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Dude, add more soil, mulch and shit around it (without necessarily like heaping soil against the stem), give it a bit extra sugars/carbs, calcium, phosphorus, vitamin E.
No jokes, it keeps the roots quite a bit warmer (like even just for water retention, other than extra insulation, which might seem counterintuitive, but aids thermal regulation), phosphorus and sugar for heat/internal energy. Calcium to "buffer" things and aid phosphorus and other uptake. Vitamin E for cold resistance.
I've grown several hundred plants through winter, sometimes as cold as -6c (still warmer than some other places).
Cheers, and good luck. I'm currently doing winter grow (germinated seeds at worst time possible, start of winter).