This is normal and a response to the change in conditions. (Mostly a change in light, being more dim indoors, but also potentially to the lower humidity and temperature differences.) The leaf shedding may continue at a gradual pace for a few weeks after a plant grown outside for the summer was moved indoors, but eventually it should wane and be minimal. Indoor plants do not receive nearly as much light intensity indoors as they did outdoors, unless they are grown under a grow light to supplement (or replace) natural light from a window. Even a sunny window indoors isn't as bright as a fairly shaded location outside, like on a balcony that doesn't receive much direct sun. Adding to this decline in light is the progressive shortening of daylength as we approach winter. As a result, the plant drops some of its older leaves because it won't be making enough energy to support keeping all of its foliage alive and healthy.

To minimize leaf drop and give the plant the best conditions while it's inside for the season, either place it directly in front of a sunny window, or provide supplemental light with a grow light. While the plant might do alright without either light source, it may go dormant and shed many more leaves while it waits out the winter. In that case, regrowth should appear within a few weeks of going back outside in April or so.

Do not fertilize much, if at all, while it's indoors (it won't need the extra nutrients) unless it's receiving very bright light. Even then, nutrient needs will be minimal, and not much fertilizer would be used by the roots, so you don't want to stress the roots with a buildup of unused, excess nutrients. Watering frequency may change due to the drier, dimmer conditions of indoors, so check the plant before assuming it needs water based on the frequency it was getting watered while outside. Feel the potting soil about two inches below the surface, and water only once it becomes somewhat dry to the touch at that depth. Asparagus fern prefers to stay relatively cool while wintering indoors, so keep the pot away from drafty heat sources like an air vent or radiator, which would also dry out the potting mix faster.

Miri

THANK YOU SO MUCH!

You're welcome!