Cold hardy species for the PNW
(Eucalyptus are limited due to Covid-19 seed import restrictions)
Eucalyptus are very unique trees and there are several things you should consider before planting. First, you must be prepared for them to grow explosively fast. Soils in Australia are very old and poor. Eucalyptus are adapted to these conditions. Even unamended soils in our region are richer in nutrients in comparison to native Australian soil. Rich soil combined with ample moisture results in Eucalyptus that thrive with little assistance from the gardener. Plant them in unamended soil and irrigate steadily through the first dry summer. In subsequent years it will be completely drought tolerant. They grow in a variety of soils, including heavy clay. Give them room to grow- this means plan ahead. They can grow up to 6’ a year. This rapid growth can leave trees susceptible to strong winter winds and combined with wet soil leave them vulnerable to wind rock. To avoid this do not amend the soil or fertilize. If growth is proceeding rapidly you may not water at all. Hardy Eucalyptus are for the most part very drought tolerant. They also tolerate regular irrigation (especially the water loving species E. parvula). Only stake plants less than 3′ tall- from that height on let it form its own sturdy trunk or trunks. Eucalyptus are completely intolerant of shade in our climate. Give them at least 6 hours a day of full sun in an open exposure for the strongest growth and sturdiest trunks.
For certain species- but not the Snow gums- unstable trees may be cut back by 1/3rd after the first season in the ground. This will ensure that top growth to root growth ratio is proportional and this will eliminate the threat of rocking.
All of the species that we grow have displayed excellent, LONG TERM survival in Western Oregon. That means that they will not be killed – or even injured in the harshest winters. That having been said there is no true arctic air in Australia. These trees are native to the naturally coldest locations in Australia, high elevations and frosty mountain valleys where winter temperatures are the same or much colder than our own. But topsy turvy freezes early and late can cause more damage and I have yet to find a Eucalyptus that will endure subzero temperatures as a tree. Several have resprouted from subzero lows but none have retained top growth. There are many Eucalyptus that will live and grow prodigiously in our climate for a few years- and they can achieve serious height, only to freeze completely to the ground or die in a colder than average winter. This is not a good way to go. Its expensive, dangerous and a royal pain in the ass to deal with a suddenly dead tree. Many of these borderline species will resprout if you are lucky but we don’t recommend this for the average gardener. Fully hardy, carefree species are what we promote.
For cut foliage:
Many Eucalyptus species have striking juvenile foliage that will turn to adult foliage with a much different look as they grow into arboreal form. The most familiar juvenile foliage is perfoliate which is the popular cut material of the florist trade. To retain this attractive juvenile foliage (or just to limit size) Eucalyptus may be cut back to the ground. “Coppicing” as this is called will result in a shrub that re-grows with multiple trunks.
This can also happen when a tree freezes to the ground and is possibly an adaptation to regeneration after bush fires. In Australia Eucalyptus with this form are called Mallees (pronounced Mollies). To achieve this it is best that you allow at least one to two years for the tree to establish a large root system. Coppicing is ideally done between March 1st and April 15th. This gives the tree a long growing season to reestablish new growth before the following winter. It may seem extreme at the time, when you find yourself staring at stumps, but new growth will begin in four to six weeks. Be patient. Once they start to re-grow it proceeds rapidly.
Snow gums the most cold hardy and reliable Eucalyptus in our region can NOT be coppiced for cut foliage. They have adult foliage from the get go and do not resprout from the base if cut. The individual plant descriptions will mention if this method if it is possible for each species.
Climate Adapted Plants for Gardeners in the PNW