ORNAMENTAL WILLOWS (SALIX SPP.) FOR ALPINE AND SMALL URBAN GARDENS

Journal of Arboriculture, Mar 2004 by Kuzovkina, Yulia A, Quigley, Martin F

Abstract. The small scale and intimate feel of contemporary urban and suburban gardens require appropriate plant material, especially woody plants with multi-seasonal appeal. Many arctic and alpine willows (Salix spp.) are ideally suited to the rock garden, alpine bed, or small urban garden. Here we evaluate alpine, arctic, and other low-growing species of willows for their horticultural merit and tolerance of U.S. Midwestern climatic and soil conditions. Detailed descriptions of 58 taxa are presented, along with photographic images that can be accessed on the Web (chadwickarboretum. osu.edu). We address taxonomy, phenology, autecology, cultural requirements, and ornamental qualities (size and habit, floral quality, bud and leaf color and shape) of selected species new to the North American market.

Key Words. Shrubs; rock garden; horticulture; groundcover; willows; Salix.

Due to their small mature size and wide ecological adaptability, low-growing and dwarf Salix species can be successfully used in small urban spaces, as well as in suburban residential rock and alpine gardens. Medium-sized shrub willows, those 0.5 to 1.5 m (1.7 to 4 ft) tall at maturity, are also suitable for limited space in confined residential gardens. Willows occupy an insignificant place in most nursery stocks, despite their variability and hardiness. The horticultural literature on Salix is sparse, and descriptions and performance information are not reliable for many willow species. Good photographic images of willow species are also difficult to find.

The small willows described here represent arctic and alpine species as well as low-growing species of temperate zones. Our purpose is to evaluate an array of obscure species available either in the trade or from public and private collections, particularly those in the United States, Great Britain, and Eastern Europe. We assess their ornamental potential for small garden landscape use, assess their performance in the continental conditions of the Midwestern United States, and verify identification and clarify taxonomy at the level of species and cultivar. The present work represents a distillation of literature searches m Salix nomenclature for North America, Europe, and Asia, and includes 3 years of growth data and evaluation of these species in central Ohio (USDA hardiness zones 4/5). Many of the taxa discussed here include species that are not well known outside specialists' collections and have only limited availability. Our hope is to reveal the horticultural potential of under-used small shrub willows, to publicize new willow introductions, and to correct some current errors in species and cultivar names.

MATERIALS AND METHODS

Between 1999 and 2002, we collected or received approximately 200 willow specimens from American and European (British, Polish, and Russian) nurseries, botanical gardens, and arboreta. These plants were installed in replicated plots at The Ohio State University Chadwick Arboretum and the Waterman Agricultural Research Facility, both in Columbus, Ohio, U.S. Performance data were collected over three growing seasons. Plants received supplemental water during the first year after planting to assist them to establish root systems, but they were not subsequently irrigated. These Salix taxa have been under observation throughout the period of study. The 60 species and cultivars described here have been selected for their relatively low growth habit. The field performance evaluations included growth rates; ornamental qualities; insect and disease resistance; and drought, heat, and cold hardiness.

The greatest challenge was the identification of species because many specimens were misnamed at the germplasm source. In an attempt to clean up some long-standing inaccuracies in Salix nomenclature, we turned to original authorities on taxonomy, including Skvortsov (1999) in Russia and Argus (1997) in Canada. Flora of Japan (Ohwi 1965) and Flora of China (Flora of China Editorial Committee 1994) were consulted in a few cases. The List of Names of Woody Plants (List of Names 2000) was used as the standard for cultivar names.

SYSTEMATICS

The genus Salix comprises three subgenera. The tree-type species belong to the subgenus Salix, the shrub-type to the subgenus Vetrix Dum., and the most low-growing and prostrate species to the subgenus Chamaetia Nasarov (based on Skvortsov 1999). All species examined here are included in the subgenera Chamaetia or Vetrix.

The subgenus Chamaetia consists of five sections: Chamaetia Dum., Retusae Kerner, Myrtilloides Koehne, Glaucae Pax, and Myrtosalix Kerner, and it is especially rich in dwarf and low-growing or creeping species; many of them are true alpine and arctic species. The members of this subgenus are typically well adapted to the extreme conditions of montane highlands and tundra. Low-growing and prostrate species can be found in a few other sections of the subgenus Vetrix. Representatives of sections Arbuscella Seringe, Glabrella Skv., Hastatae Kerner, Helix Dum., Incuhaceae Kerer, Lanatae Koehne, Vetrix Dum., Villosae Rouy, and Candidae Schneider are described, as well as hybrid species that crosses section divisions.