There have been some discussions on the forum about interfacing with DC valves. For example, take a look at this thread:
viewtopic.php?f=9&t=214&hilit=dc+relay
Pretty much the only change is to replace triacs (i.e. AC transistors) with transistors like 2N3904 or mosfets like BS170. The selection has to depend on the current rating of your DC valves. The issue with triac is that it needs a zero-crossing in the voltage to turn off, so it’s only suitable for AC power. Since DC power does not have zero-crossing, once turned on, the triac will not turn off until you cut off power. The thread above also mentioned flyback diodes. From OpenSprinkler 2.1, there are either MOVs or TVS on the board, which can act for flyback voltage protection, so you don’t need to add separate flyback diodes any more.