For a Boston fern (Nephrolepsis exaltata "Bostoniensis"), the line between growing happily and being pot-bound is a fine one. The ferns, which can grow outdoors year-round in U.S. Department of Agriculture plant hardiness zones 10 through 12 and are moved indoors for the winter in cooler zones, quickly grow to fill their pots completely with broad, arching fronds and a tangle of roots. The roots of the plant resent disturbance, but the vigorous plant eventually does outgrow its pot and should be repotted, and even divided to avoid frequent transplanting.
Signs of Being Root Bound
Boston ferns are often sold in the spring in plastic hanging baskets, already filling the pot. The plants should not be have roots extending from the drainage holes or pressing against the sides of the pots -- Boston fern roots can crack clay pots or cause plastic pots to bulge. When a Boston fern is pot-bound, the fronds have no space left to expand and the soil is so filled with roots it does not hold moisture when you water the plant. When water pours right through the drain hole as soon as you water the plant, it is a good indication it is time to repot your Boston fern.
Time of Year
Boston ferns, particularly those that live indoors, go dormant over the winter. The best time for transplanting Boston ferns into new pots is in spring, before the plants begin their most active growth period in the summer. Spring repotting is also an opportunity to trim lower fronds that have dried up and died back over the winter. The plants have a tendency to drop leaves when indoors in the dry air, leaving the twiggy centers of the fronds. These stems don't regenerate and should be trimmed off.
Pot Size and Preparation
When a Boston fern fills the pot fully, does not retain water and growth slows over the summer, it is time to repot. If you want the plant to grow larger, select a pot that is no more than 2 inches wider across than the current pot. Ferns prefer shallow pots, so the new pot should be no more than 1 inch deeper than the original. Work your fingers among the fronds at the edge of the pot and turn the fern over to lift off the pot. Watering the plant well the day before can make it easier to remove. Slicing off or breaking the pot is sometimes your only option if the Boston fern is severely root-bound. Fern grower Santa Rosa Tropical recommends just filling in around the compacted root ball with new potting mix to avoid disturbing the roots. The roots will grow into the new soil as summer growth begins.
If you want to have more Boston ferns or want a smaller plant, it is time to divide the root ball. Just as when the plant is transplanted, dividing is best done in late winter to early spring. Once the fern is out of the pot, you can cut it in halves or in quarters with a clean, serrated knife. The divisions should also go in pots no more than 2 inches wider than the diameter of the root ball.