Answer: This could be from the "shock" of the different conditions from outdoors to indoors. Your plant may just be going through an adjustment period. Yellowing leaves can also be caused by overwatering, underwatering, salt buildup in the soil, and lack of nitrogen, which produces lush, green growth. I've suggested some tips for caring for Boston ferns below. Try that for a couple of weeks and if you see no improvement, apply a houseplant fertilizer. It's best not to fertilize a plant and force it to grow when it's under stress. I hope this info helps!
Boston ferns grow best with constant moisture. Keep the soil moist, but not wet. They also thrive with humidity. The leaves will look best if you mist them daily. If your tap water contains alot of minerals/salts, use distilled water. If you can move your plant away from heat sources (stoves, baseboard heaters or radiators) that also helps. Or, you can put your plant on a tray filled with pebbles and keep water in the tray. As it evaporates it will increase the humidity in the immediate vicinity of the plant.
Flush the pot with water every few months to leach salts building up in the soil. Boston ferns need more light than most ferns. They do best in a south-facing window behind a thin curtain or in an east-facing window with direct morning sun.
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