Answer: It's difficult to know how to advise you since you didn't identify the particular cactus you're growing. However, since it is large, I suspect it has been in the pot and in the same spot for quite sometime. Moving a 5' tall cactus each winter isn't something most gardeners look forward to! I would cut back on the watering. In the summertime potted cacti can get along with one deep watering every 10 days. In the winter, their need for water is even less. I'd withhold water during this slow growth period. If there's no natural rainfall, you can water once every 2-3 weeks.
Depending upon the material the pot is made of, the roots may freeze if extremely cold or extended cold weather arrives. Plastic will freeze readily; terra cotta or ceramic will store some heat and won't be so apt to freeze during infrequent cold snaps. If you're concerned, you can insulate the pot by wrapping it with foil-backed wall insulation or even several layers of burlap (4-5" thick). I suspect, though, that the plant has been happily growing in its present location through several winters and it should be just fine this winter, too. If you can identify your cactus, send us another question through this forum and we'll be able to tell you just how low a temperature your particular cactus can tolerate.
Best wishes with your new home and landscape!
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