A Boston fern is one of the most generous houseplants you can bring into a cosy home. Given the right spot, Nephrolepis exaltata 'Bostoniensis' will throw out long, arching fronds for years, softening a windowsill better than almost anything else. But it is also a plant that is easy to buy badly. A fern that looks lush on the shelf can be hiding a soggy rootball, a pest problem or a crown that is already rotting. Here is what to check before you hand over your money.

Start With the Roots, Not the Fronds

The foliage is what catches your eye, but the roots tell you far more. Ask if you can ease the plant out of its pot — a good garden centre will not mind. Tip the pot on its side, support the stem with one hand and squeeze the sides gently. A healthy rootball holds together but is not a solid brick.

What you want to see is pale cream to tan roots, firm to the touch and faintly fuzzy with fine feeding hairs. What you want to avoid is dark brown or black roots that feel slimy, snap hollowly, or smell of decay. That is root rot, and it rarely improves once a plant is stressed by the move home.

• A few roots poking through the drainage holes are perfectly normal.
• A dense mat of roots circling the inside of the pot means the fern has been pot-bound for a while.
• Roots across the compost surface suggest it has outgrown its pot entirely.
• A pot that feels unusually heavy for its size usually means waterlogged compost.

Reading the Leaves: Colour, Texture and Warning Signs

A healthy Boston fern is a deep, even green, with fronds that arch outward under their own weight rather than drooping limply. Look for new croziers — the tight, fiddlehead curls unfurling from the centre. Active growth is the single best sign that a plant is happy where it is.

Now look more critically. Yellowing between the veins points to a nutrient shortage; overall pale or washed-out green often means too much direct sun. Crisp brown tips and papery margins are the classic complaint, usually caused by dry air, erratic watering or a build-up of salts in the compost — central heating makes this worse in winter.

Sparse, thin, stretched fronds tell you the fern has been sitting in poor light for weeks. It may recover, but it will take months to look decent again. A handful of brown older fronds at the base is normal, by contrast — ferns shed their oldest growth as new fronds come through.

The Pot, the Compost and the Drainage Test

Never buy a fern in a pot without drainage holes. It is the fastest route to root rot, however careful you are with watering. Check the pot itself for cracks, chips and hairline splits, and be wary of plants sitting inside a decorative outer sleeve where water can collect unseen.

Press a finger into the compost. It should be barely moist, springy and smell faintly earthy, like woodland floor. A sour or mouldy smell means the compost has been wet for too long. If the compost has shrunk away from the sides of the pot and feels bone dry, it has been left to dry out badly and may now repel water rather than absorb it.

• Green algae or a white crust of salts on the surface suggests the plant has sat in the same compost for months.
• Small black flies rising as you move the pot are fungus gnats, a sign of constantly wet compost.
• A tiny pot for a large fern means the plant is likely to need repotting almost immediately.

Pests, Crown Rot and Other Hidden Problems

Turn a few fronds over and inspect the undersides, and look closely at the crown — the point where all the fronds meet at compost level. That crown should be firm, pale and relatively dry. If it feels soft, looks brown and dark, or smells sour, walk away. Crown rot is very difficult to reverse.

Watch for white cottony blobs in the leaf joints, which are mealybugs, and small brown waxy bumps on the stems, which are scale. Fine webbing with pale speckling across the upper leaf surface suggests spider mites, which thrive in hot, dry air. Finally, glance at the ferns either side of the one you are considering. If the plants around it look sickly, pests and disease will already have spread through the batch.

Signs of a Robust Fern — and the First Week at Home

The best plant in the row is not always the biggest. Choose the one that is dense and symmetrical, with more new fronds than old, no bare patch in the centre, firm pale roots, compost that is damp rather than soaked, and no sign of pests.

Once it is home, keep it away from your other houseplants for a fortnight so any hidden problem has time to show itself. Place it in bright, indirect light — an east- or north-facing windowsill suits it well — and keep it clear of radiators, draughts and cold glass in winter.

Ferns adore humidity, so a bathroom or kitchen is ideal, or stand the pot on a tray of damp pebbles. Water when the top couple of centimetres of compost feel dry, using lukewarm rainwater where you can, as Boston ferns dislike hard tap water. Hold off feeding for the first month, then use a diluted houseplant feed fortnightly through spring and summer. Repot only if it is genuinely pot-bound, in spring, into a container just one size larger.