddle switch into the solar output, then overlaying the paddle with the Third Reality.