Climate Change





Hardiness - a case study (Winter 2008-2009 in the U.K.)
(Revised February 2012)

Frost-damaged Crassula ovata
A frosted and drooping Crassula ovata resembles an unpleasant-looking being from outer space. Growing in a pot close to the house it had been subject to temperatures in the region of - 3.5C. The plant survived, just, after loosing the whole of the exposed part of the trunk and subsequently re-shooting from the very base.

Although we can adjust conditions for less hardy plants in order to give that extra little margin necessary to get them through some harsh winter conditions there is little we can do when the inevitable 'big freeze' occurs. The one that springs to mind is the extreme wintry period of 1963-3 when the whole country, including the South-West, was affected. It is doubtful whether any unprotected bananas or cannas growing outside survived it. Even many Cordylines were cut down to the base or worse.

It is ironic that during the preparation of these pages and while arranging the garden for the 2009 NGS Open Days we have experienced during the night of 6-7 January quite a severe, for Torquay at least, frost: -1 to -2C is generally the lowest that the more tender plants in the garden will take without serious damage - assuming an average summer and not too damp a soil. This irony is further enhanced by the global warming hysteria surrounding us all. The max-min thermometer on the house wall showed -3C indicating that in the garden the temperature was at least -4C (quite likely -4.5 to -5C). Thankfully, unlike the 1962-3 episode, this was a very brief spell, the air was perfectly still and the soil was very dry.
Frost-damaged leaves on Phoenix canariensis
The beginning of 2009: frost-damaged Phoenix canariensis at Forches Cross, Devon. Although new leaves re-grew later in the year the setback for the plant will be very significant.
Nevertheless, a significant number of plants were scorched or had their stems (leaves, shoots & flowers) softened. While many shot from the ground during springtime some, of course, failed. I am fortunate to have placed a thick insulating mulch of wood chippings around the base of many vulnerable plants in late 2008 and I believe that in some cases this was helpful.

It is difficult to judge whether the relatively cold period which followed - intermittent mild frosts of -1C to -2C and lasting up to the beginning of March - made matters worse. But they certainly did not allow immediate recovery in cases where recovery was possible.

The cold spell experienced at the beginning of January was notable not for its duration, for it was quite short, but for its surprising ferocity. Such low temperatures following a period of very poor summertime hardening of stems and so on might be expected to cause widespread damage. I am not convinced that this is the case - at least here at Southern Comfort. One thing is sure though, it will have set back the 'exotic garden project', which some thought might botanically transform the U. K. into a Caribbean island, by several years. I dread to think how many gardeners will now be in need of counseling following the loss of carefully tended but possibly ill-conceived exotic horticultural enclaves. I can count myself very lucky because the only severe and terminal damage experienced in the garden was of easily replaceable stock. Here, no large or mature plants were lost or badly damaged except for three out of four Phoenix roebelenii (the survivor was quite small and planted in an exposed site - it was still recovering in September), three very small potted Rhopalostylis sapida in sheltered positions, a Cyathea medullaris in relatively sheltered position, and a rather splendid large pot-grown Crassula ovata with a 5" diameter trunk. This plant began to re-shoot from lower points on the trunk.

One plant of which I had several seedlings (cheaply bought for about £1.50 a tiny pot full) is the Parlour Palm, Chamaedorea elegans. All were either in pots outside or planted out. About 50% of these plants survived and many showed new growth already at the beginning of March indicating that this palm is probably much hardier than previously suspected.

Even -5C was in some ways fortunate since some places only a few miles inland had much lower temperatures that night. At Forches Cross (Newton Abbot) the temperature was reported to have been around -12C in the greenhouse of Plants Galore. Outside the greenhouse two large £1200 Phoenix canariensis in tubs had all their foliage scorched and inside all tubs of Strelitzia nicolae were fully blackened while some Aloe arborescens had the ends of their leaves damaged - similar to what happened outside at Southern Comfort. The temperature at the nursery at Trevena Cross in Cornwall fell to -8C and similar temperatures were noted at Hill House Nursery in Landscove, Devon. The normally very mild and sheltered garden at Coleton Fishacre had temperatures of -6C and some losses.

