Cold hardiness and Climatic Zone 9 plants
[Paper prepared for the International Magnolia Society 2001 Annual Meeting
held in Dublin, Ireland. © Muriel Robinson 2008
See also the additional notes made for this conference on
plant hardiness.
Although my garden at Earlscliffe, Baily, Co
Dublin
is situated at 53º north latitude it has a favourable microclimate and contains
many climatic zone 9 plants. The harsh 2000/2001 winter, the most severe since I
started taking records in 1969, provided a good opportunity to record the effect
of low temperatures on marginally hardy plants.
The 2000/01 winter
Autumn 2000 was mild and wet with temperatures well above average in early
December. However, on the night of December 26/27, 2000 the air temperature fell
to - 6º and to - 4º, - 7º, - 5.5º and - 4.5º C on the following four nights. For
the three days between the 28th and 30th the temperature
never rose above - 3º or - 4º C during the day. At the end of the cold
spell (December 30), the temperature rose 7.5 C º during the night and was +3º
by morning, so that the speed of thawing was rapid.
After a relatively mild January, another cold spell began on February 23/24,
2001 and the minimum air temperatures recorded on this and the following 9 days
were - 2º, - 2º, - 1.5º, - 2.5º - 2.5º, - 5.5º, - 5º, - 5º, - 5º and
- 1º C. A severe northerly gale blew up on the night of February 25/26 and the
snow that fell that night persisted in shaded places for a full week. For an
area that had seen little snow in the last 30 years, these conditions were
exceptional.
Apart from temperature records, the response of many plants confirms the
severity of the winter. Young plants of the banana
Musa basjoo were killed to ground
level, although they had survived all previous winters with stems intact since
this species was first planted in 1973.
Erica canaliculata the most vigorous and one of the most hardy of the South
African heaths growing at Earlscliffe, survives most winters unscathed or, in a
cold winter, a few cm of unripened shoot tips may die back. The 2000/01 winter
killed back previous year's growth on some plants by over 50 cm.
As one
of my interests is growing trees and shrubs that are close to their climatic
limit, it is inevitable that a cold winter will kill many of the more tender
plants. I do not regard plants killed in this way a tragic loss but rather as an
educational experience, an opportunity to gather fresh information and a chance
to plant up again with more frost tender plants.
To obtain an accurate picture of plant hardiness, no fleece or other
artificial means of protecting plants is used no matter how severe the frost and
no plants are overwintered under cover.
Factors influencing plant hardiness
Plant hardiness is mainly influenced by the absolute minimum temperature but
this is only one of many factors involved. Others include the duration of
freezing temperatures, the speed of drop, the hardening up process, the speed of
thaw, the moisture content of the plant, soil and air, and the soil type.
Healthy, well grown plants are better able to cope with cold than plants in a
debilitated 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. 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. Others believe that both
sugars and amino acids can protect specific sensitive proteins and enzymes from
the effect of freezing.
Because of the link between plant well being and tolerance of low
temperature, a wide range of cultural factors are relevant, including nutrition,
freedom from pests and diseases and general husbandry. The morphology of the
plants also plays a part with some plants having especially susceptible stem,
buds or roots. In addition, other climatic factors such as wind chill and depth
of snow cover may play a negative or positive role.
Previously it has been suggested that high fertility predisposes plants to
low temperature injury. With food crops this has been shown to be false. Plants
fertilised for optimum growth or yield are not unduly susceptible to low
temperatures; in contrast nutrient deficient plants and those overly fertilised
so that yield is depressed are susceptible. It seems likely that these results
are also linked to sugar levels within the plant.
Hardening up process
With the approach of winter, important differences are evident between many
climatic zone 8 plants and those in zone 9. Zone 8 plants undergo a hardening up
process under the influence of photoperiod and falling temperatures. With
temperatures in the range of 1 and 5 º C plant growth gradually ceases,
metabolic changes take place in plant proteins, soft growing tips lose moisture
and become more lignified. In contrast many zone 9 plants harden up only slowly
or not at all and many, including a large number of
Eucalyptus species continue to grow
slowly during a mild winter. Consequently they are susceptible to damage by
sub-zero temperatures.
