Hardy Eucalypts in the Pacific Northwest
This article first appeared in HPI Edition #50, May 2002.
It has been quite some time now since Alex Downie's excellent article on Eucalyptus and their potential for cultivation in the Pacific Northwest. As a follow-up, I thought I would present my own take on the cold-hardier members of this fascinating genus, based on my experience with eucalypts to date.
I have been growing eucalypts for about seven years in the South Puget Sound area. As far as climate zones are concerned, my garden can be considered as USDA zone 8b and Sunset zone 5. At present, I have approximately 115 different eucalypt species and hybrids in the garden, with another 20 or so on the way for next year. This endeavor involves obtaining a variety of different forms and provenances of each species whenever possible, to maximize genetic diversity and hopefully select some trees for superior performance and cold-hardiness. Admittedly, I'm running out of space!
Not surprisingly, cold-hardiness (or the lack thereof) is the primary limitation to eucalypt cultivation in the Pacific Nortwest. Although eucalypts have been planted here for many years, there are very few large specimens in the region to show for it today. The oldest surviving individual I know of in the area is on Bainbridge Island, Washington: an E. perriniana (spinning gum) that dates back to the 1950's, though it has frozen to the ground at least once.
Eucalypts were planted throughout the Pacific Northwest during the 70's and 80's. University of Washington professor Stan Gessel, an Australian native, tried a large number of species. E. delegatensis reached 70' tall on the University of Washington campus, only to freeze in January 1979. Also, relatively rare species such as E. mitchelliana were tried at the Washington Park Arboretum.
The freeze of February 1989 in British Columbia, and December 1990 in Washington and Oregon, cut nearly all eucalypts in the region to the ground, and many were killed outright. Some survivors I know of include E. pauciflora ssp. niphophila in Seattle, E. gunnii on Bainbridge Island, and E. parvula and E. perriniana on Vashon Island. This suggests that it is not just certain species that are hardier to cold; additionally, certain individuals of a variety of species are capable of surviving the Pacific Northwest's absolute worst freezes.
December 1998 also saw a moderately hard freeze that damaged or destroyed many eucalypts in the Pacific Northwest. The South Puget Sound area and Rogue River Valley were hit harder than other areas. In my garden, the temperature dropped to about -11°C (12°F) with highs below freezing for five consecutive days. The only seven eucalypt species I had at the time that were not cut to the ground were E. glaucescens, E. gunnii, E. gunnii ssp. archeri, E. neglecta, E. parvula, E. pauciflora ssp. niphophila, and E. urnigera. Of these, all but E. urnigera were completely undamaged. It is likely that they would have been able to survive a more severe freeze had they been larger, and I expect them to be fully hardy for me in the future. Had I been growing other hardy species such as E. coccifera and E. pauciflora ssp. debeuzevillei at the time, I have no doubt they would have survived as well.
Despite the lack of large, older eucalypts in the Pacific Northwest, I still have reason to believe they will become more successful in the future. Since the 1990 freeze eucalypts have been more readily available, and have been planted in much greater numbers in our region.
Several factors must be kept in mind when considering eucalypt cold-hardiness:
Not all eucalypts of a single species are created equal. A representative sample from a single species may exhibit tall trees and short trees, blue trees and green trees, very hardy trees and cold tender trees, etc., depending on the species. Thus, future success with a given species cannot be ruled out just because it has failed in the past. Since 1990 a wider variety of cold-hardier eucalypt provenances have been made available through local specialty nurseries. Unfortunately, many retail nurseries still carry cold-tender stock shipped up from California.
Microclimates greatly influence one's chances of success. This fact is well known to all Pacific Northwest gardeners who attempt marginally hardy plants, but must be re-emphasized for eucalypts. It is especially important to site eucalypts in a location where they will be sheltered from those cold north and east winds that occur during Arctic blasts.
Eucalypts grow very fast, and will often regrow if cut to the ground. Thus they are often worth growing even if they might be expected to freeze in a cold winter, at least for some gardener or hobbyist. A few eucalypt species that are not likely to regrow after a hard freeze include E. delegatensis, E. regnans, E. obliqua, E. fraxinoides (white ash), and some forms of E. nitens and E. globulus (blue gum); however, such species are the exception rather than the rule.
Like most plants, eucalypts are able to withstand a greater amount of cold when they are larger and more mature. Considering also that eucalypts are very sensitive to root damage and restriction, they should therefore be planted as small plants before they become rootbound, so that they will establish and grow quickly in their first year.
Some other concerns for eucalypts in our region include wind and snow. Although they are generally resistant to both problems in Australia, they can be damaged by wind or snow in certain situations in the garden. Problem trees can be dealt with in a number of ways, but the best methods are those which encourage the development of a large healthy root system and sturdy trunk base, rather than an excess of top growth. This may mean withholding nitrogen fertilizer, or cutting the tree back hard a time or two when it is establishing. I have found that if a eucalyptus falls over, it is usually a consequence of some problem with the root system, whether it be root rot, poor drainage, or uneven development from being planted to close to something else. Staking young trees is also detrimental as it weakens them.
I believe it is likely that more hardy provenances of a wider variety of species will be singled out in the future. Until recently, the range of eucalypt species that has been promoted in the Pacific Northwest has been relatively narrow. I shall continue the process of remedying that by adding some comments on my favorite eucalypts and other noteworthy species, in no particular order. (This is certainly not meant to be an all-inclusive list of possible species for our region.)
E. glaucescens (Tingaringy gum) deserves more popularity as a very hardy and attractive tree. It has intensely silvery-glaucous leaves, smooth silver branches, brown bark on the lower trunk, and a very interesting open growth habit. E. parvula (formerly E. parvifolia, small-leaved gum) is also an attractive tree, with cream colored, vanilla scented flowers in summer. Contrary to references that describe this species as small, it is actually a vigorous, formidable tree with spreading branches that may eventually reach 60' and wider. Both of these species are among the very cold-hardiest eucalypts.
