Boston fern

Identification and origin

Nephrolepis exaltata is a terrestrial or epiphytic fern, placed in the family Polypodiaceae in the broad sense; other classifications put it in a family of its own, Nephrolepidaceae. It is native to tropical and subtropical America: Florida, the Gulf Coast west to Texas, Mexico, Central America, the Caribbean and Bermuda, Colombia, Ecuador, Bolivia and Brazil, as well as Hawaii. It has naturalized elsewhere, for example in the Azores, the Canary Islands, Portugal, Morocco and Australia, and it can become invasive in warm climates. Nephrolepis comes from the Greek nephros, “kidney,” and lepis, “scale”; exaltata means “very tall.”

Its sword-shaped fronds, divided into many alternate leaflets (pinnae) with slightly toothed edges, grow upright at first and arch as they age. In a pot the plant reaches 2–3 ft tall and as wide; in the wild it can grow to about 7 ft. Fronds reach 4 ft long and 6 in wide, sometimes more in ‘Bostoniensis’. The plant also sends out thin runners that root where they touch the ground. ‘Bostoniensis’, the true Boston fern, is a natural variant found in 1894 in a large shipment of ferns sent from Philadelphia to Boston; the species has produced many other cultivars with curly, wavy or finely cut fronds (‘Fluffy Ruffles’, ‘Rooseveltii’, ‘Whitmanii’, ‘Dallas’ and more). The dark brown to black dots lined up under the leaflets are sori, which produce spores: they are neither a disease nor insects.

Light

It tolerates more light than most ferns and grows best in bright, indirect light: near an east window, or behind a sheer curtain in a south or west window, out of direct sun. A yellowish-green, washed-out look means too much light; it cannot take hot summer sun. In light that is too low, on the other hand, it keeps shedding leaflets. It tolerates some shade but will be less full there.

Watering

Keep the mix evenly moist at all times without making it soggy: always water thoroughly so the whole root ball is moist, and never let it dry out, or the fern will drop its fronds. From fall to late winter, cut back only slightly to avoid root rot, without letting it go dry. In hot weather, especially outdoors or in small containers, the root ball dries quickly, so check it often. A useful trick is double potting: set the pot inside a larger container lined with moist sphagnum moss, which holds moisture and humidity around the plant.

Air humidity matters as much as watering. Above 70 °F, stand the pot on a tray of damp pebbles and mist regularly; a bright bathroom or kitchen suits it well. In air that is too dry, the leaflets turn yellow and the tips go brown.

Potting mix

A rich, peat- or soil-based mix that drains well but holds plenty of moisture: for example a peat-based potting mix, a blend of peat and vermiculite, or half soil-based mix and half leaf mold. During active growth, a liquid houseplant fertilizer at half strength every four to six weeks is enough, and much less in winter. Too much fertilizer makes the frond ends brown and dry out.

Temperature

It is happiest between 61–70 °F, with nights around 64 °F and days no hotter than 95 °F. Indoors, do not let it drop below 50 °F, and keep it away from cold air currents. If it spends the summer outside in shade or part shade, bring it in or protect it once nights approach 41 °F. It stays in the ground year-round only in USDA zones 10a to 13b.

Repotting

When roots fill the pot, repot in spring into a pot just one size larger. Another way to limit its size: take the plant out in spring, trim away some of the outer roots and put it back in the same cleaned pot with fresh mix. A fern that looks sparse in late winter also benefits from fresh mix: keep the same pot, or go smaller, unless the root ball is packed with healthy roots. Every few years, a spring repotting is also the chance to divide it.

Pruning

Remove old or browning fronds at soil level as they appear; this encourages new fronds. Cut out right away any long, less divided fronds that occasionally develop, or they will eventually take over the plant. In spring, a tired fern can be cut back to 2–3 in, provided it can then go outdoors into bright light without direct sun. Likewise, a plant that has dropped its fronds after drying out can be cut back to about 2 in to regrow.

Propagation

Division is the simplest method: in spring, cut the root ball vertically into halves or quarters with a sharp knife, keeping as many fronds as possible on each piece. The runners also carry young plants: remove them, plant them in a pot of about 3 in, and care for them like a mature plant. Ferns can be grown from spores, but named cultivars do not come true that way.

Common problems

One symptom can have several causes: check light, air humidity, watering and temperature before acting.

• Brown tips, sparse growth in winter: air too dry, root ball allowed to dry too much between waterings, cold air currents or chilly rooms, excess fertilizer.
• Leaflet drop: not enough light, or a root ball that has dried out.
• Yellow leaflets: air too dry.
• Yellowish-green, washed-out foliage: too much light or direct sun.
• Scale and mealybugs: mostly indoors; the easiest fix is to cut off affected fronds.
• Spider mites: encouraged by dry air.
• Whiteflies and caterpillars; slugs on plants kept outdoors.
• Root rot: too much water, especially in winter.
• Regular brown dots under the leaflets: these are sori and perfectly normal.

Toxicity

No toxicity reported: the Boston fern is listed as non-toxic to dogs and cats, as well as horses.