A well-grown fern clump is one of the most generous things in a shady British garden. Give it a moist, humus-rich corner and it will reward you for years, knitting together into a dense shuttlecock of fronds that hides the bare soil beneath. But ferns are not immortal, and they are not endlessly patient. Left undisturbed for five, eight, ten years, most clumps begin to tell you they are crowded. The crown lifts above the surrounding soil, the middle of the clump goes hollow and woody, and the fronds that emerge each spring are noticeably smaller and shorter than they used to be. Flowering plants sulk in similar ways, but ferns are slower to show it, which is why so many gardeners only notice the problem when a favourite Dryopteris has shrunk to half its former glory.
Dividing solves this neatly. You cut an exhausted crown back into vigorous pieces, each with its own roots and growing points, and you replant them into refreshed soil. The original plant recovers its energy and you gain extra plants for the price of a little effort. Early spring is the moment to do it.
The window is narrow but forgiving. In most of the UK, mid-March to mid-April is ideal, though in mild coastal gardens you can start in late February and in colder inland spots you may need to wait until the first week of April. What you are looking for is the moment when the old, tired fronds of an evergreen fern have weathered the winter and can be cut away, but the new season's croziers — those tight, fiddly fiddleheads — are still furled below the surface or just breaking through.
Divide too early and the crown sits in cold, wet soil with no active roots to anchor it. Divide too late, once the croziers have opened, and you will break them, bruise the growing tips and set the plant back badly. A day or two of dry weather before you start is helpful, but avoid working on ground that is still sodden from winter rain.
Not every fern appreciates being carved up, so choose your patients with care.
Ferns have surprisingly tough crowns, particularly older specimens. You will want a garden fork, a spade, strong secateurs, a serrated bread knife or a sharp pruning knife for woody rhizomes, and a clean, flat surface such as a board or an old compost sack to work on. A trug lined with damp hessian or a bucket of water keeps the exposed roots alive while you work — this is the single most important detail in the whole process.
Prepare the new planting positions before you lift anything. Work in two or three handfuls of leafmould or composted bark per square metre, plus a little garden compost, and fork it into the top 20cm. Ferns want moisture and humus, not richness; skip the manure.
Water the parent plant the day before if the soil is at all dry, then cut away last year's fronds to within a few centimetres of the crown so you can see what you are doing. Insert your fork a full spade's width from the crown and lever gently all the way round, easing the rootball free rather than prising it up in one go. Lift it onto your board.
Now examine the crown. You will usually see obvious natural divisions — younger offsets around the rim and an older, exhausted middle. Cut down cleanly between them with the knife, following the line of least resistance rather than forcing a straight cut. Each new piece should have at least two or three growing points and a decent fan of roots; anything smaller is slow to establish. Aim for portions roughly the size of a dinner plate. Discard the woody centre on the compost heap.
Work briskly. As each division is separated, lay its roots in damp hessian or stand it briefly in the bucket. Roots that dry for even ten minutes will produce scorched, brown fronds later in the season — an unmistakable sign that the job was done in too much of a hurry.
Replant at the same depth the fern was growing before, with the crown sitting just at soil level, never buried. Spread the roots out naturally, firm the soil with your knuckles rather than your heel, and water thoroughly to settle everything in. Space divisions 30–45cm apart unless you want a colony to knit together quickly.
For the first season, water regularly through dry spells right into autumn, and mulch with leafmould to hold moisture. Slugs and snails will find the soft new growth irresistible, so set your usual defences. If a sharp frost is forecast, throw a little fleece over the new plants for a night or two. Do not feed them. Most ferns divided in early spring will produce a full, handsome set of fronds by midsummer, and you will have gained three or four new plants and a renewed centrepiece for the shade border — all from one morning's work.
The Autumn Fern shares practical, down-to-earth guidance on shade gardening, ferns and woodland plants for british gardens for readers across the UK.
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