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Selection Of Eucalyptus Species For Garden Use In Cool Temperate Conditions

International Symposium on Selection and Breeding of Woody Ornamentals. ISHS Acta Horticulturae 320, 25-30, Angers, France, 1 December 1992. Karen Foley 2016

Abstract

Between 1969 and 1976, seedlings of 102 eucalyptus species were planted in a coastal garden at Baily, Dublin, Ireland (53.4 N latitude) to test their hardiness and suitability as ornamental trees.

Although many species were killed by low temperatures, including all those from from Western Australia, Queensland and South Australia, plants of 37 species from New South Wales, Victoria and Tasmania are still alive and growing well in 1992, 16 to 23 years after planting.

The results suggests that a range of species, in addition to E. gunnii and others normally recommended, could be planted in mild areas in Ireland for garden decoration. Among those which survived temperatures down to -6 C and are suitable for this purpose are:- E. acaciaeformis (small, green, sessile juvenile foliage), E. cinerea (blue green ovate, sessile foliage), E. kitsoniana (bright green foliage and floriferous) and E. linearis (graceful habit, linear green leaves).

Introduction

Ireland has only about 1,400 species of flowering plants and ferns compared with approximately 2,500 species in Britain. However, the temperate climate enables many exotic plants to flourish. Species from Australasia are well adapted to conditions in many parts of Ireland and a number of Australian plants are 'naturalised' in some Irish gardens.

One of the most popular of these is the genus Eucalyptus. Eucalyps are widely planted in milder areas because of their many attractive features. These include beauty of foliage, bark and flowers, speed of growth and scent. Eucalyptus trees in Ireland are free from practically all the pests and diseases that affect them in Australia and so the foliage usually remains completely blemish-free except for wind scorch towards the end of the winter.

Eucalyptus gunnii is the most widely planted because of its reputation for hardiness. Other species grown in Ireland are E. johnstoni, E. urnigera, E. viminalis, E. delegatensis and E. globulus (Morley, 1979). Other species that have survived and continue to thrive in the climate of the UK and Ireland are E. coccifera, E. pauciflora, E.parvifolia, E. vernicosa and E. niphophila (Penfold et al, 1961).

Evans (1983) has classified 25 eucalyptus species that have grown in Britain into the following categories:-

  • Very hardy - likely to survive long cold spells of -10 C to -14 C. or short periods down to -18 C  E. debeuzevillei, gunnii, parvifolia, niphophila and perrineana.
  • Hardy - as above, but unlikely to survive colder than -16 C. E. archeri, coccifera, glaucescens and vernicosa.
  • Moderately hardy - likely to survive long cold spells of - 6 to -9 C, or short periods down to -16 C. E. aggregata, dalrympleana, delegatensis, nitida, pauciflora, stellulata, subcrenulata and urnigera.
  • Less hardy - likely to survive long cold spells down to -6 C or short periods down to -9 C.  E. cordata, fraxinoides, globulus, johnstoni, nitens, nicholi, pulverulenta and viminalis.

However, some 500 species of Eucalyptus are recognised (Bean, 1973) ranging from tall trees to shrubs and small, stunted plants. Because only a small number of possible eucalypts have been tested in Ireland, a wider range was examined for their hardiness and suitablilty as ornamental trees. Moreover, as pointed out by Penfold and Willis (1961), although the importance of eucalypts to Irish forestry is small, Ireland is a useful testing ground, where the behaviour of eucalypts may provide much useful information about climatic adaptability.

Materials and methods

Between 1969 and 1980, seedlings of 102 different eucalyptus species were planted in a coastal garden Earlscliffe, located on the Howth peninsula just north of Dublin, Ireland (53.4 N latitude and 6 W longitude). For details of the situation of the garden and soil type see Robinson (1992).

