Winter Protection of Tree Ferns at the Royal Botanic Garden Edinburgh

Abstract

Ten plants of six species of tree fern were trialled for frost hardiness during the winter of 2005/06 when they were planted outdoors in the ground of an interior courtyard at the Royal Botanic Garden Edinburgh. The species were Culcita macrocarpa, Cyathea dealbata, Cyathea dregei,
Cyathea smithii, Dicksonia antarctica and Thyrsopteris elegans. An additional specimen of C. dregei was planted in the main garden. The apex region of each tree fern was fitted with an electric thermometer probe to record weekly minimum and maximum temperatures. These were compared with the air temperatures of the courtyard. For thermal insulation, the trunks and crowns of the three Cyathea species were encased in straw. The prostrate rhizomes of C. macrocarpa and T. elegans were covered respectively with leaf litter, straw and a polystyrene tile. As comparators, three trunked specimens of D. antarctica were given no winter wrapping, since previous experience had shown it to be unnecessary. All ten plants survived the winter of 2005/06 which was colder than average, and put out new growth the following spring. Fronds of D. antarctica and C. macrocarpa stayed green; the fronds of the other species were withered by the coldest exposures when the air temperature reached 4.7°C.

Compared with the main botanic garden, the courtyard provided a relatively mild microclimate. It was on average 2.5 °C warmer than the air temperature measured in the screen of the main garden weather station, and 7.7°C warmer than the ‘grass’ temperature in the main garden, which went down to –13°C at its lowest. All tree fern apices registered sub-zero temperatures, the range in different plants being from –0.3 to –3.4°C. The apex regions did not get as cold as the surrounding air temperature, which ranged between 0.5 and 2.3°C. The three D. antarctica (without added insulation) had minimum apical temperatures in the same range as the species that were wrapped for the winter. The insulation effect in the apex regions was also shown by the weekly maximum temperatures, which on average were lower than those of the courtyard air maxima.

In conclusion, the combination of the locally favourable microclimate of the courtyard, plus appropriate trunk and crown insulation provided for some species, allowed the planting outdoors, of tree ferns normally grown in Edinburgh under heated glass.