By Denise Levine, U. C. Master Gardener
Polystichum
Blessed as much of Napa is to have creeks and rivers and a forested Western watershed to host cool, water-loving ferns, we are also lucky that many ferns have adapted to the drier conditions so many of us live in. Two ferns are especially suited to the variable conditions of Napa County.
The genus Pteridium has only one worldwide species, P. aquilinium, but many varieties. P. aquilinium pseudocaudatum is the variety we commonly know in Napa as the bracken fern. Bracken ferns have coarse, leathery, triangular fronds on woody stems one to two feet long and equally wide. The fronds grow at irregular intervals along a hairy, creeping stem that has deep roots and can be fast spreading.
Because of this last quality, bracken fern is a survivor that grows happily just about anywhere. It is often found in second-growth forests and fields but does as well in shady, damp lowlands or grasslands and open sunny areas. Bracken ferns are not as fussy as most ferns and can often signal poor soil.
To propagate them, divide bracken ferns' underground stems and set divisions three feet apart. If you are preparing a bed especially for bracken, they will be happy in a mixture of one part garden loam, one part builder's sand and two parts leaf mold or peat moss. A pH of 5.5 to 6.5 is ideal, but they are not too fussy.
Polystichum are robust evergreen ferns with symmetrical crowns of fronds that can grow three to five feet long. Polystichums include leather fern, Christmas fern and Western sword fern (P. munitum). This last is the species commonly found in Napa County.
Western sword ferns are also a fine choice for town gardens. Symmetrical and deep green, sword fern foliage is frequently cut for use in floral arrangements and wreaths. They are beautiful plants, useful in the garden as borders or foundation plants, or in pots or hanging baskets.
Growing in a tight clump, stalks of the Western fern spread out radially from a round base. Pairs of five-inch leaflets that look like little daggers with bristly, prickly edges grow along the stem to make the fronds.
Older clumps can have as many as 75 to 100 fronds. Sword fern fronds grow for several years before dying back to a rhizome.
Sword ferns are tough and able to withstand periods of drought, especially if they are well established. But for luxurious growth, they appreciate constant moisture, decent light and regular light applications of fertilizer.
Don't fertilize purchased plants for six months. Then feed them twice during the spring and summer months with fish emulsion at half the dilution rate recommended on the bottle for other plants.
If the Western sword fern looks familiar, it might be because our native sword fern is a cousin of the famous houseplant, the Boston fern. Did you know that all the varieties of this ubiquitous houseplant are mutations from a single plant?
In 1894, grower Robert Craig and Company sent an order of 200 sword ferns for houseplants to M.C. Becker, a distributor in Cambridge, Massachusetts. One plant stood out to Mr. Becker. It was more graceful than the others, and the fronds were longer and more delicate.
At first Becker thought it was a new species and propagated and sold about 50,000 of them under the name Nephrolepis davalliodes. Eventually the Society of American Florists sent a specimen to the Royal Botanic Gardens in England where it was decided that it was a mutant offspring of the common sword fern. They officially named it N. exaltata 'Bostoniensis,' or the Boston fern.
Boston fern mutants were soon mass-produced in commercial greenhouses. Plants were culled for special features like compact growth or deeply curled leaflets, and these aberrant specimens were then propagated. By 1920 almost 75 varieties were being propagated for houseplants, but today probably fewer than 20 are widely cultivated.
The adaptability of bracken and sword ferns makes growing them a little easier and keeps them on the list of possible plants for sun or shade. And the variety of Boston ferns still being grown can add interest and beauty to your houseplant collection as well.