Boston fern

The Boston fern is a sport of a wild sword fern from the tropical Americas, spotted in a shipment to Boston in the 1890s and grown ever since. In the wild its relatives grow in the damp shade of forest floors and on tree trunks, in air that rarely dries out. That is the whole story of this plant indoors. It asks for very little light and very little feed, and it asks for humidity that a modern heated room does not naturally have. Give it that one thing and it is undemanding; withhold it and no amount of watering will stop the fronds going crisp.

Light

Bright indirect light, and no direct sun. It tolerates a dimmer position than most ferns, which is why it does well in bathrooms and on shaded shelves, but a genuinely dark room makes it thin and sparse over a season. A north window in the northern hemisphere or a south one in the southern is ideal, and a metre back from a brighter window with a sheer curtain works just as well.

Water and humidity

Keep the compost lightly moist at all times. Not wet, and never dry: this is a narrower margin than most houseplants ask for, and it is the reason the fern has a reputation for being difficult. In practice that means watering every three or four days in summer and about weekly in winter, checking with a finger rather than a schedule. Water at the base and let it drain, and never leave the pot standing in a full saucer.

Humidity matters as much as watering. In a heated room the air can be drier than a desert, and a fern responds by browning at the tips and edges of every frond. A bathroom or kitchen suits it. Elsewhere, stand the pot on a tray of wet pebbles, group it with other plants, and keep it well away from radiators and air conditioning vents. Misting helps for an hour and is not a substitute for either.

Hard tap water browns the tips over time. Where the water is hard, rainwater makes a visible difference within a season.

Feeding and repotting

Feed monthly at half strength through spring and summer, and stop entirely in winter. Ferns have fine roots that scorch easily, and a strong feed does more harm than none at all.

Repot in spring when the roots have filled the pot, going up one size only. A pot that is too large holds wet compost the roots cannot reach, which is exactly the condition the plant does not want. Use a compost with plenty of leaf mould or coir so it holds moisture without turning to mud.

Pruning and propagation

Cut out browned fronds at the base whenever you see them; the plant looks better for it and the space encourages new growth from the crown. A fern that has gone tired and brown all over can be cut back to a few centimetres above the compost in spring, and if you keep it warm, humid and just moist it grows a full new head within a couple of months. It is a drastic looking operation and it works.

Divide a mature clump in spring, pulling it apart into two or three pieces each with roots and several fronds. The plant also sends out thin wiry runners, and pegging one down onto a small pot of moist compost gives you a new plant in a few weeks.

Ailments

Crisp brown edges are dry air. Brown tips alone are usually a pot that dried out or hard water. Yellowing fronds with wet compost are the opposite problem. A round of shedding after a move is normal and stops once the plant settles. Spider mites are the pest to watch for, and they arrive precisely when the air is too dry, so the cure for them and for the crisping is the same. The ledger below has the rest.