Ferns: hardy

Low-maintenance and tolerant of a wide range of soil conditions, ferns complement any shade plant combination. From tiny specimens grown in walls to the royal fern at six feet tall, there’s room for ferns in every sized garden.

Tree stumps and ferns at Raveningham Hall Gardens, Norfolk

Tree stumps and ferns at Raveningham Hall Gardens, Norfolk

Quick facts

Common Name Fern
Botanical Name Various
Group Perennial
Planting time Autumn or spring
Height and spread Variable
Aspect Part-shade
Hardiness H4 – hardy
Difficulty Easy

Cultivation notes

Most ferns are easy to grow and will thrive in any moist, well-drained, shady site in well-dug, humus-rich, neutral to alkaline soil. However, those such as the royal fern Osmunda regalis, require neutral to acid soil.

Feeding and watering

Ferns do not usually require feeding when planted in the open garden, but mulches such as well-rotted farmyard manure will condition the soil and give a boost to growth. Where soil conditions are particularly poor a well-balanced fertiliser, such as Growmore or fish, blood and bone can be added in spring.

Water when necessary, but apply to the roots and not directly to the fronds or crown as this can encourage rot.

Containers

Ferns make good companions for other hardy perennials in containers. Use a mix of 3 parts peat-free multipurpose compost, 1 part John Innes No 3, and 1 part gritty sand by volume. When first potting the ferns, incorporate a controlled–release fertiliser. The following year use a general fertiliser such Miracle-Gro during the growing season.

Pruning and training

Little pruning is needed but do remove any dead or unsightly fronds before the new crosiers (unrolling leaf tips) appear. Clear away any debris to encourage good air circulation.

Propagation

Division

The easiest way to propagate hardy ferns is by division. It depends on the growth habit of the fern as to how it will divide: 

Creeping rhizomes

  • In spring, cut the rhizome into segments about 5-8cm long, ensuring that each segment has at least one growth bud and a small root ball
  • Pot up individually into peat-substitute based compost at the same level at which it was growing.  Planting too deep will result in the sections rotting
  • Place the pots in light shade and keep the compost moist

Crown splitting
It can take up to ten years for a genuine ‘split’ or ‘multiple crown’ to develop. Many nursery-raised containerised plants consist of multiple plants that were initially pricked out and potted up in clumps. Tease apart mature crowns with two back-to-back forks and pot up immediately or plant out.

Bulbils

A few ferns, for example some cultivars of the soft shield fern, Polystichum setiferum, develop small bulbils along the midrib of the frond. If such a frond is pegged down onto the soil, the bulbils will eventually root into it and can then be detached and grown on. After six months the plants are ready to be lifted.

A less common hardy fern, Cystopteris bulbifera, bears numerous pea-like bulbils on the undersides of the fronds, which soon drop off, rooting quickly when they reach the ground.

Leaf bases

This process is very effective for propagating sterile cultivars, notably those of Asplenium scolopendrium.

  • Dig up mature plants  and remove the soil prior to gently peeling off the short, dead-looking leaf bases from the older rhizomes
  • Wash and then remove any frond or root remnants
  • Plant 1cm apart in sterilised compost ensuring that the attachment point (the green end) is pointing upwards
  • Place in a new polythene bag, inflate and seal
  • Keep in cool conditions with good indirect sunlight and in 3 to 4 months, young fronds should be visible emerging from the newly developed bulbils
  • Prick out into sterilised pots of compost (see below) and harden off as for spore propagation

Spores

The majority of ferns produce their spores in small heaps or lines on the undersides of the mature fronds. The royal fern, Osmunda regalis, is a notable exception in that it bears its spores on the conspicuous modified ends of the fronds.

The spore heaps (sori) are pale green when unripe and those of most ferns become a deep brown or black colour as they ripen. If they are a pale rusty brown, the spores have probably already fallen. However, as spores ripen in sequence (from tip to stem), it is often possible to find both ripe and unripe spores on a single frond, especially in mid to late summer.

To collect spores, place a small piece of spore-bearing frond in a dry paper envelope and keep for a day or so in a warm, dry place. Any spores present will then have settled in the bottom of the packet as a dust-like brown, yellow or black powder.

How to sterilise soil
Before sowing, it is essential to surface sterilise the compost to kill any stray fungal, moss or fern spores.

  1. Fill a 7.5cm (3in) plastic pot to within 1cm (½in) of the top with John Innes seed compost or a 50/50 mixture of peat and sharp sand.
  2. Place a disc of newspaper on the compost.
  3. Pour boiling water gently onto the disc until the water coming from the bottom of the pot is very hot.
  4. Cover the pot with a piece of glass or plastic, or aluminuim foil and leave until it is cold.
  5. Remove the glass and paper disc when ready to sow.

Sowing the spores

  • Sprinkle the spores very thinly over the soil surface
  • Immediately cover the pot with a piece of clear polythene or cling film, secured in place with string or an elastic band
  • Label the fern pot immediately with a permanent, waterproof marker pen
  • If sowing different batches of spores, sow each pot in a different room in order to avoid cross-contamination of batches
  • Place the pot in a cool, lightly shaded place for a month or two by which time a green film on the soil surface will be visible through the cover. This will gradually form into distinct filmy green structures (the prothalli)
  • After a further month or so the first tiny fronds will appear
  • When 2-3 fronds are visible, prick out the sporelings, in clumps, into pots of sterilised potting compost
  • Maintain a humid atmosphere by enclosing the pot in a polythene bag in indirect light for a few days, as exposure to dry air can be fatal

Midsummer sowings may well produce true fern fronds by winter, but autumn sowings may not until the following spring. In 1-2 years the young ferns will be large enough to be planted out in the garden.

Cultivar Selection

*Denotes native to the British Isles

Deciduous ferns for damp situation
Adiantum capillus-veneris*
Adiantum aleuticum AGM
Athyrium filix-femina* AGM
Cystopteris bulbifera
Cystopteris fragilis*
Dryopteris affinis*
Dryopteris dilatata* AGM
Dryopteris filix-mas* AGM
Matteuccia struthiopteris AGM
Onoclea sensibilis AGM
Osmunda regalis* AGM

Evergreen ferns for damp situations
Adiantum venustum AGM
Asplenium scolopendrium* AGM and cultivars
Asplenium trichomanes AGM*
Polypodium vulgare*
Polystichum setiferum AGM and cultivars*

Dry shade
Hardy species of the following genera are tolerant of dry shade, but will need regular watering in their first season and would appreciate a mulch of leaf mould, bark or garden compost:
Asplenium
Dryopteris
Polypodium

Very wet sites
Athyrium filix-femina* AGM (and its cultivars)
Matteuccia struthiopteris AGM
Onoclea sensibilis AGM
Osmunda regalis* AGM

Ferns with Awards of Garden Merit
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