OK - so after MONTHS of just assuming Farms and my Riften Garden never respawn - I've discovered that is NOT the case.
There's a VERY wonky bug that's working - and I'm posting this here in hopes that Beth sees it and fixes it as part of a patch 1.10, and to help those who always scratch their heads as to why crops never regrow, why revisiting Giant Camps sometimes never respawns Mammoth Cheese, and why many cave mushrooms seem to disappear when you "reset" the mine by walking in and out.
Copy/Pasting my post in another thread:
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OK -I've RECENTLY discovered what the problem is.
It's a stupid bug... but here's the deal.
When you first harvest a bunch of crops, it records it in the initial cell data for all things... but crops seem to be wonky on it.
Then it respawns in 10-30 days...
...BUT....
If you Exit and re-enter the cell - the crops REVERT back to their original save POSITION! >_<
Seriously, I've walked for MONTHS around never seeing a single crop grow.
Then, one day I saw my Riften garden alive - and I said "WOW, that finally happened!"
Went in to honeyside to drop things off, and went back out to pick them.... they were ALL gone.
That was my clue
Since then, i've discovered that happens with ANY FARM CROP in the world.
Even mushrooms in caves/mines work the same.
Mammoth Cheese in Giant Camps work the same.
Pretty much anything that's "harvestable" - When you first enter the cell, it respawns - if you exit and re-enter it reverts back to the last saved spawn data.
So, to get your crops back in Riften, soon as you walk into Riften - FIRST THING YOU DO IS PICK YOUR CROPS. You will never get a chance to do so again if you enter even a single door within the Riften Cell.
Same with farms. If you see a farm load in off in the distance before you enter that Draugr ruin, you need to high-tail it RIGHT to that farm and pick crops. Otherwise if you go into that door - the crops revert back to the original "save".