E. tenuiramis (silver peppermint) seems to be quite hardy as there is a good-sized one by the Graham Visitor Center parking lot at the Washington Park Arboretum in Seattle. It has attractive silvery leaves and smooth bark. Related, and even hardier, is E. coccifera (Mt. Wellington peppermint), which tends to have even more attractive bark, but is not extremely fast growing.
The snow gums are a familiar group of hardy eucalypts exhibiting more variation than most people realize. E. pauciflora (white sallee) officially has six subspecies, of which ssp. niphophila (snow gum) is by far the best known. Ssp. pauciflora and ssp. debeuzevillei, the latter of which is extremely cold-hardy, are also becoming more popular. The other three subspecies, ssp. acerina, ssp. hedraia and ssp. parvifructa are all very rare in cultivation. There are many more forms which do not fall directly into any particular subspecies. Snow gums are slower growing than most eucalypts and always have thick leaves with parallel veins, and very attractive patchwork bark in shades of orange, red, white, tan, cream and even shiny silver. These generally range in hardiness from about 10°F all the way down to -8°F: quite a large range, so it is important to obtain a superior provenance. Closely related is the very rare, attractive and hardy E. lacrimans, which has strongly weeping branches.
Swamp gums such as E. aggregata (black gum), E. rodwayi (Rodway's black gum), and E. cadens are very hardy and tough and should be used more. They are excellent for difficult soils and situations, including shallow, waterlogged soils. E. subcrenulata (alpine yellow gum) is not quite as resilient, but it does have attractive deep green leaves and bark that peels in strips to reveal green, orange, cream and yellow patches. E. vernicosa (varnished gum) is a small, shrubby relative of E. subcrenulata that inhabits very exposed alpine areas in the wild. All of these are at least as hardy as the well known E. gunnii.
E. crenulata (Buxton gum) is an attractive small tree whose outermost growth is very silvery, while the older growth is green: it looks very interesting when viewed from a distance. It is also somewhat shade tolerant, for a eucalypt, and will tolerate poorly drained sites.
"Ashes" are an interesting eucalypt group that includes some hardy species. E. kybeanenis (Kybean mallee ash) is a small, often multi-trunked tree and very attractive in flower, as is E. stricta (Blue Mountains mallee ash). E. triflora (Pigeon House ash) and E. paliformis (Wadbilliga ash) are especially fantastic-looking flat-topped trees with smooth bark, but they remain little known in cultivation. These species are a little bit on the cold tender side, and are probably suited for sheltered gardens only.
Surprisingly, there are even some very tropical looking smooth-barked eucalypts that will grow in the Pacific Northwest. E. mannifera (manna gum), E. elliptica (brittle gum) and E. scoparia (Wallangarra white gum) come to mind: moderate sized trees with an attractive habit and powdery white bark. E. dalrympleana (mountain gum), E. rubida (candlebark gum) and the rare E. chapmaniana (Bogong gum) are also quite hardy and frequently have attractive smooth bark. While some of these species don't have a great track record for cold hardiness, I believe hardy provenances of most will probably be established here eventually.
E. urnigera (urn gum) and E. morrisbyi (Morrisby's gum) are two Tasmanian species related to E. gunnii (cider gum) and about as cold-hardy, but still relatively uncommon in cultivation. From the pictures I have seen, I think they both make more attractive garden specimens than does E. gunnii. E. morrisbyi is especially magnificent, but it is very rare both in the wild and in cultivation.
E. regnans (Australian mountain ash) is frequently a subject of interest among eucalypt enthusiasts, as it is the world's largest non-coniferous tree, capable of growing taller than 360'. Although it is generally a bit on the tender side for our region, it is not out of the question that some provenances may be successful in milder areas. Mature, cultivated trees of this species in New Zealand have withstood 7°F, but it must be considered that this was a primarily radiational freeze with very little wind, unlike all of our Arcitic blasts: the same temperature here may produce very different results. (Note that, despite the common name, this tree looks absolutely nothing like our Sorbus spp., the mountain ashes of the Northern Hemisphere.)
Other large timber species such as E. nitens (shining gum), E. delegatensis (alpine ash) and E. obliqua (messmate) may eventually grow here as well, but it may take some time for hardy provenances to be developed. Thus far success has been limited. The stringybarks also include some good timber species, some of which are cold-hardy, including E. youmanii (youman's stringybark) and possibly E. laevopinea (silver-leaved stringybark).
E. amygdalina (black peppermint), E. nitida (Smithton peppermint), E. radiata (narrow-leaved peppermint), E. dives (broad-leaved peppermint) and other species belong to a diverse group called the peppermints. Not surprisingly, their leaves smell of peppermint, and in general they tend to be attractive weeping trees. Hardiness varies somewhat in this group, and most remain untested.
Another small group of smooth-barked eucalypts includes E. stellulata (black sallee), which is reasonably well known, E. mitchelliana (Mt. Buffalo gum) and E. moorei (little sallee). These are all quite hardy, and E. mitchelliana is probably the best-looking of the bunch, having an attractive, open, weeping habit.
This concludes a very brief summary of some eucalypts that might be grown in the Pacific Northwest in the future. Indeed, many of them are being tried right now. I should have some exciting findings from in my own garden a few years down the road, which will hopefully pave the way for future research.
I welcome anyone wishing to know more about eucalypts to visit my Hardy Eucalyptus Page, where a complete list of cold-hardy species can be found with more detailed comments and pictures, along with general information about growing eucalypts in cold climates.