Hardy New Zealanders for Pacific Northwest Gardens

The mild climates of western Oregon and Washington and southwestern British Columbia have always provided the adventuresome gardener the opportunity to cultivate a great diversity of plants, including many from climates generally considered less rigorous than our own.

The reputation of the Pacific Northwest as a bastion of conifer, Japanese maple, and rhododendron culture has obscured the accomplishments of numerous innovative horticulturists throughout the region’s history. Perhaps the best tribute to their accomplishments is that, today, a new generation of innovators in the Northwest have made it a Mecca for plant enthusiasts, and have led to a tremendous increase in the range of plants now cultivated.

The number and quality of nurseries catering to this trend has increased in recent years, and there are now few areas of the world, outside of the tropics, that don’t offer new gems for local gardeners. Nevertheless, of the multitude of plants cultivated in Pacific Northwest gardens, surprisingly few representatives from New Zealand may be found. This is unfortunate, as much of the flora possesses the highly desirable quality of evergreen foliage, and displays hues, attractive patterns, and textures not found in plants native elsewhere, such as a divaricating branch structure and a deep brownish-purple foliage coloration. Such coloration has the unfortunate effect of disguising from the uninitiated the fact that a plant has died–in some instances perhaps weeks or months earlier! Corokia cotoneaster is unrivalled in this regard. Despite this characteristic (and many more worthwhile ones), perhaps the only genera with any commercial name recognition in this area are Hebe, Leptospermum, and Phormium.

Provenance is the Key

Why this should be so is primarily a consequence of geography. The main islands of New Zealand lie in the South Pacific Ocean, mostly between 34 and 47 degrees latitude. The tempering influences of latitude and ocean mean that few areas, outside of the alpine regions, experience more than a few degrees of frost in winter, even in the extreme south of South Island. Consequently, much of the flora has not evolved to be particularly cold hardy and is vulnerable to the kinds of continental freeze events occasionally visited on the Pacific Northwest. In fact, the term “hardy,” which in our area exclusively describes an ability to survive low temperatures, is in New Zealand also applied to plants that are drought, wind, or salt-spray resistant. Nevertheless, the distribution of many New Zealand plants extends over the full range of latitudes and from near sea level into the montane zones; this results in a genetic range of individuals adapted to these varied “provenances” or specific geographic origins. These individuals may be subject to considerable differences in cold exposure. As with plants from other warm-temperate areas that we wish to grow here, attention to the provenance of each seed source in the colder parts of New Zealand is critical to success.

If there was one thing that the exceptional cold spell of December 1998 did, it was to sort out the provenances of New Zealand species growing here at that time. For most of the region, this was the coldest winter since 1990. Official temperatures in Seattle dropped to as low as -12^oC and in Portland to -9^oC for several consecutive nights, and did not rise above freezing during the day. Many low-lying areas were even colder, but coastal areas, especially in southern Oregon, remained much milder. Still, the temperatures were above historic lows, which have reached -11^oC in Portland. The damage after the 1998 freeze involved the usual mix of surprise and dismay at what had lived or died. Among the many selections of Hebe, several of the commonly planted cultivars such as ‘Amy’ and ‘Patty’s Purple’ suffered badly or required removal; ‘Autumn Joy’ jettisoned its leaves. Many other plants in which gardeners had invested a lot of (misplaced) hope, were severely damaged, including Pittosporum eugenioides, Coprosma robusta, and Olearia avicenniaefolia. Yet, a great many plants have survived these significant freezes and continue to enrich our landscapes.

Northwest gardeners would benefit from a list of recommended plants, refined through a systematic and region-wide review of the results of cold spells like that of December 1998. A surprising amount of information does exist, with notable collections of New Zealand native plants at the University of British Columbia Botanical Garden in Vancouver and at Washington Park Arboretum in Seattle. The Friends of the University of Washington Arboretum published, back in 1972, “Ornamental Plants Hardy in the Pacific Northwest,” which listed many species of New Zealand plants, including, for example, forty-three species of Hebe. How many of these were subsequently proven less than hardy after the devastating freeze of December 1972 is unknown. Still, the publication is a valuable reference for general cultivation outside of these exceptional winters. Such reviews are only as good as the range of species and cultivars evaluated. While many plants remain to be tested in our area, the performance of others too often goes unreported (except for tearful stories in the pubs!). The diversity of climates within our area complicates the discussion, as significant climatic differences exist in both north-south and east-west directions. Damage to plants of any given species may only represent an inappropriate provenance, not an inherent unsuitability of that species for our area. What we need in the Northwest is proven selections that can be named and propagated clonally.

