in the fic in question, which i otherwise am quite enjoying:
the isekai protagonist OC is a very minor noble on marginal land trying to keep her peasants from starving in the six year winter immediately following the end of House of the Dragon/Fire and Blood
which: so true bestie! agricultural yield WAS a matter of life and death for premodern peasants, we should have more fantasy fiction about agricultural problems
OC invents clear pane glass. dubious but ok sure its cool
now to be fair, a lesser writer would say "and then we planted wheat and everyone lived happily ever after". but wheat is a bad crop for greenhouse cultivation bc its not very useful in small planted areas. your best greenhouse crop will be greens or similar, for micronutrients.
again, writer has done their research. crop rotation schemes, discussion of preservation techniques prior to canning, rotting manure as a heat source for greenhouse beds, etc. i see we have both read about the paris market gardens
"however!" author says proudly. "do you know what has high caloric yield per unit land, excellent micronutrient characteristics, and tolerates low soil temperatures? the humble potato!" or, "winter apple", "from bear island". you know what sure, isekai protag can have little a seed potato, as a treat
except even day neutral potatoes are not totally indifferent to day length! there's a basic energetic issue at play, which is that high latitude winter sunlight is weak and there's not much of it - winter days are as short as 7-8 hours in much of the northern US and Europe. many plants shut down vegetative growth below 10hr days, regardless of temperature, presumably because vegetative growth requires abundant energy. (i am not a botanist, but, like. thermodynamics.)
EXCEPT. planetos winters are canonically magic, not related to axial tilt. which brings up the possibility that even in winterfell, winter days are canonically twelve hours long. (it would be so easy for GRRM to accidentally fuck this up and talk about the long northern winter nights or whatever; I do not know if he did)
however! if this is true then the northern biomes should be adapted to these conditions. we should see an entire food chain built around evergreens or even lichen. wild mammals gotta eat something. and probably something in there is domesticatable if humans have been living in the temperate regions of Planetos for millennia. there's probably a lot of complexity there about low temperature chemistry making this plant matter not very rich or abundant calorically, but nature abhors waste of solar energy and would probably find a way. I don't know nearly enough about tundra/boreal ecosystems to write this. There's also something there about carrying capacity and human population densities; not sure what without trying to do math about it.
Notably, this doesn't bring us to the equilibrium we see in canon, where peasants are trying to survive on warm season grains for... no reason? I would say this would make the First Men recent settlers without useful foodways, but canon claims they've been there for eight to twelve thousand years! What are they doing?
What about the children of the forest? The wiki says they cultivated fucking oats and barley, which means unless they domesticated something winterhardy and called it oats/barley, they have the same supply problems as the First Men. (Not touching the treatment of indigeneity here. GRRM racist, more at 11)
