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Paper presented at Federation Of Zoological Gardens Of Great Britain And
Ireland, Fifth Annual Plant Group Conference, Zoological Society Of
Introduction to the Irish climate
The Gulf Stream and the North Atlantic drift, originating in the Gulf of
Mexico, have a major influence on the climate in Ireland and consequently on our
exotic flora. This warm current and the predominantly south westerly winds that
blow over it give
lies nearer to the North Pole than to the Equator and is further north than
Summer temperatures are comparatively low. The mean daily temperature for the warmest month (July) is 14.5° C - 16° C for most lowland areas. Prolonged summer heat and extreme winter cold are uncommon. In the south west of the country there is only 8° C difference in temperature between the means of the coldest and warmest months and only 10 to 11° C difference in most of the rest of the country.
Rainfall varies from 650 to 1000 mm in the drier east and midlands and from over 1,000 to over 2,000 in the south and west. Rain falls in every month of the year, although there is a tendency for the period March to June to be the driest and December the wettest. Each of the 12 months has been the wettest or the driest in some year.
Native and exotic species in
the mild climate and ample rainfall encourages the growth of a wide range of
Many Zone 9 plants have been planted in a garden at Earlscliffe, Baily, Co Dublin between 1969 and 2001 and the remainder of this contribution deals with a few of these plants. A more extensive list of the plants grown is given in the following paper. As in most mild gardens in Ireland some of the Zone 9 plants grown here come from the northern hemisphere, notably the Canary Islands, the majority come from South America, South Africa and Australasia.
The garden at Earlscliffe, Baily, Co.
Earlscliffe is a coastal garden on the Howth peninsula just north of
The soil at Earlscliffe is derived from Cambrian shale and quartzite and
contains approximately 25% clay and 4.5% organic matter in the top 8 cm. It is
mostly acid to neutral (pH 5.0 - 6.5) in contrast to the alkalinity of the
limestone derived soil of most of
Self sown seedlings
When a exotic plant spreads by means of self sown seedlings it indicates that
local conditions are suitable. As in other mild areas in
The kangaroo apple (Solanum laciniatum) with violet blue flowers up to 5 cm wide followed by orange yellow egg-shaped fruits, can grow to a height of over 3 metres. All plants were killed by - 7º C in December 2000 but large numbers of seedling appeared during the following season and made rapid growth.
The most notable self seeder at Earlscliffe is the Giant Borage,
Echium pininana. Although this is an
endangered species in its native
E. pininana, the more beautiful red
flowered Echium wildpretii is not on
the endangered list in the
Although grown since 1990, Echium
wildpretii failed to self seed until spring 2001, when scores of seedlings
appeared in areas where the plant had been grown the previous year. This
suggests that natural selection is producing a more cold hardy strain of this
species also. Hybrids between E wildpretii
and E. pininana occur
occasionally. These are also striking plants being intermediate in height,
colour and type of inflorescence between the parents. The hybrids appear to be
hardier than either of the parents as leaf wilting during periods of low
temperature is less severe than with either of the parents and none of the
hybrids has been killed by winter frost.
other climatic zone 9 plants self propagate at Earlscliffe, although in a less
profligate manner. These include Dicksonia
Eucalyptus species at Earlscliffe
Several Eucalyptus species also
self seed including E. gunnii, E.
globulus, E. cordata, E. urnigera and
E. pulverulenta. These are only a few of approximately 130 species that have
been planted at Earlscliffe to test for hardiness and suitability for cut
foliage since 1969. About 60 species mainly native of
The beautiful leaf-scented E.
citriodora from latitudes 20 to 25º in
The red flowering gum Corymbia ficifolia (formerly Eucalyptus ficifolia) is a most striking tree carrying masses of brilliant red flowers. In addition the leaves are attractive being like a Ficus but with red mid-rib and margins. It is severely damaged when the temperature falls below - 4º C. At Earlscliffe this eucalypt survived the 1974/75 and 1975/76 winters well and flowered between September and November 1976. The tree failed to set seed, was severely injured the following winter and died in spring 1977.
In an attempt to produce a eucalypt with red flowers and which would survive
the Irish winter, a six week visit was made to Western and
Unlike many eucalyptus species, C. ficifolia has a very restricted natural range and occurs only in
two small areas - inland about 30 miles north east from the town of Walpole near
Mount Frankland in south west Australia and right on the coast about nine miles
east of Walpole. The area near
The seed, sown at Earlscliffe in cold glass in April 1996, germinated
readily. Some seedlings were planted out in autumn and the majority in spring
1997. Subsequent winters were mild and all seedlings grew well without injury
until December 2000 when many were about 3 metres tall. The severe frost from
December 26th to 30th killed back all seedlings although
the damage did not show until late January. In May 2001 about 5% of seedlings
resprouted from the base and an attempt is being made by staff at
In addition to this attempt to allow the climate to select out a hardier
strain of C. ficifolia, a small
breeding programme is also underway with the same purpose.