Temperatures of -4.5 to -5C in our part of Torquay are so rare that the plants in column A (below) in my view most certainly worth the risk - especially since not every plant was severely or terminally affected. But it is a very clearly a risk. In the case of damage, replacement of these plants is probably the best option since recovery, if it occurs, may be very slow. If possible, the over-wintering of 'insurance plants' taken from cuttings and placed the greenhouse is probably a good idea. The plants in column B are a better bet and the risk with them becomes significant only at even lower temperatures; these are very infrequent indeed.

Plant damage @ -4.5C to -5C (Meadfoot, Torquay. Devon) - climatic zone: 9b. Provisional list with constant update.
These plants previously remained undamaged @ -1C (brief periods).


Plants lost:


Plants showing severe to medium damage but expected to make a full recovery:


Plants showing some signs of stress; normally evergreen plants becoming defoliated; minor leaf-tip injury etc.; but no real damage:

Aloe 'Lizard Lips' (immediate 'meltdown')
Cuphea ignea
(plant lost; a neighbour has reported that his plant has survived)
Cyathea medullaris (total loss of mature fronds; unfurled fronds resumed growth in mid-April; growth not vigorous and plant appeared severely weakened; by September it is becoming clear that the plant will probably not survive)
Ensete ventricosum - type (this plant was in a pot and did not resume growth in the spring, unlike the 'Maurelii' variety which was in the ground and resumed growth relatively early)
'Thalia' (roots appear undamaged but plant not re-shooting)
Grevillea johnsonii (this plant was not initially planted in an ideal position; all top growth lost - plant began to re-shoot from base in June but growth was weak and the plant was finally lost; this plant was expected to survive easily; it will be tried again in a very open, sunny and well-drained position)
Haworthia retusa (delayed 'meltdown' after a few days)
Ipomoea learii
Lotus berthelotii
Lotus maculatus
Passiflora x exoniensis (a small plant, not fully established)
Phoenix roebelenii (three plants lost; one survived also plant in neighbour's sheltered garden survived)
Protea cynaroides (all top growth lost; began re-shooting from base but finally lost. This was an unestablished plant - less than a season in position and the soil was satisfactory for it but not ideal.)
Rhopalostylis sapida (three small plants; larger 1m plant survived in neighbour's garden)
Tillandsia cyanea (loss not unexpected even during a mild winter)


Some of these plants could be used as summer bedding with the clear assumption that they will survive only an exceptionally mild winter. This would make sense in the case of those which are inexpensive or those whose cuttings are easy to overwinter: Cuphea, Fuchsia, Ipomoea, Lotus.

Plants like Rhopalostylis sapida, Phoenix roebelenii, Cyathea medullaris are too expensive in terms of purchase price and care to risk losing and therefore not recommended but the somewhat lower price of the Ensete ventricosum (which does sometimes survive) may make it tempting.

Tillandsia cyanea, the more tender aloes and Haworthias are probably not worth bothering with; they all proved to be very sensitive.

The remaining plants - Protea cynaroides, Grevillea johnsonii and Passiflora x exoniensis would probably stand a very good chance if they were give ideal soil and position. I shall certainly continue with them.

