Pacific Plant Promotions: Adiantum xmairisii

By: Richie Steffen

Richie Steffen is curator of horticulture for the Elisabeth C Miller Botanical Garden in Seattle, where he manages the rare…

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Mairis’s maidenhair (Adiantum x mairisii). Author’s photograph

The delicate foliage of a fern adds a softness and grace to any garden. Maidenhair ferns (Adiantum species), in particular, are known for the beauty of their fine lacy fronds. According to Thomas Moore in his The Nature-Printed British Ferns (1863):

The name Adiantum is a Latinized form of the Greek adianton, derived from adiantos, dry or unmoistened, and seems to apply to a property possessed by these plants of repelling water from the surface of the fronds, so that they cannot easily be wetted.

Unfortunately, most Adiantum species are tropical and are only reliable in the frost-free garden or as a container plant to be protected in the winter. So, it is exciting when a new winter-hardy species becomes available. Mairis’s maidenhair (A. xmairisii), is a rare hybrid, first raised before 1885 in Mairis & Co’s nursery in England. Author Thomas Moore, a well-known Victorian fern collector and curator of the Chelsea Physic Garden during the height of Pteridomania, named the hybrid in the latter half of the 1800s. Mairis’s maidenhair is a sterile fern, thought to be a cross between the Southern maidenhair (A. capillus-veneris) and an unknown species (perhaps the tender A. cuneatum). Surprisingly, this chance hybrid is vigorous, easily grown, and more cold tolerant than either of its suspected parents. Over the last one hundred years, it has been carefully divided and shared among fern enthusiasts, eventually reaching Naud Burnett, owner of Casa Flora wholesale fern nursery, the largest producer of ferns in North America. The nursery’s research team was able to coax the plant to reproduce through tissue culture, thus making it widely available for the first time.

The Elisabeth C Miller Botanical Garden, working with the Hardy Fern Foundation, offered this fern, grown by Casa Flora, to readers of Pacific Horticulture in 2012 through its Pacific Plant Promotions program. The Miller Garden advocates the use of new, unusual, and well-adapted plants for Northwest gardens; the Hardy Fern Foundation encourages education, exploration, and the garden use of temperate ferns throughout the world. Mairis’s maidenhair is an exciting and adaptable addition to the fern palette and has the potential to grow well along the entire West Coast. Due to its limited availability in the past, most of the cultural information is compiled from growers in Texas, Britain, and, more recently, Washington’s Puget Sound region.

Mairis’s maidenhair grows best in rich organic soils, but tolerates both clay and sandy soils. Plants remain evergreen in mild winter climates. In the Northwest, the foliage is retained late into autumn, gradually being defoliated by cold weather as winter builds. Fresh fronds emerge bright green in late spring, contrasting with the charcoal black stems. Mature plants reach twelve to fifteen inches high and slowly form a patch about twenty-four inches across. Most current literature lists its hardiness as USDA zones 8-10, but it seems to be safely grown in USDA zone 7. Plants I have grown have survived and thrived after experiencing lows of 11°F. As with many ferns, this maidenhair likes a location where it will receive regular watering during dry periods and protection from hot afternoon sun. New fronds are slow to emerge in spring, often not appearing in the Northwest until late May or early June. In mild climates, it would make an excellent container plant.