The instructions I have read for refrigerator stratification say to put them into slightly moist sterile potting medium (probably perlite or vermiculite), so if you do it outdoors over winter, I'd say water once or twice and then let your rain and snow take over. Either should work, but be aware that iris seeds can be slow to germinate, sometimes finally sprouting two or three years after planting. So don't discard pots just because they don't germinate the first spring.