Aloe arborescens (tips of many leaves browned)
Aloe bainesii (slight, some damage to leaf tips)
Aloe cooperi (slight, some older leaves lost)
Aloe perfoliata
Aloe pretoriensis (slight)
Aloe reitzii (slight)
Aloe striata (slight)
Aloe vera (extensive 'meltdown' of leaves; this plant is almost impossible outside here in Torquay at the best of times; most plants lost)
Aeonium undulatum (damage to top growth but re-shooting from stem and from base)
Aeonium arboreum (some stems frozen through)
Begonia luxurians (all top growth lost; unexpectedly plant re-shot from base late May)
Billbergia nutans (10-20% of clumps lost)
Canna spp. (all top growth lost - this is generally not unusual; one newly planted C. musifolia lost)
Carpobrotus (terminal damage on 80% of all growth, depending on location)
Cestrum nocturnum (all top growth killed; re-shot from base in mid-April)
Clerodendrum bungei 'Pink Diamond'
Clerodendrum philippinum
(all top growth lost; re-shot from base in July - growth was healthy and vigorous but did not make up for the winter loss of bulk)
Chlorophytum comosum
'Vittatum' ( about 10% of plant survived)
Clivia miniata (most top growth lost)
Crassula arborescens
(all leaves and ends of branches lost - re-shooting from lower stem)
Crassula ovata (plant lost to base of trunk - base damaged but re-shooting)
Dahlia imperialis
(plant in pot, sheltered, some leaf damage only)
Echium pininana (70-80% of all plants lost; some tiny seedlings survived)
Ensete ventricosum 'Maurelii' (plant in pot lost but plant in open ground survived; leaf loss total)
Gunnera magellanica (90% of small clump lost)
Hedychium coronarium (very rapid recovery after seemingly severe loss of foliage. However, even after a few years, this plant has never flowered.)
Heliconia scheideana
(several, but not all, shoots cut to ground level; re-shot from centre of one damaged shoot and also numerous shoots from base. Looking normal by September)
Hoya carnosa (90% of top growth lost, recovery from base)
Impatiens tinctoria (all top growth lost, re-shooting from base to full recovery)
Lampranthus spp. (various degrees depending on plant, but most recovered rapidly)
Lapageria rosea (slight, younger growth lost)
Musa basjoo (all leaves lost; one plant cut down to ground level)
Musa sikkimensis 'Red Tiger' (all top growth lost)
Musa (=Musella) lasiocarpa (all leaves scorched)
Nerium oleander (slight, some plants)

Ornithogalum longibracteatum (large bulb lost; small offsets unaffected)
Pandorea jasminoides (some growing tips lost - bulk of plant undamaged)
Phlebodium sp
Phoenix roebelenii (one small plant in open part of garden survived, three lost)
Sparrmannia africana (extensive damage, plant re-shot from root and more than made up for lost growth)
Strelitzia reginae (one plant lost in exposed position. Main plant, probably as a result of the frost, did not flower in 2009)
Tibouchina urvilleana (All top growth lost - re-shot from base in April and more than made up for lost growth)
Washingtonia filifera (leaf damage on plants in more exposed sites; one plant lost all leaves but recovered fully by September)



Of the succulents growing outside only the following eventually recovered successfully and continued to grow during subsequent years: Aloe arborescens, Aeonium arboreum, A. undulatum, Carpobrotus.

Agapanthus sp.
Agave attenuata (pot, under cover)
Aloe brevifolia
Aloe plicatilis
(some leaf tip scorch)
Beschorneria septentrionalis (patches of mould in some leaf bases)
Bismarckia nobilis (pot, under cover - tips of older leaves browned; expected a loss)
Cycas revoluta
(slight scorching of leaflets)
Dicksonia antarctica

Euphorbia rigida
Euphorbia mellifera
Fascicularia bicolor
x Fatshedera lizei 'Annemeike'
Furcraea longaeva
Hedychium spp. (except H. coronarium)
Jasminum polyanthum
Jasminum officinalis

Ledebouria socialis (leaf loss of potted plant)
Melianthus major (bulk of leaves scorched)
Myosotidium hortensia
Phoenix canariensis (very slight almost unnoticeable damage: some ends of leaflets browned)
Tillandsia aeranthos (possible damage to flower only, plant untouched)
Zantedeschia aethiopica (all top growth lost)

The following plants, and there are some surprises, appeared to be unscathed. They are all obviously low-risk plants and worth trying.

'Exotics'/semi-tender/marginally hardy plants seemingly unaffected by -4.5C to -5C (selected plants only)


Butia capitata
Chamaedorea microspadix
Cordyline australis
Cordyline indivisa

Eucalyptus pauciflora niphophila
Magnolia garandiflora
Olea europaea
Phoenix dactylifera
Sabal minor
Syagrus romanzoffiana


Cestrum 'Newellii'
Correa alba
Crinodendron hookeri
Desfontainia spinosa
Mahonia lomariifolia
Schefflera taiwaniana
Yucca spp.

