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Xeric Ferns

I know, it should be "Xerophytic", but the diminutive is so commonly used that I thought I'd use it here. In either case it refers to a group of ferns that live in very arid places where you would never expect a fern to live. One thinks of ferns as living in damp shady spots but many of them live in the sands of the desert or in the rocks and rubble of barren hillsides. They use a number of different stratagems to accomplish this.

Many xeric ferns live in quick draining sandy soil or rocky shale which delivers the occasional water to their roots and also prevents the formation of stagnant water which can just as quickly rot their roots A number of the xerics live in areas where the annual rainfall is condensed into a couple of months in the dead of winter. These plants produce new fronds very rapidly after the first rain, then the sori will mature and dehisce, shedding their spores, before the heat of summer can shrivel the plants in preparation for the next year's rains. Many of these plants live on the shady side of rocks or boulders where their roots can seek the cool damp shade under the rocks.

Some xerics have the entire plant covered with short hairs or scales which, by reducing air motion, create a little humid microclimate next to the pinnules so that the dryness does not so badly affect the plant. Many of these same plants also curl their pinnules inward when the dry weather comes to further reduce the area of evaporation. Some of these, like the resurrection fern, Pleopeltis polypodioides (until 1993 Polypodium polypodioides) will curl up tightly and then spring back to their normal shape in the presence of water.

A few of the plants in this group have the underside of their pinnules coated with a powdery substance known as "farina." This coating is usually white or gold or silver, so that it increases the reflectivity while sealing off the pores from the low humidity.

The members of this group of drought resistant ferns are very difficult to classify. In 1979, Dr. John Mickel of the New York Botanical Garden lumped most of them together into the single genus Cheilanthes. As a result, they have come to be known as Cheilanthoids. More recent studies have established several genera and assigned them to the families Adiantaceae or Pteridaceae. A great deal of work remains to be done before a single consistent classification system is achieved. The term Cheilanthoids, however, remains in general use.

Members of this group, however assembled or named, are found on every continent where ferns are found, except Antarctica. Some are found in the true dessert. Others may be found on cliffs or open hillsides but all seek places where there is bright light and usually, very little rainfall. Not unexpectedly, many are found in the Southwestern United States and Mexico and at least ten species are native to San Diego County. You can find several of them as close at hand as Mission Trails Park and this is fine time of year to do so. Wait until summer and there will be nothing left but dry stalks.

Following the outline in the Flora of North America, which is based on the family Pteridaceae, we find there are some 40 genera world-wide with about 13 of them in this country. The largest group is still found in the genus Cheilanthes with about 150 species world-wide, although it may be further divided at a later date. Recent studies have already established that some of Mickel's Cheilanthes belong in separate genera, thus forming Aspidotis, Argyrochosma, and Astrolepis. The genus Notholaena has been reestablished but in a highly restricted fashion. In the United States we also have the genera Bommeria, Pellaea, and Pentagramma. Elsewhere in the world we must include at least the genera Adiantopsis, Doryopteris, Gymnopteris, Hemionitis, Paraceterach, Pityrogramma, Pleurosorus, and Quercifilix. Some of these, like the Pellaeas, are not so staunchly drought-resistant as the others. These ferns prefer locations with partial sun and good drainage, but not long term drought.

Our climate here in Southern California gives us a great opportunity to grow these fascinating plants. The major requirement is reminiscent of an old real estate axiom. It is 'drainage, drainage, drainage!' Members of this group should be planted in volcanic scree or in coarse sand mixed with gravel - any mix which will drain quickly - and should have good run-off from the sub-strata. If planted in containers, the pot should be deeper than for most ferns with many drainage holes. They should be fed sparingly. Many like alkaline soil and will want the occasional addition of lime. Interestingly, although these plants have adapted to arid conditions with only occasional precipitation, they do not demand these conditions. They will be happier with more regular watering, but the roots must never be allowed to stay wet. The books all caution against overhead watering and getting the leaves wet, but the experience of nurserymen and growers in the Pacific Northwest would indicate that this is not that important. It is the underground part of the fern that is fussy.

The most popular of these plants in cultivation is probably Pellaea rotundifolia, along with its companion form, Pellaea falcata. These are marginal Xerics. They like a lot of light, maybe a little sun, but are otherwise more like a 'normal' ferns. We occasionally have other forms like Pellaea ovata and Pellaea cordifolia on the plant table. They, too, are only semi-xeric in habit. The true Cheilanthoid types are seldom available in the local nurseries, but a selection of Cheilanthes is often available from both Fancy Fronds and Foliage Gardens nurseries in Seattle.


References:            
Hoshizaki, Barbara J.The Fern Growers Manual
Jones, David L. Encyclopaedia of Ferns
Witham, HelenFerns of San Diego County
--------- Flora of North America, Vol. 2. Pteridophytes & Gymnosperms
Mickel, John T."The Fern Genus Cheilanthes in the Continental United States." Phytologia 41:431-437, 1979


Bob Halley, Frequent Contributor,
San Diego Fern Society Fern World
April 2001

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Last Updated 4/7/02