The Opposite of Freezing: Plants Have Upper Limits Too

It’s Sunday, July 30, and 87º outside, our forecasted high.  We’re at the front end of a forecast that is calling for two days over our record highest temperature ever recorded in Portland.  I’m looking at it now, Monday, the 31st calls for 92º, August 1 for 99º, 108º, a record, on the 2nd, 110º, another record, on the 3rd, before ‘cooling’ to 105º on the 4th and 95º the next day.  Our average high for this time of year is 82º.  The current record is 107º set on Aug. 8, ’81 and matched on Aug. 10, ’81.  That may not seem that high to people in the SW, but it is here and here is what matters.  Temperature is a local phenomenon.  It’s okay if we whine about it.  It’s hotter than we’re used to.  Hotter than what the local native flora and fauna are ‘used’ to.  For native species it’s not just about preferences, though we may use that word when we talk about their requirements and limits.  

If you hadn’t noticed the local flora and fauna aren’t like us.  They have different responses to changes in their environment than we do.  We wear clothing to which we can add or remove layers to help us moderate our internal temperature.  Our bodies have some ability to slow or speed up the metabolism that takes place within the mitochondria of our cells, producing more or less heat.  We can move in or out of the sun or ‘heavy’ weather and have built shelters we can retreat into, inside of which we have the technology to control the temperature and humidity for our comfort…we can even ‘live’ for a time in the void of space.  Or, we can pick up and move to an entirely different region and climate at relatively short notice, if we can afford to, if the government of the district we are moving to will allow it.  Plants? Not so much, them being rooted and all.  Most of our native wildlife can’t do much about it either.

Plants require water to grow and keep their metabolism going, they also transpire, pulling water from the soil, moving it throughout their structure and losing it out their leaf stomata, cooling their leaf surfaces.  Those plants that follow the C3 pathway, the bulk of photosynthetic plant species, lose much more water in this way than they use internally and increasingly so as temperatures rise.  To a large extent they are at the mercy of the ambient air temperature.  At best they have a very limited ability to cool the air around them and like all evaporative cooling, it diminishes as humidity increases…not our problem here.  The problem for many plants suffering excessive heat levels is that they also transpire more water, rapidly depleting the soil’s water reserve putting the plant in danger of desiccation.

High temperatures can reach levels where they become damaging in themselves as well.  Excessive heat, defined as that beyond a plant’s ability to cope with it, can cause plants to shut down their metabolic processes and, their ability to take up water.  In many plants high heat can damage the mitochondria, the ‘power generators’ within each cell…and this damage can be permanent, the only ‘cure’ being the replacement of that cell with a healthy one.  Intense direct sunlight can damage a plants epidermis on its leaves which can cause a plant to lose water more quickly, its protective layer now compromised.  This type of ‘sunburn’ shows itself as a burning toward the center of the leaf along the midrib.  Proper and appropriate shading then is helpful…how much shading depends on the plant, whether it has the ability to grow ‘protections’ of its own within its structure, e.g., thicker epidermis, heavy cuticle, dense light reflecting hairs, etc. and if it has been growing in an exposure that promotes their full development.


Musa ventricosa ‘Maureli’. I took this picture on day 7 of our heat wave. The older leaves are showing considerable burning on their margins, the remaining tissues are hanging more limply and are duller in color than the newer growth. The lowest leaf has collapsed. The under hydrated cells lack the strength of fully hydrated cells.

Drought symptoms in plants begin more subtly.  They do not go from healthy to dead in a single step.  As soil water levels drop so do they within the plants and cells themselves.  Under hydrated cells begin to heat up even more, speeding up their internal processes, but with insufficient water, while in some drought adapted plants, the plant takes this as a signal to slow down its metabolism and thereby conserve the water that it has.  Under hydrated cells, ‘shrink’ and the tissues that they comprise begin to wilt.  Moisture is drawn back from the leaf margins and those tissues furthest from the active growing points.  Leaf margins begin to scorch, destroying the tissue, while other tissues, under hydrated, lose turgidity and because of this, have compromised structural integrity.  They begin to sag unable to support themselves against gravity.  Woody plants with their woody perennial structures are different as their cells not longer contain the quantities of water that actively growing and dividing cells do.  Instead they are comprise of sturdy cellulose and lignin.  As the plants attempt to minimize their losses and conserve their most vital tissues, older leaves and less important tissues are sacrificed.  Plants, which are unable to conserve their internal water levels, begin to collapse.