Low temperatures can kill plants in different ways. In highly susceptible
plants, the contents of the cell may freeze and death occurs rapidly due to
disruption of protoplasm. In other plants, however, ice crystals are first
formed outside the cells in the walls and intercellular space as a result of the
antifreeze effect of the cell contents. Further cooling leads to the growth of
these crystals with the water being drawn from the cells. When this occurs the
plant may die slowly as a result of desiccation and this is now believed to be a
more common cause of plant death than cell disruption by ice crystals (Levitt,
1972).
Contrary to previous widely held beliefs, it is now thought that the
formation of ice in the intercellular spaces of itself is not responsible for
plant death (Kramer and Kozlowski, 1979). The view that plants tolerate low
temperatures because the freezing point of protoplasm is lowered as water moved
out of the cell has also now been largely discredited.
When plants are killed immediately due to intracellular freezing, e.g. many
succulents such as Aeonium tabuliforme,
the plant collapses quickly as mushy pulp. Within a few weeks the remains of
many non-lignified plants killed in this way have virtually disappeared.
Plants that are killed by cell desiccation, e.g. many plants with small to
medium sized, firm leaves such as Metrosideros excelsa, Psoralea pinnata and
Eucalyptus ficifolia, do not show the effects of low temperature injury for many weeks. Their leaves
gradually wilt and turn brown and the plant shows typical symptoms of drought.
At sub lethal temperatures, some plants with large fleshy leaves, e.g. the Giant
Borage, Echium pininana, wilt
repeatedly when the temperature drops at night below approximately - 3º C and
regain turgidity when the temperature rises again during the day, presumably as
water moves in and out of cells into intercellular space. With experience it is
possible to gauge approximately the temperature between -2 º C and - 8 º C by
the extent of the flaccidity of the Echium
leaves.
Because so many plant processes are involved when a plant is severely damaged
by frost and many different parts of the plant may be affected, it is not
possible to be certain whether a particular plant will live or die under
specific temperature conditions. Evergreen plants that are damaged by low
temperatures may absciss their leaves cleanly or, in cases of more severe
injury, may hold onto the dead foliage for many months. In general, plants that
absciss their damaged leaves appear to have a better chance of survival than
those that hold onto them in a dead condition.
Leaf abscission is controlled by plant hormones and it is generally believed
that auxins prevent abscission as long as the leaf blade is healthy and growing
(Leopold and Kriedemann, 1975). In the case of deciduous species in the autumn
and evergreen plants with damaged leaves, auxin flow diminishes and the plants
produce ethylene, which triggers leaf fall.
Where death or severe injury occurs suddenly, the plant's hormonal control
mechanism is destroyed and so foliage dies but does not fall. In these cases the
chances of recovery by resprouting from the above ground part of the plant are
very remote. However many plants affected in this way can resprout strongly from
soil level. The likelihood of regrowth from adventitious buds on stems and
branches appears to be greater where leaf abscission has occurred.
Effects of the 2000/2001 winter on
Zone 9 plants at Earlscliffe
Zone 9 plants at Earlscliffe reacted in many different ways to the 2000/01
winter. Many fleshy succulents, that contain much water, were especially
susceptible to freezing conditions and collapsed quickly during the cold spell
in December. Plants included in this group were:-
Aeonium tabuliforme, A. arboreum 'Atropurpureum'
and many other Aeonium species.
Aeonium balsamiferum, although
damaged, appeared to be the most hardy
of the species tested and many plants survived. Another
Canary
Island
succulent, Greenovia aurea, was more
tolerant of freezing conditions than
Aeonium spp, although its response varied from no discernible effect to
severe blackening of foliage and death in different parts of the garden.
Solanum laciniatum, the soft subshrub
Solanum quitoense (z 10) and young
plant of Musa sikkimensis and
M. basjoo were killed back to ground
level.
Not all plants with a high moisture content were sensitive to low temperature
damage. A number of plants with fleshy leaves, including some
Agave species,
Aloe striatula, A.aristata, and
Beschorneria yuccoides, all survived
completely unharmed. Among the Agaves, A
parryi, A.ferox and A.
americana
were uninjured but all the outside leaves of
A sisalana were desiccated, although
the youngest leaves in the heart of the plant remained green.
Furcraea longaeva, another succulent
related to Agave, reacted differently;
the older leaves were undamaged but some of the younger leaves turned
temporarily chlorotic with a few dead patches but the plant quickly recovered
when temperatures rose in the spring.
