Cold Hardiness of Five Eucalypts in Northern California

Thirty years ago, I began planting eucalypts in my garden in Concord, California, located about thirty miles inland from San Francisco (in Sunset zone 15). California’s mediterranean climate is ideal climate for temperate and near sub-tropical gardening. Rainfall is usually confined to the winter months, and, in most years, frosty nights are sporadic and rarely cause any major damage.

The problem in growing most eucalypts in Northern California is that, unlike Australia, the San Francisco Bay Area does experience, every ten years or so, a severe freeze when nighttime temperatures drop into the low 20s F (‑4° to ‑6° C). Temperatures this low can destroy many Eucalyptus species. If they are not killed outright, they will be reduced to stumps and the new growth will resprout along major branches, along the trunk, or from below ground level. This behavior, generally, is not acceptable to most gardeners, who expect mature trees to remain, at least, the same size year after year.

Of the many eucalypts in my garden, five are of particular interest because of their beautiful foliage and flowers: Eucalyptus erythrocorys, E. macrocarpa, E. grossa, E. gillii, and E. citriodora. None of them are common in Northern California gardens. The trials and tribulations of these five are characteristic of the challenges in growing many eucalypts in regions experiencing occasional freezes.

Five Eucalpyts

Eucalyptus grossa is a compact shrub, occasionally a small scraggly tree, with thick, ovate, dark green leaves. Flowers are yellow and the fruits (woody capsules) are reminiscent of clusters of fat little sausages. It is native to Western Australia, south of Perth. I started my plant from seed in 1975 and planted it in the ground two years later. It has yet to reach a height of six feet.

Eucalyptus gillii is a small tree with attractive glaucous, cordate leaves. Flowers are yellow and are produced in dense clusters. It is native to the North Flinders Range, north of Adelaide, South Australia. I purchased my tree from a local amateur grower in 1985.

Red-cap gum (Eucalyptus erythrocorys) is possibly the most beautiful of all small eucalypts, with long lanceolate to falcate, dark green leaves, brilliant vermilion red flower bud caps (reminiscent of a cardinal’s hat), and large yellow green flowers turning butter yellow as they mature. Capsules are large and bell shaped (one-and-a-quarter by two inches). It is native to Western Australia’s coastal area south of Shark Bay and north of Perth. I purchased my tree in 1975 at a University of California Botanical Garden plant sale. It has never attained more than twenty feet in height.

Eucalyptus macrocarpa is a spreading, sprawling shrub with multiple trunks. Leaves are broadly ovate to elliptical and the size of a human hand. The overlapping gray leaves, like loose scales clustered along the branches, have a reptilian look. The flowers are the largest in the genus, being over three inches in diameter when completely opened. The pale rose stamens with golden anthers are a brilliant contrast to the ash gray background of the foliage. The capsules are squat, conical and can attain two-and-a-half inches in diameter. It is native to interior parts of Western Australia, northeast of Perth. I started my tree from seed in 1975 and planted it in the ground two years later.

The four Eucalyptus species described above are all mallees: shrubs or small trees, generally with many stems arising from ground level. All have a lignotuber, a woody swelling at the base of the trunk that contains dormant growth buds. The significance of the lignotuber will become obvious later.

Lemon-scented gum (Eucalyptus citriodora) can grow into a tall slender, single-trunked tree of seventy-five feet or more in height, with a white and gray dimpled trunk and dark green, narrow lanceolate leaves. The flowers are pale yellowish white but are barely noticeable on mature trees because they are fifty feet or more in the air. The leaves contain citronella, an essential oil; even when completely dry, the leaves give off a strong citrus aroma when crushed. Lemon-scented gum is commonly grown in Southern California as far north as Santa Barbara, where Highway 101 is lined with beautiful mature specimens. It is native to coastal Queensland, from Mackay to Maryborough, straddling the tropic of Capricorn. I purchased mine from an amateur grower in 1985. This species is occasionally planted in parks and on campuses nearer San Francisco Bay.

The five profiled eucalypts share my garden with approximately twenty-five other members of the genus, several grevillea, four deciduous fruit trees, six citrus trees, many hardy palms, and several temperate bamboos.

The Big Freeze of 1990

By 1990, all of these eucalypts had been in the ground for many years. Because they had all been planted as seedlings and were never watered after their first year in the ground, they had sent down deep roots into the heavy clay adobe soil characteristic of this region. Adobe is notorious baking rock hard in summer and becoming wet, heavy, sticky, and almost impossible to work with in the winter. However, about two feet below the surface there is always some moisture in the summer and the water table in our area can be just twenty feet below ground level. There had not been a major freeze since 1972, and even the drought years in the late 70s did not slow down the growth of my eucalypts.

In mid-December of 1990, California experienced the most severe freeze in recorded history (about 150 years). A massive body of cold air that had accumulated over central Alaska was pushed south along the coast in late December. What made this freeze particularly deadly was the extremely dry conditions in California at the time. There was virtually no moisture in the air to form frost. Many plants were “freeze-dried” in one night. When the sun came out in the mornings, more damage was done. From December 19 through December 25, the nighttime temperatures in Concord were in the mid-20s F (-3 to -5° C), with a low of 19°F (-7° C) recorded in my garden on December 22. The temperatures were slightly higher near San Francisco Bay but even colder in some pockets away from the water; The Ruth Bancroft Garden in nearby Walnut Creek recorded temperatures in the low teens. By Christmas, there were few green plants left in Concord except for conifers. Mexican fan palm (Washingtonia robusta), Canary Island date palm (Phoenix canariensis), acacias, blue gum (Eucalyptus globulus), and citrus trees were all brown and lifeless.