French Trials

One of the most extensive field evaluations of New Zealand species for cold hardiness was undertaken by Warwick Harris from Christchurch, New Zealand, working with Luc Decourtye, and Alain Cadic, both horticulturists from Angers, France. Their goal was to evaluate plants for their suitability for general cultivation in France. Angers lies in the Loire Valley, approximately 150 miles southwest of Paris. The climate there is comparable to, although more humid than, much of the Pacific Northwest and has similar bouts of intense cold, although they can last as long as three weeks—far longer than is typical for our region. In 1987, plants representing seventy-five New Zealand species were set out for long-term evaluation at Angers. A cold spell in the winter of 1990-91, marked by twenty-seven consecutive days of frost and a low temperature of -12.5^oC, severely damaged sixty-three of the species and killed many. A subsequent prolonged cold snap in the winter of 1996-97, with a low of -15.7^oC, left only six species undamaged.

Among the six species that have escaped damage in France are Discaria toumatou and Melicytus alpinus, two spiny plants that the French consider useful for defensive hedging. The others are Plagianthus divaricatus, a divaricate shrub suitable as an unusual feature plant or as hedging, and Podocarpus halli, P. nivalis, and P. totara. Others that have survived and recovered well at Angers are Coprosma propinqua, interesting because of its habit and fruits, Corokia cotoneaster, distinctive for its habit, flowers, and especially its fruit, Phormium colensoi (syn. P. cookianum) and several montane species of Hebe. As it turns out, most of these species also have performed well in the Pacific Northwest.

The Northwest Winter of 1998-99

Temperatures at UBC Botanical Garden did not drop below -8^oC last winter, but the garden’s collection includes many selections planted in the early 1970s, whose hardiness is beyond doubt. Among them are beautiful expanses of Muehlenbeckia axillaris var. minor and Coprosma areolata (syn. C. petrei var. atropurpurea), as well as Pimelia prostrata, all of which are mat-forming dwarf shrubs. Another shrub of dwarf, spreading habit that has thrived in the garden is Cyathodes empetrifolia. UBC also has excellent examples of Podocarpus nivalis and P. totara, as well as a four-meter specimen of Hoheria lyallii. Most of the hebes there are only recently acquired and are as yet unproven, but a few, such as ‘Blue Mist’, have survived for several years.

In the Seattle area, temperatures for the cold spell of December 19-23 ranged down to –12^oC. At Washington Park Arboretum, the low of -9.4^oC was sufficient to kill some highly desirable species, including Carmichaelia glabrescens (syn. Notospartium glabrescens), and caused substantial damage to both leaves and stems of Leptospermum scoparium, Fuchsia excorticata, Cordyline australis, and C. indivisa. However, a survey by curator Randall Hitchin in May showed that a number of species survived relatively unscathed. As at Angers, Podocarpus hallii and P. nivalis suffered no damage; the survival of the larger-growing P. hallii is particularly encouraging. Two southern beeches (Nothofagus solandri var. cliffortioides and N. menziesii) dropped their leaves but showed no stem damage. Among several selections of Griselinia littoralis, a dense evergreen tree, damage ranged from none to significant, illustrating the potential for selection of hardy clones. Dan Hinkley at Heronswood, across the sound in Kingston, found that G. littoralis ‘Variegata’ was severely damaged.

