Plant tolerance of dry heat. Part 2.
Patrick Mills / This article was first published in The Mediterranean Garden, the journal of The Mediterranean Garden Society, Nº 40, April 2005.

In the first part of this article (TMG 39), I proposed a dry-heat resistant coefficient for grading the ability of garden plants to withstand summer conditions in the Mediterranean. Let us now look at some of the difficulties and objections to this system, starting with the biggest one, an alternative system.


An alternative: heat-zone maps
“Wouldn’t it be simpler to have some sort of system parallel to the cold hardiness zones set up by the U.S. Department of Agriculture?” This is what I used to think too, and I was surprised that nobody had done it, until my attention was drawn, by our colleague David Streeter of Palm Springs, to the book published in 1998 called Heat-Zone Gardening (Marc Cathey, Time-Life Books, ISBN 0-7835-5279-8). This book is based on a heat-zone map of the United States devised by the American Horticultural Society. The country is divided into twelve zones according to the average number of days per year with a temperature of 30C or above, with Zone 1 having an average of less than one and Zone 12 an average of 210 days or more. The book is basically a dictionary of plants, giving their cold hardiness zones and their heat zones. This promised to be very interesting indeed, so I got my hands on a copy post haste and read it. I was disappointed. Although the book will be of some use to gardeners in the USA, I now have my reservations about the whole idea.


Firstly, the system does not distinguish between a desert and a swamp, between dry heat and steamy heat, between, say, Arizona and Florida. A genus that likes dry heat, such as rosemary, might have the same heat-zone allocation as a tropical or semi-tropical plant such as canna. I am not sure of the extent to which this is helpful.


Secondly, I begin to doubt whether just the number of days above 30 is such a valid measure by itself. According to this measurement my summer garden conditions are similar to those in Virginia, for example, or parts of the California-Oregon border, and this certainly does not feel right. What about the number of days over 40? The number of days with a humidity of 30% or less? The number of hours of wind at speed such-and-such? All these factors affect the transpiration rate of the plants. Two geographical areas may both have, say, 50 days of temperatures above 30 but if one of them has seven days of hot-desert conditions and the other does not, this is enough to change everything. It is the sum of all the negative factors over a summer that matters – what you might call the ‘Total Desiccation Effect’ – not simply the number of warm days.


My third reservation is that I now believe that cold hardiness zones and heat zones are not so parallel as I once thought. When reading about cold hardiness, we learn about the importance of microclimates in the garden and so you have probably done as I have done: put thermometers out at night in the middle of winter in different parts of the garden to find out how much the ground temperature varies. As a result of this, I consider that any plant supposedly hardy to -15C will be safe anywhere in my garden (except perhaps for that freak winter once in a decade or two); those said to be hardy to -10 I place with care, making sure that they have protection from winds and get some extra heat from a wall or a big rock; and those that will stand only -8 I no longer invest in: they all seem to succumb, and sooner rather than later. This range is interesting but not enormous.


Now let’s do something similar in the middle of summer. At midday towards the end of July, with the thermometer on a shaded wall of the house showing just 25C, I measured the temperature in two places in the same garden bed, one in the dappled shade of a tree (Koelreuteria paniculata) and the other just two metres away in full sun. In both cases the thermometer was placed under two inches of mulch. In the first case, the temperature was 30C (the previous few days had been hotter) and the soil surface was damp – from rain some time before, not from watering. In the second case the soil was dry and the reading was 50C! Under the mulch, remember. And it can only get worse as the summer goes on. (Two weeks later I checked again and the readings were 28, 34 and 54.) We could make matters even worse by adding, in the 50+ area, a big boulder or stone wall plus a blasting westerly wind. You can begin to see the extreme conditions that some plants have to suffer and the surprising differences that may exist between one part of the garden and another. We humans head for a shady tree in the middle of a hot summer’s day; if garden plants had legs I would not be surprised to find most of them under there with us.


These rough-and-ready, unscientific measurements, together with other personal observations, make me suspect that the specific microclimate is much more critical in summer than in winter, that what matters is not so much which heat zone you live in – whether the local weather office registers 40, 50 or 60 days over 30C – but rather the very specific conditions in different parts of your own garden, where the variation might even be equivalent to the effect of crossing several of those heat zones.


I now do not think that the heat zone alone is a great help after all.


Further comments and caveats
Let’s turn back to the dry-heat resistance grading system and its problems. Here are four of them:


1. “Why do we have to reduce everything to numbers? Why not some simple self-explanatory system like extremely/ very/quite/somewhat/not at all resistant to heat and drought?” Indeed, we do often talk like this and no doubt we always will. Such descriptions are useful, to the point and have the advantage of being in the vernacular. For some people this will be perfectly satisfactory, just as for some the use of common names for plants is enough, whereas for others the use of Latin names allows for more precision and better understanding. For me, at least, the coefficient system works better, giving me more information (and making me observe better too).