With the exception of Eucalyptus gunnii and several cold hardy palms (Sabal, Chamaerops, Trachycarpus, Butia), my entire yard was left in several shades of sepia by the freeze. I thought that eighteen years of gardening had been destroyed in a few days. I purchased a chain saw, expecting to harvest my eucalypts for firewood. There was nothing to do but wait. I knew from the experience in the Oakland Hills after the freeze of 1972 that few of the blue gums had actually died during that freeze. Most recovered, or would have had they not been cut to the ground by hasty home owners or municipal employees. I hoped that my small eucalyptus grove would recover in a similar manner.

In February, I began to notice life returning to some of the eucalypts and other trees and shrubs. By April, it was clear how much of each tree was really alive and which plants were completely dead. I started removing dead branches and, in many cases, entire trees. I cut all five of these eucalypts to the ground, because it was obvious that there was no above-ground life in the small trees. Much to my surprise, all five eventually resprouted from the bases of the trunks or from below ground level. The lemon-scented gum sent up seven stems that eventually became trunks. I left them all for two years until I was certain which were the dominant ones; then I removed all but two. In three years, I had two trees in place of the one I had cut to the ground. By mid 1991, the other four species had recovered and, in three years, there was no noticeable difference except for the red-cap gum, which never regained its attractive shape. Eventually one stem became the dominant trunk, and, with some help from supporting wires anchored to the side of my porch, the tree attained its former height. When it flowered, the blossoms were just as magnificent as they had been years before.

The ability of mallees to recover from below ground level by sprouting from the lignotuber is a survival strategy for dealing with fire damage in Australia. Fire is a way of life for eucalypts there; freezing winters, however, are not. However, the damage to the plant is much the same: all living tissue above the soil level is destroyed. Recovery has to come from the roots and undamaged lignotuber below ground level.

The Next Onslaught

By the summer of 1998 there was no evidence of the damage done in the winter of 1990. Actually, the garden had recovered within three years; I had lost only a few trees (more palms than eucalypts) and in the intervening years, everything in my garden had added height and girth. The winters had been mild all through the decade. There had only been occasional freezing nights and some winters were so mild that even bougainvillea and canna suffered little or no damage.

There was nothing different about the approaching winter of 1998 until December 21, when the temperature dropped to 28°F (-2° C). The next four nights saw temperatures in the mid twenties; the lowest was December 24 when I recorded 24°F (-4.5° C) in my garden. The daytime temperatures were always in the mid-40s F (6-7° C). It was the most severe winter freeze in eight years, and every morning, I checked my garden for damage. There did not appear to be much: no damage to my citrus or any of the large eucalypts or any of the palms (including Chamaedorea radicalis and Phoenix reclinata). There was some noticeable frost damage to the red-cap gum and to Eucalyptus macrocarpa but nothing to worry about. I began to formulate a theory about cold hardiness in my neighborhood. It appeared that at 24-25°F (-4°  C), damage would occur and at five degrees colder, damage would be severe.

About one week later, it was obvious that the much milder freeze of 1998 actually did much more damage to my five exotic eucalypts and a few other trees than I had originally thought. All five eucalypts were severely frost damaged; it had just taken longer for the damage to become evident. There was not a green (or gray) leaf left. The only other frost damage I experienced was to Phoenix reclinata. Not one of the other eucalypts showed any sign of frost damage nor did any of the other palms. By mid-January, it was clear that these five eucalypts had been killed to ground level, just as in 1990. So much for formulated theories. Since there was no pressing need to rid my garden of dead trees as in the spring of 1991, I left the five eucalypts alone to see what would happen.

Spring of 1999 was one of the coolest that I can recall and probably delayed the recovery of all of the damaged trees. The first species to show any signs of life was Eucalyptus grossa, which leafed out at the base of its trunk. Eucalyptus macrocarpa initially resprouted about four feet up on the main trunk, but these sprouts aborted three weeks later; then came new growth from below ground level. Eucalyptus gillii also resprouted from below ground level. Red-cap gum resprouted from the base of the trunk and again became a straggly mess of a shrub that needed to be supported.

The real surprise was lemon-scented gum. I expected it to resprout from its base or from below ground level as it had in spring of 1991. However, in April, I noticed new growth almost at the top of the trunks, about thirty feet above ground level. The new stems and leaves emerged directly from the two main trunks; all of the side branches were dead and eventually fell off. By the middle of the summer, the two trunks were encased in new green foliage right down to three feet above ground level. I wondered if the tree might have responded in the same manner, had I not cut the original tree down in the early spring of 1991.

Most gardeners would not tolerate trees that are killed to the ground every decade, but I have so much time and effort invested in these plants that I cannot part with them. I think of it as an investment in lignotubers and root stock, which would be extremely difficult to replace; who could re-supply twenty-five-year-old root stock? The recovery of these eucalypts is really rapid and I have had the pleasure of their evergreen growth in nine years out of every ten.