Publications

RHS Journals

The Garden
April 1999

Horticultural Techniques

Frond thoughts

This second extract from Propagating Plants, the comprehensive new RHS manual edited by Alan Toogood, gives details how to increase stocks of ferns by sowing spores and outlines a range of other techniques

Ferns are primitive plants that, lacking flowers, reproduce by spores rather than seeds. Increase from spores is the usual method of propagation when many plants are wanted. However, it is fiddly and not always possible: spores may not form when cultural conditions are less than ideal; some ferns are sterile; and many crested or plumose cultivars do not come true from spores. Many ferns also reproduce by vegetative means, such as rhizomes, bulbils or plantlets.

The fern life cycle (above) has two phases; a sporophyte (spore-bearing) asexual stage, familiar as the fronded plants we grow, and a sexual, gametophyte stage called the prothallus, produced when spores are dispersed from the fern and germinate. It is at this stage that fertilisation takes place, enabled by water, since the male sperm must swim to the female egg; this is why ferns grow in moist places. An embryo develops, then a recognisable fern; when mature this fern will produce spores, continuing the cycle.

Propagation

Collecting spores

Right: 1 Select a frond with ripe sporangia. In this photograph, fronds of brown-spored Adiantum raddianum'Fritz Luthi' are (from left to right) unripe, ripe and over-ripe

Cut off the frond with a clean, sharp knife. Place it in a clean folded sheet of paper or envelope in a warm, dry place for 2-3 days to collect the spores

Spores of most temperate fern species ripen in mid- to late summer; those of many tropical ferns ripen less seasonally through the year. The sori, or spore-bearing bodies, are visible on the underside of the fronds 1. A few ferns, such as Onoclea, produce special spore-bearing fronds. Unripe sori are usually pale green or pale brown, with a granular surface. As sori ripen, their colour darkens and the sporangia within swell and split to shed spores. When just a few of the sori are open and appear shaggy, the frond is ready for propagation.

To collect spores, place a fertile frond, or section of frond, in a clean envelope and keep in a warm, dry atmosphere. Do not use plastic bags: they encourage dampness and moulds. When the spores are released, they have the appearance of fine dust. Before sowing, separate any debris, such as scale remnants or leaf hairs, which can contaminate the spore culture.

Examination with a hand lens will reveal minute particles of uniform size: these are the spores, and the rest is debris. Either use a fine sieve, or tip the dust onto a clean sheet of paper. Hold the paper at an angle of 45 degrees. Debris will travel rapidly down the surface while spores move slowly; with practice the spores can be kept on the paper while the debris falls off.

Contamination with algae, mosses and fungi is a major cause of poor viability and death of prothalli. If you are having problems, try sterilising the spores for 5-10 minutes in a 10 percent solution of sodium hypochlorite (standard household bleach) in distilled water. Drain, rinse in sterile, boiled and cooled water, and dry the spores on filter paper for 24-48 hours.

Green spores, as in Lygodium and Osmunda, have very short viability and must be sown within 48 hours of collection. Only spores that are brown when ripe can be stored; they may remain viable for 3-5 years if properly prepared: to store spores, transfer to a labelled plastic film canister containing a sachet of desiccant; keep in a refrigerator at 4-5°C (39-41°F).

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