2. Choose fully-expanded, fertile (i.e. spore-producing) fronds from a mature plant.
• Check that the fern is growing wild. This key will not identify foreign species that have been planted.
• Avoid fronds that are sterile (i.e. are not producing spores), damaged or are smaller than the majority of fronds present. With practice, ferns can often be identified from sterile fronds, but beginners will find this more difficult.
• If a frond is removed for examination, it is important to detach it at the base so that the whole of the stipe is present.
Note 1: Whether in ferns, flowers or humans, every species is variable; not all individuals within a species are identical in appearance. Some species are more variable than others. Where the species is distinct from all others, this does not interfere with accurate identification. For example, fronds of Bracken (Pteridium aquilinum) vary considerably in the degree of division of the fronds, but are readily distinguished from all other species. However, it can cause difficulties when separating closely related species with similarities in appearance, such as Hard Shield Fern (Polystichum aculeatum) and Soft Shield Fern (P. setiferum). In addition, there are occasional ‘freak’ individuals that fall outside the normal range of variation for the species. It is difficult to allow for all variants in a simple key. This key is based on ‘typical’ individuals for each species (i.e. the majority that are close to the middle of the range of variation); it may not work so well for the occasional extreme variants.
Note 2: In many species, sterile fronds differ slightly in morphology, usually by being ‘leafier’ with broader subdivisions, from fertile fronds. In a few species, the differences between the two types of frond are so conspicuous that the fern is termed ‘dimorphic’. Among native species, this is most marked in Hard Fern (Blechnum spicant), Parsley Fern (Cryptogramma crispa), and Marsh Fern (Thelypteris palustris). Among naturalised aliens, dimorphy is a striking feature of Ostrich Fern (Matteuccia struthiopteris) and Sensitive Fern, (Onoclea sensibilis), in which the fertile fronds become brown and woody. In all these species, the fertile fronds are not only more finely divided but also more upright than the sterile fronds. There is also a discernible interval between the flushing of sterile fronds and the later first appearance of fertile fronds. As a consequence, fertile fronds may not be present in, for example, Marsh Fern and Ostrich Fern, early in the summer even though some sterile fronds are fully expanded.
Note 3: Young plants and individuals stunted by crowding or adverse environmental conditions may have atypically small or distorted fronds. Cultivated examples, and individuals in unusually sheltered or shady conditions, are often atypically large.
Note 4: Immature plants not only lack the diagnostic characters of the fertile fronds but, when very young, have juvenile foliage that is different, often very different, morphologically from that of older plants. The first few fronds of a sporeling are not miniature versions of those produced by adults. No identification key to juvenile foliage has yet been produced.
Note 5: A few species, especially Athyrium distentifolium, Blechnum spicant, Ophioglossum vulgatum, Pteridium aquilinum and Thelypteris palustris, sometimes remain sterile even when mature and grown to full size. As this is particularly characteristic of populations at high latitudes and altitudes, the cause is probably adverse environmental conditions.
Note 6: Some species are deciduous and fresh fronds are not available in the winter. Wintergreen fronds can be found on the following species, although in some cases only in more sheltered localities: Asplenium species, Blechnum spicant, Dryopteris aemula, some types of D. affinis, D. dilatata, D. expansa, D. filix-mas, Hymenophyllum species, Pilularia globulifera, Polypodium species, Polystichum species, and Vandenboschia speciosa. The other British species are deciduous.
Note 7: It is illegal to pick any material from, or otherwise disturb, certain designated rare and protected species including Cystopteris fragilis var. dickieana, Vandenboschia speciosa, Woodsia alpina, and Woodsia ilvensis. If in doubt, don’t pick it. If you need evidence for others to confirm the identification of one of these species, photograph it carefully with minimal disturbance. For all other species, remove the smallest amount of material necessary for identification. There is rarely a good reason to dig up a whole plant and to do so for even the most common species requires the prior permission of the landowner.
3. Go to the key and make the first choice after considering the alternatives.
4. Follow the key until you arrive at a name.
5. Check your fern against illustrations and other information for that name in published guides.
6. If the specimen does not match the description, repeat 3-5, carefully checking each step.
7. If there is still no match, it may be that your specimen does not belong to one of the 37 more common species. Compare your specimen with the descriptions of rare and scarce native species. Another possibility is that it is a naturalised alien fern, of which there are an increasing number. Check in a book of garden ferns, or send a picture to What’s that Fern
8. If your specimen shows similarities to two species, it might be the intermediate hybrid; check that the putative hybrid is known and then consult a more detailed identification guide for confirmation.