2. “Why these numbers? Why not 1 to 10, 1 to 100 or even A,B,C,D?” Yes, why not? The only cautionary comment I would make is that while reading Heat-Zone Gardening and seeing for every entry something like


Cold: Zones 5 to 9
Heat: Zones 9 to 2


Cold: Zones 6 to 9
Heat: Zones 9 to 2


Cold: Zones 5 to 10
Heat: Zones 8 to 1


my head was beginning to spin. I felt that they had made a mistake using more or less the same range of numbers for the heat zone as for the cold-hardiness zones. For clarity we need a scale that looks entirely different. Anyone who is familiar with Olivier Filippi’s catalogue of dry-garden plants knows how well he used the range 1 to 5 (with decimals) to describe the reactions of different oleander cultivars to the cold. Its effectiveness obviously influenced me in deciding on my classifying scale but I don’t think there is any great danger of confusion between the two scales.


3. Leaves that wilt, curl, turn yellow and then drop may be the sign of an emergency: the plant does not like the conditions and is taking the first steps on a path that might lead to death if such conditions continue or get worse. Or it could be just the opposite: defoliation, partial or total, is a matter of course for this species, which is totally adapted to the conditions, and in this way ensures its survival. You may have noticed that the grading system presented here prizes those plants that get by quite well without this tactic. It evaluates not just survival, but “graceful survival”, a horticultural appreciation rather than a botanical one, based on the assumption that, in general, gardeners prefer a fully-green-leaved rosemary or fully-grey-leaved saltbush to a brown-leaved lilac, a totally bare-legged rose bush or a half-yellow cistus. This raises problems. First, is this really the right way to go? This worried me considerably until I realised that after all we do belong to the Mediterranean Garden Society, not to the Mediterranean Botanical Society. Even so, the point is still up for debate, since not recognising the difference between a sign of stress and an indication of stress-avoidance might turn out to be a serious handicap. The other problems raised are more practical ones, mainly created by the introduction of a subjective element by asking “How good does it look?” We won’t always agree. (This question is, of course, the central axis around which all gardening discussion turns anyway!) As our colleague Olivier Filippi pointed out to me, loss of leaf is not always unattractive. He quoted as an example Phlomis lycia, which in June starts to lose all its winter leaves, leaving it with fewer, smaller ones, and looks better for it.


Similarly, I wonder whether people might not think that a defoliated Thymus capitatus, with its weird, ghostly-white, twiggy stems, is more fun than when it had all its leaves. Awkward decisions keep cropping up: what do you say about a black mulberry that keeps its green leaves but drops its fruit? A disaster as a fruit tree but fine as an ornamental. And spring bulbs? They are just invisible in summer. And how do you evaluate a pampas grass that has perfect green leaves but fails to send up its “feathers”? These are no more than annoyances, problems to be solved, not insuperable obstacles.


4. If you decide to try using a scale like this one – and I hope you do – you will find that you sometimes have to “fudge” a bit in your gradings. You have a sample of plant X in the garden; you probably haven’t got forty of them spread over five different sites. So, when grading it, you have to take this into account: is this an “average” site? Or is it favoured in any way? Or particularly difficult? Depending on the answer, you might have to lean a little in one way or the other in your ratings. If a plant looks like being a star performer, you might like to plant another of the same kind in a nastier part of the garden to see how it fares, and whether the grade needs to be revised upwards.


One should note, too, that a plant newly placed in the garden has to receive special care and there is no point trying to rate it until it has become established. Even then, a plant could be in trouble for some reason other than heat and drought: is it in the right soil? Has it got a disease or a pest? Was it too weak from the beginning? Is some beastie chewing up its roots? Remember also that dead flowers and flower stalks (Nepeta, Lavandula, Stachys etc), which would have been the same whatever the climate or weather, are really irrelevant.


If grades are revised year after year, we slowly approach something close to a “truth”, and the more people making measurements, the more reliable the information would be.


In conclusion
In this long ramble through the hot, dry countryside, my objective was to make the following points (though not in this order):
• Mediterranean summers can be a stressful environment for a variety of reasons
• It is important that MGS members as individuals and the MGS as an institution study plants’ resistance to these stressors. The knowledge so accumulated can help to reinforce the guiding principle of “choose the appropriate plant” rather than “throw more water at it”.
• Existing classification schemes are sometimes useful but not totally satisfactory. The MGS is in a position to create something new and better. One possible system was presented at length in order to show the advantages and difficulties in such schemes.
• Studying the behaviour of one’s own plants and classifying them can be a rewarding learning process. Being forced to decide on the merits of one plant in relation to others increases one’s knowledge and understanding of both the plants and the garden sites.

If, as a result of this article, some members of the MGS actually go out into their gardens in high summer and observe and take notes, I would feel very pleased. If a local branch decided to do this next summer and collate their efforts, it would be terrific. And if the MGS sponsored a movement to create a scheme for grading dry-heat resistance in plants, I would be highly delighted. The least I would hope for as a result of this article is a certain amount of enlightened discussion of the subject which might lead to better gardens and happier gardeners in the Mediterranean.


"And what about those roses?"… Ah yes, the roses. Maybe some other time.

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