Saturday, February 7, 2015

Spanish Moss Cold Hardiness

Tillandsia usneoides grown outdoors in Connecticut and brought inside on December 24 (left) and January 12 (right).
 Tillandsia usneoides, or Spanish Moss, is a rootless epiphyte with inconspicuous greenish flowers, in the family Bromeliaceae (the pineapple or bromeliad family), and one of the emblematic plants of the American South. It is native to an enormous geographical range in the tropics and warm temperate regions of the New World, from coastal southeastern Virgina and possibly Maryland, down almost to the southern tip of South America in Argentina and Chile. The whitish appearance of the plants is the result of a dense coat of umbrella-shaped (peltate) hairs, which are involved in trapping and absorbing the rainwater and trace nutrients.

Tillandsia usneoides grows naturally in locations that experience considerable frost, and would appear to be the most cold hardy of the epiphytic angiosperms (flowering plants). I have grown Spanish Moss outdoors in the branches of trees in Mansfield, Connecticut many times and it always does quite well from spring to fall. Plants that are left out over the winter have never survived, though, even in mild years.

This winter I had left out a number of clumps of extra T. usneoides, but after they had been thoroughly frosted I decided to bring some in to the UConn greenhouse to see if they could be revived. Plants retrieved on December 24 looked a little dehydrated at first but recovered quickly in the greenhouse, with negligible die back. November-December was warmer than normal, but the plants had experienced numerous hard freezes, some multiple-day stretches of sub-freezing temperatures, and absolute lows of about 10°F (-12°C).

Another clump brought inside on January 12 experienced much harsher weather, including week long periods with daytime highs below freezing and absolute lows of -2°F (-19°C). Although this plant probably lost well over 90% of its shoots, a smattering of stem tips with a couple of leaves and internodes have definitely survived and are resuming growth in a cool but frost-free greenhouse. Still, the plant was clearly near the limits of survivable conditions.

Three different collections of Tillandsia usneoides: typical USA form, gathered in Charleston, SC by R.W.A. Opel (left); fine Carribean form from the Dominican Republic (center); robust form from Peru (right).
As might be expected from a plant with such a wide native range, T. usneoides is variable in appearance from place to place; the photo above shows three forms collected in different localities but grown in the same greenhouses. This year's hardiness tests were carried out with cultivated Spanish Moss originating in the southeastern USA; in previous years I have tried the Peruvian material outdoors, but it succumbed rapidly during the first hard frosts in autumn. So, there seems to be some genetic variability in winter hardiness in T. usneoides. It might be interesting to experiment with outdoor cultivation of material from the northern limits of the range of Spanish Moss, Virginia or the inland edges of the Carolina Low Country. Success would seem unlikely in Connecticut, regardless of the wild source of the plants, but I wonder if some adventurous gardener might be able to establish T. usneoides in a mild, protected coastal location in New Jersey or Long Island, or possibly even the more maritime parts of Massachusetts, say Martha's Vinyard or the Outer Cape.

Addendum: I rescued some more clumps of T. usneoides from outdoors in mid-February, after more extended periods of sub-freezing weather. Some shoots still looked pretty green initially, but this was the green of a package of frozen spinach; everything was clearly brown and dead a few days after thawing out. 

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