Protecting and otherwise ‘coddling’ a plant can preclude the development or ‘hardening’ that is necessary for full on intense summer exposure.  By ‘pushing’ their growth with nitrogen, by providing conditions that allow the plant to push or stretch, or by having a period of very mild temperatures throughout the early summer, puts your plants in a condition less able to protect themselves from harsh conditions…should a sudden and sustained spike in temperature occur.  Rapid growth produces ‘soft tissues’ in terms of heat and sunlight toughness, but we all already know this from our days of seeding veggie starts indoors and later preparing them to be set out in the garden;)  Before an extensive period of heat arrives is critical in ‘preparing’ our plants to better endure any heat and drought stress they may later be subject to.  Gradual increases in stresses allow a plant time to adapt as different responses are triggered and enabled within the plant.

While properly hardened off plants are more ‘durable’ we don’t want to enter a period of abnormally high temperatures with water depleted soils and plants that are already suffering drought stress.  Tough and healthy growth is what we ‘want’.  Additionally, simply turning on the spigot and trying to water our way through the heat to replace the lost soil water is important, but when soil is already dry, we can find ourselves in a near impossible situation as we try to bring soil moisture levels back up from an already inadequate level while experiencing abnormally high evapotranspiration rates.

How any plant will do depends on two things: being well hydrated and conditioned going into the heat event, as described above, and and whether our plants are adapted to hot climates.  You can’t expect a plant to meet the extremes you might experience, or to exceed them, if they come from milder regions, just because you want them to.  Here, in our summer/dry mediterranean type climate, adapted plants would include many sclerophyll, plants whose leaves have adaptations that make them resistant to water loss, heat tolerant succulents, though not all succulents are such, I have Sempervivums in too much sun that curl and dry before we hit our hottest, and having the bulk of our plants which utilize the C4 or CAM photosynthetic pathways as these, in general, are better adapted to drought and heat.

Many heat/drought adapted plants have leaves that have trichomes growing from their outer epidermis.  These can be in the form of ‘hairs’ or scales and can be spaced widely or densely.  Many species have such trichomes on their leaves, but they are too sparse to aid a plant in this respect.  When densely covered with such hairs, the leaf surface is often described as having formed an ‘indumentum’, a ‘furry’ covering, and maybe described as ‘tomentose’.  On many other plants the trichomes are ‘peltate’ and form ‘scales’ or tiny flat plates on top of the leaf surface attached by a ‘foot’ or short stem.  These are tiny and cannot be seen by the unaided eye.  Both give an additional layer of protection to the simple epidermis shielding or reflecting away the sun’s rays from the softer tissue below.  (In some cases, as in many epiphytic Bromeliads, these peltate trichomes have been modified to absorb water that falls on their leaf surface.)  Just to complicate matters, these can also occur on species that aren’t particularly heat and drought tolerant, but they never-the-less serve to protect the leaves from excessive water loss.  This includes the lepidotes of the genus Rhododendron.  On these the tiny scales occur on the leaf undersides, which on some species are obscured by the fine hairs that form above them, their indumentum.  Rhododendrons are commonly divided by aficionados into two groups based on this characteristic, the lepidotes, those with scales and the elepidotes, those without, which include all Azaleas.

Whatever the species, plant leaves all have stoma, through which water vapor escapes and atmospheric gases move back and forth.  How many of these there are, their particular structure, even the timing of their opening and closing, varies among species, but the mechanism of how they open and close is essentially the same.  ‘Guard cells’, which bound the stoma, which are essentially holes in the leaf epidermis for gases to pass in and out, swell and shrink based on their level of hydration, the more fully hydrated they are, the more they swell ‘opening’ the stoma.  As moisture levels drop in the leaf, they drop in the ‘guard cells’ shrinking them and closing the stoma, reducing a leaf’s water loss.  Each stoma serve as one of many thousands of regulatory valves on a plant.  This is the basic ‘pattern’ for all C3 and C4 plants.  Plants that follow the CAM photosynthetic pathway have modified this having guard cells that remain closed during the heat of the day and opening only at night when ambient temperatures fall.

