Good question!
Two things: The base your greenhouse is in also needs a water supply module attached to it (either adjacent, or connected via conduits). Then you need to collect and place ice into the water supply module.
While plants are growing, you'll see the total amount of water in your base slowly getting consumed. I think that greenhouses that are "watered" have a different (darker) soil texture, but that may still be a bit buggy at the moment.