As with plants in general, some types of ferns are fairly simple to identify due to their distinctive features. Others can be difficult, sometimes even for experts. Most fall somewhere between. With a knowledge of the key features to look for and a good fern guide, you'll be well on your way to successful identification of ferns in gardens and in nature.
Leaves or Fronds
A logical place to start when identifying types of ferns is with leaf characteristics. The leaves, or fronds, of a fern are the structures that arise from the roots. So, typically, leaves are the entire above-ground parts of the plant. Some ferns are evergreen -- keeping their leaves all year -- while many are not.
Fern leaves can be simple or compound. Hart's tongue fern (Asplenium scolopendrium), which is perennial, or hardy, in U.S. Department of Agriculture plant hardiness zones 5 through 9, is an example of a fern that has a simple leaf, one shaped a bit like a sword or knife blade. Noting this leaf characteristic eliminates a vast number of ferns from the list of possibilities in fern identification. Although the hart's tongue fern is an example of a fern with a simple leaf, it happens to be rare in the wild. Therefore, you are unlikely to encounter it outside of botanical gardens and private plantings.
Other ferns -- those considered more typical -- have compound leaves. Compound leaves are divided into sections. Some kinds of ferns with compound leaves have one level of division into leaflets while others have leaflets that are further divided into subleaflets. Still other types of ferns even have subleaflets divided into lobes. Those ferns have quite a lacy texture.
Even though many types of ferns have compound leaves of one sort or another, placing an unidentified fern into one of the compound-leaf categories narrows the possibilities of that fern's species. So look at the basic leaf characteristics first.
Unlike flowering plants, ferns do not produce true fruits or seeds. They do, however, form spores, and those spores develop in structures that are useful in fern identification. The size, shape and other features of the spore-bearing parts of a fern are unique to each fern species.
In some ferns, the spores occur in so-called fruit dots on the leaves' undersides. In this category are ferns that have leaves with spore structures and those without on the same plant. Still other ferns have specialized spore-bearing leaves that appear very different from the leaves without spores, which are called sterile leaves. An example of that type is the cinnamon fern (Osmunda cinnamomea, USDA zones 3 through 9). Its spore leaves are spirelike and cinnamon colored.
Rhizome and Stipe
Besides the leaves and spore structures, another key feature of ferns is the rhizome. A fern's rhizome is a stem, often underground and usually horizontal, that produces leaves above and roots below. The size, shape, orientation and other features of the rhizome and roots vary among different types of ferns. Even though they may be less obvious than the leaves, these parts of a fern's anatomy can be helpful in the plant's identification.
The very bottom section of a fern's leaf is called the stipe or stem. It is the part between the rhizome and the main part of the leaf. The stipe may be smooth, may be covered with scales, may have just a few scales or may be hairy. All of these features, combined with other characteristics, help in fern identification.
Habitat and Range
In the case of wild ferns, knowing their natural ranges can aid in their identification by eliminating non-native ferns from consideration. Of course, this method isn't foolproof because ferns, like other plants, might escape cultivation and show up in more or less wild areas. In general, though, a glance at the natural range information for a fern can narrow the possibilities when identifying it in a wild setting.
Some ferns can appear decidedly not fernlike. The water fern (Azolla filiculoides, USDA zones 7 through 10) is an example. It is a tiny, aquatic fern that floats on the water surface and tends to form masses of many individuals. This little fern can be an addition to a water garden.