The most problematic situations are the ‘soft’ leafed C3 plants from mild maritime and other mild climates, those adapted to regular summer rains.  These can include many woodlanders and plants from summer warm/mild monsoonal climates that normally don’t experience the regular summer drought periods, that we do here, nor the high temp/low humidity periods like we are in currently.  Drought and heat are a double whammy for these type of plants.  Pushed to our expected extremes this week…there will be failures with these without serious supplemental watering.  Some may just have their leaves toasted, but recover with a push later in the season or next spring.  Other plants from such areas may have foliage that appears much ‘tougher’ but fail catastrophically, quickly collapsing, dead.  Many of these, such as some of the maritime natives of New Zealand and Tasmania simply don’t have the ability to tolerate these temperatures and they collapse.  Talking with gardener friends in the mild maritime climate to our north of Victoria and Port Townsend vicinity, plants like the Aciphylla spp. and  Celmesia spp. simply can’t take anything much above 90º, and there are others that will struggle as well.  Every plant has its limits.  Others may survive, but under perform due to the additional heat stress.

We all know that plants in pots are prone to damage within an even narrower temperature range.  This is a concern for any potted plant as both freezing and high temperatures can more quickly have a deleterious effect.  This is a constant concern of those who grow Bonsai especially in summer hot/dry climates.  This same quality is exactly why we sometimes choose to plant particular plants into pots.  Plants like Cacti, Agave, and various other ‘desert’ denizens that cannot tolerate having their roots continuously wet or even moist.  The danger with these is with novices who fail to appreciate that in order to grow these they too will need periodic summer watering or they will stress and struggle.  Pots that quickly warm their soil in spring can give certain plants like Tomatoes and Basil a ‘jump start’ on the season, while our garden soil lags behind slowing the germination and growth of these same plants. This is due largely to the loss of buffering  that the Earth provides their root systems.  This reduced soil volume not only warms up faster than do our garden soils, it can also dry out more quickly, both potentially leading to a plant’s premature death.  High temperatures, combined with low humidity, can ‘draw’ moisture more quickly out of an unglazed pot as the air temperature rises and the humidity drops, as it does here.  With their limited soil volume and exposure to the air, pots are more susceptible to this than the soil of our gardens and landscapes.  Glazed and plastic pots are not a panacea either though it slows the evaporative losses common to terra-cotta.

Additionally, growing in ‘hot’ soil is an unnatural condition for most plants.  Many of our plants come with the prescription to grow them with their ‘heads’ in the sun and their ‘feet’ in the shade.  They’ve evolved with cooler root zones.  Again, normally, the Earth buffers the soil temperature.  These plants can have the metabolism that goes on in their root system, metabolism occurs within all living tissues, disrupted and if serious enough, it can lead to cell death.  These same atmospheric conditions increase a plant’s transpiration rate causing it to draw more quickly on the soil moisture than it will in lower temperatures.  The higher the temperature, the smaller the soil volume, the quicker a plant can desiccate.  Desiccation equals death in plants.  Choices. Choices.

During periods like this then, plants can die as a result of desiccation, of hot temperatures high enough to cause cell death directly and the quick collapse of the plant, or because of our anxious attempts to keep them hydrated and rotting away their roots, a condition most likely to happen when potted plants are repeatedly, and excessively, watered.  This last group of plants are essentially unable to effectively utilize the water available to them.  Check their soil moisture before watering them…again.  Remember that leaves wilt when they cannot get the water that they need and their need goes up with temperatures.  It is vey common to over water pots, especially during hot periods, as anxious gardeners keep watering keeping the potting soil at near saturation levels, resulting in insufficient pore/air space.  Healthy roots require air for gas exchange.  This is necessary because root cells are actively metabolizing and need to release CO2 and take in O2.  Too much water supports rot organisms like Phytophthora and Pythium which is well positioned to attack the now stressed roots.  Healthy new roots are ‘white’.  Roots are constantly being shed and replaced.  If your roots are tan to brown to gray, they are dead.  Check your soil moisture if you find yourself being compelled to water every day.  Of course, if a plant has out grown its pot it may have a soil volume that is insufficient to hold enough water to support healthy growth and they plant wilts.  Check your soil moisture.  Pot it up one size if need be.  Don’t up the pot size too much or the over large volume of soil will tend to keep the roots too wet.

