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?