Among the shrubs, Ozothamnus vauvilliersii (syn. Cassinia vauvilliersii) suffered no damage nor did O. fulvida, at Heronswood. Most species and selections of Hebe were unharmed, including H. amplexicaulis, H. haastii, H. ochracea, H. subalpina, H. pimeleoides ‘Quicksilver’, and H. ‘White Gem’; not surprisingly, all of these species are from the mountains of South Island as is the equally hardy Heliohebe raoulii (syn. Hebe raoulii). Other, lesser-known genera fared surprisingly well. Hymenanthera alpina and H. angustifolia (syn. Melicytus alpinus and M. angustifolia, respectively) had only minor stem damage while Pachystegia insignis (syn. Olearia insignis) experienced some leaf damage. Among the species of Olearia in the arboretum, O. nummulariifolia, O. macrodonta and O. ilicifolia, suffered only slight defoliation. One that did well at Heronswood was O. x haastii (O. avicenniifolia x O. moschata). Another pleasant surprise was the minimal damage to Pittosporum tenuifolium; this species showed variable damage at Angers suggesting that hardier selections could be made. Herbaceous plants that fared well included several species of Aciphylla, a genus of perennial herbs with yucca-like mounds of slender, spine-tipped leaves. A. aurea, A. glaucescens, and A. subflabellata suffered no damage. Other plants that survived with minimal damage were the mountain flax (Phormium colensoi), and the snow tussock grass (Chionochloa conspicua).

In Portland, the temperature at Hoyt Arboretum reached -8^oC. There was no damage at the arboretum to Podocarpus totara ‘Aurea’ nor to the aforementioned podocarps thriving in the Seattle area. Another survivor was Dacrycarpus dacridioides. In the city, casualties included most plants of Pseudopanax, Coprosma robusta, and Pittosporum eugenioides, (although P. eugenioides ‘Variegata’ survived), as well as Corokia buddleioides and C. x virgata ‘Cheesemanii’. Other selections of C. x virgata, such as ‘Sunsplash’ suffered little or no damage, nor did species with a more decumbent growth habit, such as C. petriei. Coprosma rhamnoides, whose leaves turn purplish in winter, did well.

Most hebes in the city survived unscathed, including Hebe pinguifolia, H. salicifolia, H. pimeleoides, H. albicans, H anomala and H. prostrata. Although some cultivars suffered damage, others like ‘Snow Wreath’ did not. Several selections of Pittosporum tenuifolium performed well, including ‘Irene Paterson’, ‘Silver Magic’, and ‘Marjorie Channon’. A hybrid selection, ‘Garnetii’ (P. tenuifolium x P. ralphii), also suffered no damage. Among other shrubs that survived were numerous species of Olearia, including O. ilicifolia, O. virgata, O. moschata, O. x haastii, and O. nummulariifolia. A close relative, Brachyglottis monroi, showed no damage, nor did B. spedenii, a very pretty mountain shrub with leaves that become purplish in winter. Others that show promise are Carmichaelia odorata and the diminutive Teucridium parvifolium. Clematis petriei, C. paniculata, C. afoliata, C. quadribracteolata, and C. marmoraria were all in fine condition after the freeze, as were Clematis hybrids such as C. x cartmanii ‘Joe’.

Damage to New Zealand flaxes (Phormium) varied greatly, with cultivars such as ‘Maori Maiden’, ‘Tricolor’, ‘Tiny Tiger’, and ‘Pink Stripe’ performing well with little to moderate damage (from which they recovered); others such as ‘Yellow Wave’ were badly damaged or killed if not protected. Selections of mountain flax were generally uninjured. Surprisingly, even some tree ferns, like Dicksonia fibrosa, came through this cold snap with surprisingly little damage.

Far from being cause for gloom, the winter of 1998-99 has shown that there is quite a range of New Zealand native plants suitable for cultivation in the Northwest. It also illustrates that for many of the genera, notably Hebe, few of the many species and cultivars with potential have been evaluated, as they are not widely cultivated. Other less familiar genera, like Olearia and Brachyglottis, show great promise for our area. Hardier representatives of Olearia, notably O. virgata, are known, but are rarely cultivated locally. Several selections of Pittosporum tenuifolium are proving hardy—exciting considering their diversity of form and their fragrant flowers. Other pittosporums, in particular P. dallii and P. patulum, also with fragrant flowers, are considered to be even hardier. The case of the Griselinia at Washington Park Arboretum perfectly illustrates the need for selection of hardy clones whose performance has been evaluated locally. Certainly the experience of the last few years has provided enough information to begin this process, which should lead to more widespread cultivation of these beautiful and useful plants.