Because of shorter day length, most houseplants, even those which don’t lose their leaves, will not be actively growing now, even citrus with ripening fruit. So this is the time to re-pot if it’s been a couple of years since the last time. Move your plant into a slightly larger container; or trim roots ½- 2” around the sides, 1-4” across the bottom, and put back in the same container with fresh potting mix to maintain size (the larger amount to trim is for a 20” diameter container, or larger, the smaller amount is for a 6-8” diameter container).
After re-potting, your indoor plants will also benefit from pruning and shaping; delay pruning citrus branches which still have some fruit on them until after harvest. Pruning potted fruit trees or shrubs is similar to pruning plants in the ground; make heading cuts where you want more branching, thin out branches that are getting too crowded. Citrus can be maintained in an open center or central leader tree form and be sure to remove rootstock suckers. Pineapple guava and pomegranate are multi-stemmed shrubs, both benefit from thinning out older shoots to make room for new ones. Pomegranate bears fruit on new growth, and is best pruned just before new growth begins; pineapple guava bears fruit on last year’s wood.
Lemon trees may start to look a little ragged in January. So long as they are not over-watered, they will burst out with new growth next month and replace the previous year’s old tired leaves. Keep plants away from windows that can be rather chilly over night in colder climates. Provide supplemental lighting if plants are looking leggy or more yellow. Begin feeding when new growth starts to show.