Herbaceous succulents

Aeonium arboreum 'Zwartkop'
Aloe striatula

Aloe aristata
Aloe ferox
Aloe humilis
Aloe krapohliana
Aloe maculata
Aloe peglerae
Aloe variegata
Agave (all except A. attenuata)
Crassula sarcocaulis
Echeveria spp. (most)
Euphorbia characias
Haworthia attenuata

Sedum praealtum

Herbaceous other

Amaryllis belladonna
Aspidistra elatior
Astelia chathamica
Blechnum chilense
Begonia sutherlandii
Begonia evansiana
Crinum moorei
Crinum x powellii
Cyperus alternifolius (in water)
Dasylirion texanum
Geranium palmatum
Helleborus lividus
Hippeastrum (Red, ex Hill House)
Phormium spp.
Rhodocoma capensis
Woodwardia radicans

The strange Colocasia - is it hardy in the UK?

Colocasias (and Alocasias) are currently in vogue. The size, shape and colour of their leaves imparts a strong exotic flavour to their immediate surroundings. They are plants, which in the garbage-talk of modern garden designers, "make a statement".

What interests me are the conditions they require to grow and thrive. I have only recently tried growing Colocasias and have discovered very contradictory recommendations. On the one hand, they are bog plants (or, at least, require very damp soil) and on they other they must be kept dry during winter in order to avoid rotting away. It is not the cold that kills them but the fungal infections resulting from prolonged contact with damp at a low temperatures. At these low temperatures the plant is pushed into dormancy and becomes particularly susceptible to infection. So far, so good. Dig them up and keep them dry over the winter period and replant outside in mid-spring (or later). If temperatures are high enough sooner or later (in my experience, later) the plant will break out of its dormant state and resume growth. This works admirably but involves digging the plants up and replanting them - something which I wish to avoid.

It is reported that in the United States, where summer temperatures are higher, Colocasias are able to survive quite low winter temperatures, much lower than those we experience in the UK. If this is true it does raise some interesting questions. Is it because of the higher temperatures in the summer that the Colocasias survive the winters or is it because the effects of damp and fungus decline with the further lowering of temperature? Or perhaps a combination of both?

I was interested to observe that several corms of Colocasia esculenta which I had in pots outside (and in the ground) during the winter 2008-09 appeared to survive the cold spells of January 2009 despite being in very damp conditions. The corms remained firm. They were left outside and continued to remain firm for a couple of months when quite suddenly they began to rot at a galloping rate. Although the degree of damage was high, some material was easily salvaged, dried out, replanted and grew normally. This would appear to indicate that rot and not temperature is the culprit. However, two plants (one C. esculenta 'Fontanesii' and one species) appear to have over-wintered in the ground successfully to produce leaves in June; no protection had been provided but - and this is probably very significant - the soil was very well-drained.

Growing Colocasias in a very well-drained porous medium would certainly give them a better chance of over-wintering outside. Also, the occasional dousing of the corm with fungicide during the cold months would go some way to increasing the plant's chances of survival.


The above tables do not take into account size of plant or position of plant in garden. Thus a Strelitzia reginae growing close to the south-facing wall suffered few ill-effects while one clump in an open position was killed outright.

Outlook and tips for the future

It is always disappointing to see plants - especially those which are diligently looked after for years, structurally integrated into a landscape and wonderfully mature - succumbing to a terminal strike during a viciously cold spell. This is precisely why I would not recommend either spending vast sums on mature but only marginally hardy palms or pinning too many hopes on similarly delicate young plants the for the long term. Not taking heed may be particularly risky today when we are probably approaching a period of general cooling of the climate (see above table of 'facts about global warming'). There is a fairly wide base of reliably hardy (in the South-West at least) feather and fan palms to choose from. The genera Phoenix, Brahea, Butia, Sabal, Washingtonia and Syagrus all have species which resist several degrees of frost. Chamaerops humilis and Jubaea chilensis are both hardy here, all species of Trachycarpus are extremely frost-resistant, and Chamaedorea microspadix seems unaffected by -5C. Whether they will all grow at the levels of heat we have is another matter and experimentation here is probably less traumatic.

