The relay used on the board is a very small device. It would be better to make this little relay drive a bigger relay (yes I know it is cumbersome) and have that drive the solenoid.
There are two reasons for saying this.
1). the little on-board relay might do the job for a longer or shorter while, maybe forever. However if it does fail then you just lost your master valve so the entire irrigation system would stop.
2). It is easy to replace components that are not on the board. If the on-board relay fails then you will have to either replace the entire board or do some soldering iron surgery.