Seed, obtained mainly from Kew Gardens London, Mr F.W.Walker, Department of Agriculture, Hobart, Tasmania and Mr A.H.Crane, Chief Commissioner, Forestry Commission, Hobart, Tasmania was sown in a peat compost at Kinsealy Research Centre, Dublin usually in February or March. The seedlings were transplanted to peat pots when large enough to handle and were planted out in their final position usually in June or July of the same year when they were 150 - 300 mm high.

The eucalypts tested included species from Tasmania, Victoria, New South Wales, South Australia, Queensland and Western Australia. The following species (usually three seedlings of each) were planted:-   E. acaciaeformis, aggregata, alba, albida, amygdalina, andreana, astringens, baxteri, bicostata, blakelyi, blaxlandi, bridgesiana, caesia, calophylla, camalduensis, campaspe, camphora, carnea, cinerea, citriodora, cladocalyx, coccifera, cordata, cordieri, cornuta, dalrympleana, deanei, delegatensis, diversifolia, doratoxylon, elaeophora, eremophila, erythronema, falcata, fastigata, ficifolia, forrestiana, glaucescens, globulus, gomphocephala, goniocalyx, grossa, guilfoylei, gunnii, huberiana, jacksoni, johnstoni, kitsoniana, kruseana, laevopinea, lansdowneana, lehmanni, leucoxylon, linearis, longifolia, macarthuri, maculata, maideni, McKieana, macrorrhynca mitchelliana, neglecta, nicholi, nortoni, notabilis, niphophila, nitens, notabilis, nova anglica, nutans, obliqua, oblonga, obtusifolia var. Dendromorpha, occidentalis, ovata, parvifolia, pauciflora var. Bothwell, peperita, perrineana, polyanthemos, preissiana, pulverulenta, punctata, radiata, regnans, resinifera, risdoni, rubida, saligna, simondsi, steedmanii, stellulata, stricklandi, stricta, stuartiana, sturgessiana, tenuiramus (tasmanica), tetraptera, urnigera, viminalis, wandoo and woodwardi.

The eucalyptus were kept weed free mainly by two applications of simazine at 1.7 kg/ha each year applied in April and July, plus spot treatment of other appropriate herbicides as required. [*]The heights of surviving trees were recorded in the early years. It became necessary to reduce the size of most of the trees because of their rapid growth and height measurement was discontinued.

Minimum air temperatures were recorded at Earlscliffe and meteorological data were recorded at Danesfort, Baily, about 400 m due north of the test site.

Results

For general information on the climate in Baily see Robinson (1992). The lowest temperature recorded during the period was -6.0 C, which occurred on a number of nights in the period 31 December 1978 - 3 January 1979 and on 13/14 January 1987. Low temperatures of - 5.0/5.5 C were recorded on a number of other occasions, but some winters were very mild. Typical figures for lowest minimum air temperatures for a five year period are shown in Table 1.

Table 1 - Lowest minimum air temperature ( C) recorded each month at Danesfort, Baily, Co Dublin, 1969-1973.  

Yr

 

 

 

Jan

 

 

Feb

 

 

Mar

 

 

Apr

 

 

Ma

 

 

Jun

 

 

Jul

 

 

Aug

 

 

Sep

 

 

Oct

 

 

Nov

 

 

Dec

 

 

1969

 

 

-3.3

 

 

 

-5.5

 

 

-0.3

 

 

 

-0.3

 

 

3.1

 

 

5.3

 

 

7.0

 

 

7.5

 

 

3.4

 

 

3.1

 

 

-1.6

 

 

-1.1

 

 

70

 

 

-5.5

 

 

-2.8

 

 

-2.5

 

 

-0.8

 

 

7.0

 

 

9.2

 

 

6.7

 

 

7.8

 

 

6.1

 

 

2.8

 

 

-0.5

 

 

-2.0

 

 

71

 

 

-1.7

 

 

-1.1

 

 

0.6

 

 

1.7

 

 

3.9

 

 

4.2

 

 

6.7

 

