Stardew Valley – Can ‘The Beloved Farmer’ Achievement Be Obtained After Marriage?

It takes two days to upgrade the watering can. Since you can use the watering can both on the day you take it into the shop and the day you pick it up, there's normally only one day that you won't be able to water your plants. So you want to ensure you don't need to water your plants that day. There are a few easy ways to ensure this without having to resort installing a bunch of sprinklers.

• Upgrade the watering can on a day when the weather forecast for the next day is for rain
• Upgrade it on Spring 27, Fall 27 or Winter 27. You don't need to water your plants on the 28th day of these months because none of your plants will survive the change in seasons. Summer is an exception because corn will survive into Fall.
• Upgrade it on any day in the Winter (except Winter 28) if you're not growing one of the few crops, all Winter forage, that can grow then.

Finally you should also ensure that you'll be able to pick up the watering can on day the it will be available. Clint's store is closed or otherwise inaccessible on most festival days.

I should also I point out I discovered the last problem the hard way. As it turns it out not watering your crops one day isn't the end of the world. None of them died, it just delayed their growth by one day.

I believe you can only reach (and subsequently see) the 10-heart cut-scenes if you give each marriage candidate a bouquet. Here's a quote from the developer on a thread from Reddit:

wait, you can get to 10 hearts without giving a bouquet? That was the whole reason for the bouquet mechanic... to prevent players from seeing the 10-heart event unless they show their romantic interest in someone. whoops

From his comment at the end, it sounds like it might be possible (via a bug) to see those cut-scenes without the bouquet, but that will likely be fixed in a future update if history serves.

Update: Here's another comment from the developer on this (again, from Reddit):

I think the ability to non-romantically befriend the marriage candidates to 10 is a great idea, the only thing stopping me from doing it is the large amount of work involved. It might happen some day though. Now if only I could get my hands on Maru's cloning device...

Update 2: I spotted this in the change log for the upcoming 1.07 patch (available April 4, 2016):

The last two hearts are now greyed-out for marriage candidates until you give them the bouquet.

That confirms to me that hearts 9 & 10 are indeed unreachable without a bouquet.