How would you farm crops that get their energy through darkness in a post-apocalyptic world ravaged by monsters?

Question

In a post-apocalyptic world, monster-like creatures have taken over most of the planet, and twenty years later, only a few clusters of humanity still remain with somewhat functioning governments, and with "towns," as in places with apartments/buildings that have somewhat functioning water and electricity and other necessities that people live in, that are scattered across what's left of America. In addition, near-total darkness enveloped the planet, blocking out the sun with clouds of darkness. However, through means no one knows how, instead of killing all life on earth, the darkness only made the world in general colder, though not nearly ice age or nuclear winter level cold, just making it so that anywhere between 20-45 degrees Fahrenheit is now the average "warm" temperature throughout the year. It also changed almost every plant on a biological level, so now they convert the magical darkness that's everywhere into energy they can use to live. As a result, the plants can now survive in the much colder temperatures, and so long as they have access to some water and decent soil, unless the species of plant never needed either before the apocalypse happened, can grow without any issues.

The only other problem this world faces are the monster-like creatures that took over most of the planet; however, the remnants of humanity have mostly figured out how to fight them off/protect their own homes, and the monsters only try to attack their "towns" once they gather a large enough force, which usually takes anywhere from a month to two. But if they spot humans outside their "towns" trying to get weapons, reclaim lost territory, or even grow food, they will try and attack them.

Given all of this information, how could humanity effectively set up a system of farms, or any equivalent, centered around these darkness-eating plants in a way that ensures the least likely chance of them being destroyed by monsters?

Answer 1

Hydroponics. You don't even need to illuminate them anymore. Stack several layers in a garage somewhere and the food issue is solved 3 times over.

Answer 2

If temperature and daylight are no longer issues, you can grow anything anywhere, and grow multiple crops per year limited only by growing time.

All the farmland you need can be enclosed within town and city walls.

It's already 20-45F in some places, not sure where you have in mind or how much of a change.

Answer 3

Urban farms in the safe zone

Hydroponics, aeroponics, warehousing, and vertical farming, there are tons of modern ways to farm in a small area. Entire skyscrapers could be converted to huge farms. More exotic solutions like farming mushrooms, seaweed, or algae could also be used. Even animals can be raised in buildings. There's nothing stopping them from farming in parks inside the city too (so long as it can be made safe).

Isolated farms

Farms could be built in isolation, far from the town. They would be manned by a skeleton crew for long periods of time. This would make them less likely to be attacked, and if they are only 1 farm is lost. Periodically a convoy could (riskily) travel to the farm, collect the produce, deliver supplies, swap out staff, then return to the colony.

Thinning the herd

It doesn't seem exactly clear from your description, but if the monsters only attack when the humans build-up, or randomly if they happen to see a human, then that behavior could be exploited. Highly armed small groups of humans could roam a large area, killing individual monsters before they can build up.

Running

Depending on the monster's intelligence, it may not even be necessary to drive them away from the land. Farms and ranches can be huge affairs. If monsters don't hunt animals then cattle could be let to range over a grassland, and harvested when needed. Lookouts can spot monsters in the distance, and when they are seen the humans can run. Similarly, if a monster wanders onto a field, leave the harvest for another day.

Luring

The farms could be built nearby the town, but in a less accessible area. Due to the low density of humans, monsters won't build up and attack it. Instead, they will attack the high-density living areas (which have been suitably fortified).

Hunting and gathering

I'm not sure what's going on with wild plants and animals in your setting, but in the absence of humans, both will be growing out of control. Not enough to support millions, but the comparatively few humans may find hunting and gathering to be productive if they can keep out of view of the monsters. The formerly sparse urban parks may become filled with deer and edible plants, the barren rivers filled with fish.

Specialists

These duties may not be performed by the average person. Special traders may develop, gathering food from hunters, farmers, etc. They can then secretly get to the towns, and sell the food for a premium. Normal humans may be locked in secured buildings, awaiting a sign that a food seller has approached the area with much-needed supplies.

Answer 4

It depends on the location.

Hydroponics can be used anywhere and have a good yield for relatively little space.

More natural options can include securing a mountain valley (helps with natural defenses) such as those found in Swiss or Austria.

Considering the nature of your apocalypse it mostly boils down to everything people do normally with the only exception being that they're limited to what they can realistically defend.