 

7.5

 

 

7.0

 

 

6.1

 

 

-1.7

 

 

1.4

 

 

72

 

 

-3.6

 

 

-2.5

 

 

0.0

 

 

2.0

 

 

2.2

 

 

4.1

 

 

7.5

 

 

7.2

 

 

2.8

 

 

1.5

 

 

-2.5

 

 

-0.6

 

 

73

 

 

-0.6

 

 

-2.5

 

 

1.1

 

 

-1.9

 

 

1.1

 

 

5.0

 

 

6.7

 

 

8.3

 

 

5.0

 

 

0.6

 

 

-3.2

 

 

-1.7

 

As expected, a number of the Eucalyptus species failed to survive their first winter after planting. This applied particularly to those species from warmer parts of Australia, such as E. citriodora from Queensland .

Plants of the very attractive Western Australian eucalypt E. ficifolia planted in June 1972 were killed in the following winter. One of two plants overwintered under glass and planted out in May 1973 blossomed between September and November, 1976 with flowers of a good red colour. This tree was badly damaged during the 1976/77 winter and, although it made some recovery during the 1977 summer, it died subsequently.

Many other tender Eucalyptus species were killed by the unusually prolonged cold spell during the period 31 December 1978 - 3 January 1979 .  This cold period killed all the surviving species from Western Australia except E. kruseana and cornuta but these species also died subsequently. The Western Australian species that died during this period or earlier included E. astringens, calophylla, campaspe, carnei, cornuta, doratoxylon, eremophila, erythronema, falcata, forrestiana, gomphocephala, grossa, jacksoni, lehmanni, leucoxylon, nutans, occidentalis, preissiana, steedmanii, stricklandi, tetraptera and woodwardi.

Many other species died in the early 1980s, but in June 1992, 16 to 23 years after planting, 37 species still survive.  Of the species tested in Baily the following appear to be the most hardy:-  E. acaciaeformis, aggregata, blaxlandi, camphora, cinerea, cordata, coccifera, cordieri, dalrympleana, delegatensis, fastigata, glaucescens, globulus, goniocalyx, gunnii, johnstoni,  kitsoniana,  linearis,  niphophila,  nitens, ovata, parvifolia, pauciflora, perrineana, pulverulenta, risdoni, stellulata, tenuiramis (tasmanica), urnigera and viminalis.  Somewhat less hardy, although still surviving after 20 years are:- E. baxteri, blakelyi, camalduensis. elaeophora, laevopinea, nicholi and polyanthemos.  All the species in the most hardy group are native of Tasmania, Victoria and/or New South Wales and those in the less hardy group are native of Victoria and/or New South Wales . No species endemic in Western Australia, South Australia or Queensland survive, although E. pauciflora and viminalis occur in South Australia as well as Victoria and New South Wales .

Growth of most of the species that survive is rapid compared with native trees. Some tree heights measured in October 1975 from seed sown in February of a previous year are as follows:- after 4 seasons E. parvifolia 4 m; after 5 seasons E. maideni 8.4 m, E. cordata 9 m, E. pauciflora var Bothwell 7 m, E. dalrympleana 7 m; after 7 seasons E. goniocalyx 6.7 m, E. delegatensis 8 m, E. johnstoni 8 m, E. urnigera 8 m.

No serious problems occurred with pests or diseases. Occasionally aphides and the Blue Gum Psylla (Ctenarytaina eucalypti) occurred, particularly on E. pulverulenta, but caused little damage.

Discussion

The major factor influencing the survival of eucalypts is the minimum winter temperature and not spring frosts, but many other factors are involved (Martin, 1948). Seed provenance is important, but so also are soil conditions, for plants will stand a lower temperature on a well-drained site better than on a poorly drained one.

Weather conditions prior to the onset of a cold period are also relevant. If the eucalypts are gradually exposed to low temperatures in the autumn and early winter, they will withstand frost conditions more effectively than if the temperature drops rapidly before any 'hardening off' process occurs.