The foliage of a 4 metre high plant of
Brugmansia sanguinea collapsed immediately the first severe frost arrived.
However, although the entire top growth was killed, much of the root system
remained alive during the winter and, as after previous hard winters, the plant
began to resprout strongly from the base in mid-June 2001. In contrast, plants
that were slow to die but are unlikely to recover include the blue flowering
South African tree Psoralea pinnata, Corynocarpus
laevegata from
New Zealand
.
Frost hardiness and sugar content
The link between frost hardiness and sugar content was demonstrated by the
variable response of seedlings of Echium pininana, the Giant Borage from the
Canary Islands , naturalised in a
Betula woodland. This plant is hardy
in a normal winter but in 2000/01 many plants were killed in the middle the
woodland but survived in the woodland fringe. It seems likely that sugar levels
were low in plants in the shade inside the wood and plants died even though the
canopy of branches would have given extra protection from frost. The higher
sugar levels of the plants grown in the open and in the woodland fringe probably
accounts for their greater tolerance.
Differences within genera
As a result of their evolutionary history, big differences often occur in the
susceptibility of species within a genus.
Euryops virgineus started flowering in December and continued until June at
least without showing any signs of damage. Some plants of
E. pectinatus suffered moderate leaf
scorching and dieback but survived while E. chrysanthemoides died quickly. The roots of this plant appeared
dead in January and recovery was not expected.
Metrosideros robusta was not affected
in any way by low temperatures but plants of
M. excelsa were killed back to ground level. Among the tree ferns,
Dicksonia
antarctica
, D. squarosa and
Cyathea dealbata showed only slight signs of damage but
C. cooperi was killed.
As would be expected from a genus covering a vast geographical area,
different species of Eucalyptus
responded differently to the low temperatures. All
Eucalyptus ficifolia saplings raised
from seed collected from the wild in
Western Australia
in January 1996, were killed back to ground level. Several hundred plants
survived the mild winters between 1996/97 and 1999/2000 in good condition but
were unable to withstand temperatures of -6º and -7º after Christmas 2000. About
6% of these saplings showed some regrowth from their base in June 2001 although
their survival is far from assured. A few plants raised from seed
collected from higher elevations, have been severely damaged but appear to be
somewhat less susceptible to low temperature injury than the progeny of seed
collected in the plant's areas of natural distribution.
E. curtisii ffrom the
Brisbane
area of
Queensland
was also severely scorched and is unlikely to live. Approximately 70 other
species of Eucalyptus, mainly from
New South Wales
,
Victoria
and
Tasmania
, survived in good condition. These include
Eucalyptus crenulata from
Victoria
which continued to grow throughout the winter without any significant injury to
the expanding young foliage.
Leaf abscission and plant recovery.
With few exceptions, no regrowth from branches occurred on evergreen plants
that held onto their dead foliage after the cold spells in December and
February. Plants in this group included:-
Casuarina equisetifolia, Erica x
hiemalis, Erica versicolor, Agathosma ciliata, Cineraria 'Purple
picotee' and Psoralea pinnata.
However, many plants which had their top growth killed, resprouted strongly from
the base such as Pelargonium
papillionaceum and Sonchus arboreus.
Nerium oleander was an exception insofar as epicormic growth
occurred in June although most of the dead foliage was still attached to the
plant.
Other normally evergreen plants which abscissed all their foliage cleanly
after low temperature injury included Itoa orientalis, Tetrapanax papyfera and
Grevillea robusta.. Plants in this group have produced leaves from
adventitious buds on branches although those of
Grevillea robusta are still weak in late June.
Root system
The location of roots in the soil is important both with regard to plant
health and also freezing injury. Examination of the root system of many plants
growing at Earlscliffe shows that a large proportion have most of their roots
close to the soil surface. This is due to the absence of severe drought,
relatively shallow soil and the routine use of herbicides to control weeds so
that the soil surface is not disturbed.
Stem tissues are usually less sensitive to low temperature injury than root
tissue, but roots of trees and shrubs are injured less in winter because the
soil (and snow) cover protects them to some extent from exposure to freezing
temperatures. In theory plants with their roots close to the soil surface,
should be especially susceptible but there was no evidence of this under
conditions at Earlscliffe. Nevertheless when soil freezes, roots are often
damaged, especially the small physiologically important ones.