From a gardening point of view the above selection would probably cover most needs and more. All the other species will, sooner or later, end up being 'zapped'. The larger they are, the greater the disappointment. Therefore my advice is: plant sensibly and use Trachycarpus where you can. The good old 'Trachy' will not let you down and will even become a weed if you are lucky.

Looking through catalogues I can see that there is a vogue for a variety of species which are most unlikely to survive anything below -1 to -2C. Have a go by all means but blame only yourself when you feel you need counseling. Having said this, I must admit that I have one or two, now sad-looking, objects floating around; they once proudly bore the name of 'palm'. They may recover but I fear that eventually they will go. Their place would be better filled by a more robust species which would reliably grow and thrive for years to come.

Herbaceous plants and shrubs are another matter. They are more easily replaced and propagated. They are generally cheaper as well.


Winter 2009-2010
(updated 1 April, 2010)

I hope that I am being optimistic by describing the nights December 17-21 in Torquay as exceptionally cold for the time of year. In the more open parts of the the garden temperatures dropped to about -2C, there was little wind and the temperature did not rise much above zero during the day. On the night of 18-19 December it dropped to as low as -3 to -4C. This is not very much warmer than the temperatures of -4C to -5C which were experienced here in January of this year. The cold was due to a clear skies with a north-easterly wind which brought snow to eastern parts of the UK and bitterly cold temperatures coupled with snow-induced chaos in Eastern Europe. Ironically again, the cold made itself felt in Copenhagen during the 'Global Warming Summit' where delegates were supposedly seeking to combat rising temperatures at the planet's surface by the introduction of draconian legal mechanisms.

Damage was expected but it became clear very soon that the extent of the damage was very considerably less than in January 2009 when the temperature was 1C less. This is good news especially after a 'lousy' summer where not much ripening of growth would have been expected. However, this was only the beginning. For about 2½ months night temperatures dropped regularly to between minus 2-3C and barely rose above zero during the day. On several occasions there was lying snow. The length of the cold spell and the associated dampness during this period was no doubt more aggressive the the colder, but briefer, spell last winter.


Winter 2010-2011

This winter is now the fourth in a run of bad winters and the third in a run of exceptionally bad winters. Although Southern Comfort escaped the worst of the the British West Country weather the low temperatures - every bit a severe as those of 2008-2009 - set in very early. At the end of November there were rapid drops to -2C and even to -3C with temperature barely rising during daytime. This got progressively worse during December with night temperatures dropping to almost -5C in the run-up to Christmas. In comparison to other parts of the country where lows of up to -20C were recorded this was indeed 'mild'.

I see no point in carrying out another case study of the effects of the cold. However, I shall include information on any losses of established palms, bananas, tree ferns and other relatively cold-resistant exotics. This year I have given my vulnerable tub plants inside protection and have dug up some aloes and aeoniums and also placed them under cover. There they have been so far assured of a minimum of 0C. This is not something I like to do but it is preferable to losing them. I am sure that other prudent gardeners would have done the same.

I have written elsewhere that the evidence for expecting cooler temperatures in the future is more robust than that for warming. In any case, there is no harm in exercising some caution now and certainly there will be financial savings if resistant species are planted outside. All species of Trachycarpus (and there are at least eight) will survive all but the most grueling lows we can expect in this country; the magnificent Jubaea chilensis has a trunk diameter of up to 180 cm (6 feet) and, while painfully slow-growing, is also very hardy - a couple of specimens in a garden in Hesketh Road (around the corner from Southern Comfort) have been there since Victorian times; Phoenix canariensis is a particularly good bet because it grows at a reasonable rate and in my experienced has not reacted negatively to heavy feeding whilst growing in the ground. It is a very large palm with a crown of up to 12 metres(40 feet) in diameter and a trunk diameter of up to 90 cm (3 feet) and should therefore be sited with care.

Links related to plant hardiness - refer to section on Hardiness in Practice (see bar at top of page)

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