None of the species listed as hardy in Evans' (1983) classification was killed in this study and the results of the trial are in general agreement with his groupings although not all the species listed by Evans were included in this trial.

The results suggest that a number of additional species are worthy of testing in mild areas of Britain and Ireland to obtain further information on their hardiness.  The following should be included in future trials:-

  • E. acaciaeformis, blaxlandi, camphora, cinerea, cordieri, fastigata, goniocalyx, kitsoniana, linearis, ovata, polyanthemos, risdoni, and tenuiramis, as all have survived temperatures as low as -6 C.

In addition to the species normally grown in Britain and Ireland and to those listed by Evans (1983), there are several relatively hardy eucalypts that would appear to have especial merit as garden plants. Four attractive species not previously mentioned in the available literature on Eucalyptus in Britain and Ireland are:-

  • E. acaciaeformis, cinerea, kitsoniana and linearis.
  • E. acaciaeformis. In Ireland this eucalypt grows into an attractive tree with small glaucous leaves about 50 x 20 mm. Although it grows up to 25 m in its native New South Wales , it can easily be maintained as a small ornamental garden tree 3 to 4 m tall by judicious pruning.
  • E, cinerea. This species, from New South Wales and Victoria , has attractive glaucous, blue-grey foliage and rough fibrous bark on the trunk, but not on the branches. it does not make a large tree and is highly ornamental.
  • E. kitsoniana. This tree grows to about 5 m and is notable for its bright green foliage and prolific flowering from sessile buds usually in large clusters of seven.    
  • E. linearis. This attractive species has light green, long, linear leaves, approximately 70 x 5 mm and with a strong peppermint smell. It makes a graceful tree about 6 m tall with a light crown.

The State of Western Australia is noted for the large number of attractive ecualypts with brilliant coloured flowers including red, pink, orange and yellow. The results in this trial suggest that none of these would be sufficiently hardy for growing in Ireland . Nevertheless the fact that E. ficifolia did survive for several years and flowered in 1976 suggests that there might be opportunities for hybridising it with a more cold tolerant species. The natural climate could be used to eliminate the less hardy seedlings, and the survivors screened for their horticultural qualities.

The results show that a wider range of eucalyptus species than had previously been tested will grow satisfactorily in mild areas in Ireland and planting of further Eucalyptus species continues.

References

  • Bean, W.J., 1973. Trees and shrubs hardy in the British  Isles . Eight Edition. M.Bean and John Murray (Publishers  Ltd.) 50 Albemarle Street, London W1X 4BD , 125-138.
  • Evans, J., 1983. Notes on growing Eucalyptus in Britain . Forestry Commission Booklet 50. HMSO, 49 High Holborn, London WC1V 6HB. pp31.
  • Martin, D,, 1948. Eucalyptus in the British Isles . Australian Forestry 12, 64-74.
  • Morley, B.D., 1979. The contribution of southern hemisphere plants to Irish gardens. Irish gardening and horticulture, (Eds. C. Nelson and A. Brady), Royal Horticultural Society of Ireland, Dublin. 153-166.
  • Penfold, A.R. and Willis, J.L.,1961. The Eucalypts. Botany, chemistry, cultivation and utilization. Leonard Hill Books Ltd. 9 Eden Street, London NW1:123-125.
  • Robinson, D.W., 1992. Increase of cold hardiness of Echium pininana through natural selection. ISHS Symposium on Selection and breeding of woody ornamentals. Acta Horticulturae (In press).

[* Footnote. Since this article was written, the herbicide, simazine, has been banned in Europe under Commission Decisions 2004/141/EC(3), 2004/248/EC(4), 2004/140/EC(5) and 2004/247/EC(6), taken within the framework of Council Directive 91/414/EEC of 15 July 1991. This came into effect on 26th April 2004.]

 

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