They will eventually start doing it on their own. I had villagers trying to water the field before they had creek water so nothing happened. But they'd only do it once then go off, do something else, come back. Same when the creek was open. I didn't MAKE them do it and I had a lovely field of flowers after about 24 hours of waiting and concentrating on other things. I guess as long as you have the population and food supply up there should be at least one or two who are doing it on their own, maybe when you're not looking
Some of these things will take time. Patience is good with this game! Unless you want to up the speed and make EVERYTHING go faster, then you might see results quicker.