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:
- 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
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.
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.
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
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.
- 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.
- Place a disc of newspaper on the compost.
- Pour boiling water gently onto the disc until the water coming from the bottom of the pot is very hot.
- Cover the pot with a piece of glass or plastic, or aluminuim foil and leave until it is cold.
- 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.