Climatic zone plants at Earlscliffe, Baily, Co.
Paper presented at Federation Of Zoological Gardens Of Great Britain And
Ireland, Fifth Annual Plant Group Conference, Zoological Society Of
September 5th - 7th 2001
© Muriel Robinson 2009.
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 Ireland
an essentially maritime climate of mild winters, cool summers and all-the-year
lies nearer to the North Pole than to the Equator and is further north than
. If it were not for the warm westerly winds and the
Gulf Stream ,
would be icebound for part of the winter, somewhat similar to the
Labrador coast of
. In contrast the mean temperature of the coldest month (January) ranges from 7°
C in the south to 4° C in the north. The west, south and much of the east
coastal areas of
are classified as Climatic Zone 9, the same as the Mediterranean coast of
. The remainder of
is Climatic Zone 8.
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
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
has a very restricted natural flora due to the early breakdown of the land
Europe . Consequently
has only two thirds of the 1349 native flowering plants found in
. Only three evergreen tree genera (Arbutus,
Ilex and Taxus) and 13 deciduous
genera (Alnus, Betula, Corylus, Crataegus,
Fraxinus, Malus, Populus, Prunus, Quercus, Rhamnus, Salix, Sorbus and
Ulmus) are native. However the climate encourages the growth of a richer and
more diverse range of exotic plants than any other country of similar latitude
and species from all continents are found in Irish gardens. Of the most commonly
used 1,400 tree and shrub species about 30% were introduced from Asia especially
from Japan and the Himalayas, around 21% from North America, about 20% from
Europe and the remaining 23% are mainly introductions from the southern
hemisphere (Hannan, 1970).
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
city. It has a good microclimate being sheltered on the north by the 180 metres
high Hill of Howth and extending on the south side down to high watermark. Air
frosts occur regularly generally between early December and mid April. The
absolute lowest winter minimum is usually about - 4º C but more severe frosts
occur from time to time. Temperatures of -7.5 º C were recorded in December 1995
and -7.0 º C in December 2000. Except for the 2000/01 winter, freezing days when
the temperature does not rise above 0 º C, are rare. Because of the proximity of
the sea, summer temperatures are moderate, seldom exceeding 23º C. The absolute
maximum recorded since 1969 was 27.5º C in August 1995 during a prolonged warm
spell. Average annual rainfall is only 650 mm but this is spread fairly evenly
throughout the year and periods of drought are rare.
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
, many Climatic Zone 9 plants, such as
Luma apiculata, Solanum laciniatum and
Cordyline australis, propagate themselves freely. Hundreds of
seedlings of these species occur each year, often in areas well away from the
parent plants. As a millennium project, advantage has been taken of the
fecundity of Luma apiculata to establish a small woodland consisting of 100
plants of this species, one for each decade of the 20th century. The
plants for the new woodland were all self sown seedlings dug up from various
parts of the garden, the last tree was planted as dusk was falling on New Year's
Eve 1999. Luma apiculata with its
beautiful cinnamon-coloured bark, grows as happily on the Howth peninsula as it
does in its native Chile and Argentina This tree becomes more beautiful as it
ages and the new woodland should be an attractive feature in about 25 years'
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
Canary Islands , it self seeds freely in many coastal
. Plants can grow from over 1 m to a height of 6 m between February and early
June. The plant flowers in its second or third year and, being monocarpic, dies
after flowering. E. pininana is cross pollinated and produces seeds prolifically (in
excess of 200,000/plant), which germinate readily and dense carpets of
Echium seedlings are commonplace.
Plants developing from these seedlings survive most winters but in hard winters
many are killed by cold. On two occasions (1978/79 and 1986/87) during the last
24 years all plants were killed by temperatures less than - 6º C in January.
Following such severe winters seedlings develop from the seed bank in the soil.
It seems possible that natural selection is producing a more cold hardy strain
of E. pininana, as in two recent cold
winters (1995/6 and 2000/01) when temperatures fell to -7 º C, many seedling
E. pininana, the more beautiful red
flowered Echium wildpretii is not on
the endangered list in the
Island EN-GB">, although in
EN-GB">it is much more tender. Many plants are killed in a cold winter and all
may succumb in an exceptionally cold season. In the 2000/01 winter about 20
plants of potential flowering size were killed and only one survived. Seed is
now being collected from this plant, for sowing in spring 2002, as it may be
inherently hardier than the rest of the population.
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.
climatic zone 9 plants self propagate at Earlscliffe, although in a less
profligate manner. These include Dicksonia
, Lomatia ferruginea, Callistemon
citrinus, Erica arborea, Phormium tenax and
Dodonaea viscosa 'Purpurea'.
EN-GB">does not self propagate in soil but in peat sods stored in open polythene
bags stored behind a north wall.
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
New South Wales
have survived. No species endemic in
survived for more than a few years, although
E. pauciflora and
viminalis occur in
as well as
New South Wales
The beautiful leaf-scented E.
citriodora from latitudes 20 to 25º in
survived for a few years until it was killed by the severe 1995/96 winter.
Growth of most of the surviving species was rapid compared with native trees.
Some tree heights measured in October 1976 from seed sown in February of a
previous year were:- 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.
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
Southern Australia in January/February 1996 to collect
seed of Corymbia ficifolia. Although recognising that the chance of success
was not high, it seemed worthwhile to collect seed of
C. ficifolia where it occurs naturally and also from other less
favourable areas. Seedlings could then be raised and planted out in
and, although most would inevitably be killed by winter cold, one or two
seedlings might prove hardy. These could then be propagated by tissue culture.
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
is less than 300 metres above sea level so that there might not be a great
difference in the harshness of the climate between the two areas. Seed was
collected from these areas of natural occurrence and also from trees that had
been planted in other colder parts of
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
to propagate these few survivors by tissue culture.
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
as they belong to different sub-genera. However, one of the red flowered
E. caesia belongs to the same subgenus
as E. pulverulenta and
E. gunnii and these species will
hybridise. About 100 flowers of E.
pulverulenta were pollinated with pollen of
E. caesia flown in from
in May 1996. Tests for viability on this pollen showed that it was lacking in
vigour on arrival in
. Only five capsules set seed following pollination and the subsequent seedlings
were weak and did not survive the first season. A tree of
E. caesia has been planted in a
greenhouse at the National Botanic Gardens and it is planned to use pollen from
this tree when it flowers in further crossings.
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 2001although 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
New South Wales
in the family Monimiaceae. This tree
is interesting, not so much for its aromatic foliage or other attributes but for
the fact that it grows in association with
Wollemia nobilis (the Wollemy pine) in the remote canyon where this 'fossil'
tree was discovered east of
in 1994. D. sassafras was planted at
Earlscliffe 1981 and is now 5 metres, tall. As it has never been damaged by
winter frost, this suggests that the Wollemy pine may also survive in mild parts
and should be widely tested when it becomes available.
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
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
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
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
: A rare opportunity to visit the Private Hacienda,
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.
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