C.ficifolia will not hybridise with
any of the hardy Eucalypts that survive in
Palms at Earlscliffe
In addition to the widely grown Trachycarpus fortunei and Chamaerops humilis, several other members of the family Palmae grow at Earlscliffe. The rarest of the palms is the Chonta (Juania australis) from the Juan Fernandez islands (Robinson Crusoe's island). Planted in September 1995, Juania has grown well and has shown no signs of cold or wind damage throughout this period and is pest and disease free. It is now about 2.5 metres tall with leaves up to 2.2 metres in length. Juania australis is cited in the literature as 'the ungrowable palm' (Morici, 2001), 'difficult to grow' (Endt, 1996) or 'most efforts to bring it into cultivation have failed (Gunter and Mahalik, 1990). Morici (2000) states that the cultural needs of this palm are quite peculiar and that only one mature specimen survives away from its native home (in Santiago de Chile).
With this reputation, the ease with which Juania australis is growing at Earlcliffe is surprising. During the 6 years since planting, it has received no attention apart from an occasional directed spray of glyphosate to remove any small weeds developing at its base.
Jubaea chilensis (the Chilean Wine palm) is reputed to be a slow growing, long lived tree with a life span of 700 to 1,000 years. Planted around 1971, it is still only 1.4 metres in height in August 2001 although the circumference at ground level is over 70 cm. The tree is growing at approximately 7 cm/year which is less than a tree in Torquay (South Devon) which grew to a height of 7 metres tall in approximately 72 years (about 9.6 cm/year) (Taylor, 1990). The tree at Earlscliffe has survived all winters since the early 1970s without obvious signs of injury and continues to grow slowly. Because of its apparent hardiness, it should make an interesting contribution to the palm collection at Earlscliffe in the 22nd century!
Doryophora sassafras is an evergreen tree from
Despite its location at 53.3º N latitude, Earlscliffe has a remarkably wide range of Climatic Zone 9 plants (and some that are erroneously classified in the New RHS Dictionary of Gardening as Zone 10). These include Cyperus alternifolius, Ochagavea carnea, Araucaria bidwillii and Itoa orientalis. The favourable microclimate is the major factor contributing to the survival of these plants. However, it seems likely that the method of soil management contributes to some degree to tolerance of low temperature injury.
Since 1969 weeds have been controlled almost entirely by herbicides, mainly glyphosate, paraquat and simazine* and the soil is not disturbed by cultivation after planting. No fertilisers are used on trees and shrubs and plants are not given artificial protection from low temperatures during the winter. Simazine is applied as an overall spray on all tree and shrub plantings except where Protea and Banksia are grown. These genera are susceptible to low doses of simazine but most woody plants are tolerant. The use of a herbicide programme without cultivation encourages plants to develop an extensive surface root system.
Numerous root excavations suggest that many plants will develop preferentially roots close to the soil surface, if moisture and temperature conditions and absence of mechanical damage permit them to do so. This occurs because the surface soil tends to be richer in oxygen and nutrients than deeper layers.
Many experiments between the 1950s and 1970s show that fruit crops grow more vigorously and crop more heavily on herbicide-treated plots than on those cultivated, mulched or grassed down (Robinson 1963, 1978). Depressed yields with the latter treatments were attributed to repeated root damage on cultivated plots, to lower temperatures where mulches were used and to competition for moisture and nutrients on grassed plots. It seems likely that the response of ornamental trees and shrubs would be similar to that of woody fruit crops.
It is widely recognised that well grown plants are better able to cope with cold than plants in a weakened condition. A correlation between the development of frost hardiness and an increase in sugar content has been demonstrated for a large number but not for all plants (Kramer and Kozlowski, 1979). The exact role of sugar is still debated but it has been suggested that high sugar decreases the freezing point by accumulating in the vacuoles and decreasing the amount of ice formed (Levitt, 1972). Because of the link between plant well-being and tolerance of low temperature, many cultural factors are relevant, including nutrition and method of soil management.
Excavation of the previously undisturbed root system of a South African heath Erica canaliculata showed that most of the root system was in the top 5 cm of soil. Hoeing around this plants or hand pulling established weeds of annual meadow grass (Poa annua) and other fibrous-rooted weeds would damage the root system of the Erica and result in a less cold tolerant plant.
Herbicides are not merely an alternative method of weed control to cultivation. Used judiciously they are also, indirectly, growth promoting substances and contribute to the successful cultivation of tender plants in marginally suitable areas.
- Endt, D. (1991). The native palms of
Chile Las Palmas Principes, 35 (1), 19 - 21.
- Gunter, B. and Mahalik, P. (1990). Juania australis in Habitat.
Principes, 34 (2), 79 -81.
- Hannan, A. (1979). Personal communication.
- Kramer, P.J. and Kozlowski, T.T. (1979).
Physiology of woody plants. Academic
- Levitt, J. (1972). 'Responses of plants to environmental stresses.'
- Morici, C. (2000). Paschalococos and the disappearing palms. Chamaerops 40, 5 - 7.
- Robinson, D. W. (1963). Investigations on the elimination of cultivation in bush fruit crops. Proceedings XVIth International Horticultural Congress, 1962, III: 270 - 275
- Robinson, D. W. and O'Kennedy, N. D. (1978). The effect of overall herbicide systems of soil management on growth and yield of apple trees. Scientia Horticulturae, 9, 227 -236.
[* 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.]
[* 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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