A large number of roots were examined on
February 18, 2001 and as far
as could be ascertained root damage on many plants was minimal compared with the
severe injury to the above ground portion. Despite extensive damage to foliage,
stems and branches, the roots of many plants appeared to be turgid, moist and
fresh. Plants in this category which eventually recovered included:-
Brugmansia sanquineum, Tetrapanax papyfera,
Sonchus arboreus, Strobilanthes pentstemonoides, Metrosideros excelsus and a
small number of saplings of Eucalyptus
ficifolia. Plants with apparently healthy roots in February which failed to
recover included:- , Cyathea cooperi,
Psoralea pinnata and many saplings of
Eucalyptus ficifolia. With some plants it was clear that both top and root
were severely damaged e.g. Solanum
quitoense, Europys chrysanthemoides and
Aeonium spp and that recovery was
unlikely.
It is clear from the way so many plants resprout strongly from the base, six
months or more after the entire top growth has been killed that, in many plants,
the root system is less susceptible to low temperature injury than the above
ground part of the plant.
The fact that a large part of the root system of most plants at Earlscliffe
is close to the soil surface may be an overall advantage rather than a
disadvantage. Roots growing in the surface soil layers seem to be especially
important to plants, due partly to the fact that this region is richer in
nutrients and oxygen. It has also been shown that young roots are the main
production sites in plants for some essential growth hormones such as
gibberellins and cytokinins (Scott Russell, 1978). Gibberellins promote
stem elongation by stimulating cell division. Cytokinins stimulate cell
enlargement as well as cell division and so promote leaf expansion. On balance,
the horticultural advantages of allowing roots to exploit the soil surface seem
to outweigh any possible disadvantage due to increased risk of low temperature
injury.
Survivors
Although large numbers of plants have been killed or severely injured, even
larger numbers of other zone 9 plants have apparently survived unharmed or with
slight damage only. All palm trees appear to have come through the winter well.
These include Trachycarpus fortunei,
Chamaerops humilis,,
Phoenix
canariensis, Jubaea chilensis and
young plants of Washingtonia robusta, W. filifera, Brahea armata and
Butia capitata. The rare palm from the
Juan Fernandez Islands
Juania australis showed no signs of
low temperature injury and continues to grow strongly.
Many tender conifers have also survived in good condition, such as the Rimu,
Dacrydium cupressinum, D. franklinii,
Callitris rhomboidea and C. oblonga.
Other non coniferous survivors include:-
Pseudopanax arboreus, A. ferox and A. laetus, Telopea speciosissima,
Bromeliads including Ochagavea carnea
and Fascicularia bicolor and many
species of Puya. The aromatic tree
Doryophora sassifras is also in good
condition. This plant has assumed a special interest in recent years as, in its
native
New South Wales
, it grows alongside the soon to be released 'fossil tree' the Wollemy pine,
Wollemia nobilis,
Protea lacticolor showed slight
scorching only of unripened shoot tips and the King Protea,
P. cynaroides has also survived
although with some leaf damage Banksia
spinulosa flowered in November 2000 and was little affected by winter cold
but the combination of a wet blanket of snow and gale force winds on the night
of February 26/27, split off several main branches. The grass tree, Xanthorrhoea preissii appeared to be in as good condition in
spring as it was when it entered the winter. It was planted in August 1995 and
has been slow to recover from transplanting shock.
Increase in hardiness of
Echium pininana by natural selection
Although
Echium pininana, the Giant Borage is
an endangered species in its native
Canary Islands , it self seeds freely in many coastal
areas of
Ireland
. areas of
Ireland
. 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 low temperatures of -6º C in January. After these severe winters new
seedlings develop from the seed bank in the soil.
It seems possible that natural selection from cold stress 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 survived.
Conclusion
It is obvious that a large number of climatic, physiological, genetic and
cultural factors are involved in plant tolerance to low temperature. In
addition, marked differences in microclimate can occur in a small area, even on
a garden scale. Consequently, it would appear that the only way to find out if a
zone 9 plant is hardy in a warm temperate areas is to plant it and see what
happens.
The response of some tender plants to the severe 2000/2001 winter.