Other plants, among them the sclerophyll native to chaparral plant communities, like Manzanitas, and others from the mediterranean regions of the world, can be compromised or killed by supplemental watering in summer.  Some are susceptible to foliar fungal diseases which are supported by overhead watering during warm periods.  Keep in mind that drought adapted plants, like sclerophyll, evolved in dry summer conditions, the adaptations they’ve made to survive them may actually impair their ability to survive had no reason for the defenses that regular summer watering requires of summer/wet adapted plants.  Such conditions can push them to an extreme.  Watering in summer changes the conditions within the soil for the micro-organisms there, allowing some to flourish that would not normally.  These can be fatal to some plants.  This situation isn’t directly related to high heat spells.  It is more a result of the plant’s adaptation to winter/wet summer/dry soil conditions.  This is most likely to occur to plants

Plants, as an overall group of organisms, have made many adaptations to their sessile, fixed to place, reality…they’ve had to to be successful over the many millions of years that they’ve existed.  Heat and drought are common occurrences to all plants.  Each species has different tolerances linked directly to the conditions they’ve evolved with.  A network of interconnected cellular stress response systems is a prerequisite for plant survival and productivity.  This is a complex stress response network with a wide array of mechanisms for adapting to plants’ changing environments at the physiological, biochemical, and molecular levels to increase their tolerance to the stresses.  As a science this work is being done by crop scientists, agronomists.  That is where the money is, not in decreasing the pressures caused by human population growth or our overall consumptive economy that has lead directly to the threat of climate change.  Increasing human population, the continuing loss of prime agricultural lands and the warming of the world’s climates is driving this work as abiotic stressors like heat, drought and soil salinity work together resulting in increasingly devastating crop failures.


[I borrowed this diagram from an article I labored through to give you some idea of the complexity of a plants response to heat stress.  It comes from, “The Plant Heat Stress Transcription Factors (HSFs): Structure, Regulation, and Function in Response to Abiotic Stresses”.  Schematic representation of HSFs as key components in transcriptional regulatory networks during abiotic stress. The scheme integrates both positive (arrows) and negative (bars) regulatory mechanisms. Abiotic stresses provoke a rise of cytoplasmic calcium, ROS accumulation and proteins denaturation inside the cells which convey stress-induced signals to responding genes, directly targeting HSF proteins marked with an asterisk. HSFs induce the activation of various genes playing a central role under abiotic stress conditions, thereby enhancing the abiotic stress tolerance. ROS, reactive oxygen species; CaM, Ca2+–calmodulin; TFs, transcription factors; Hsp, heat shock protein; sHsp, small Hsp; HSE, heat stress element; Hsa32, heat stress- associated 32-kD protein; Rof1, FK506-binding proteins; GST, glutathione-S-transferase; RD29A, drought-regulated gene 29A; APX2, ascorbate peroxidase 2; GolS1, a galactinol synthase; HSBP, HSF binding protein.

Ultimately, even for us, temperature is about more than ‘comfort’.  We are living organisms, amongst the relatively small group of warm blooded organisms on the planet.  Plants have evolved very different strategies in response to stressors like heat and drought, because they aren’t mobile.  These systems are species specific in their details with redundancies.  Like other features and and functions in plants, plants have evolved similar ways to meet a need with different genetics in a process not unlike convergent evolution which we typically think of in terms of their physical characteristics.  Many differences are internal and not readily visible to the eye.  Given this, all of us, all living things, can only exist within a relatively narrow range of growing conditions, we can adapt to and tolerate only so much.  Each species, plant and animal, has its optimum range and though it may survive in the short term outside of it, is not equipped to do so in a way that will assure the survival of the next generation.  Change the conditions that a plant or animal must contend with or move it someplace where it must live at the margins, and it may begin to decline or die outright.  That is the nature of living at the margin.  When you’re living at your margin, even a very few degrees difference, can be critical.

[I thought that I’d add this link to a site discussing the commonly made claim that watering a plant on a hot clear day can result in leaf scorch, via some process of ‘magnification’…untrue.  Sunburnt leaves: the myth debunked]

These are the temperatures we reached at my house in inner SE Portland. PDX temps are NOAA recordings at the airport.  Most recent day of measurable rain: June 16.
Aug. 1, high of 98ºF; high at PDX 97º
Aug. 2, low of 67ºF; high of 102ºF; temps at PDX 65º – 103º
Aug. 3, low of 71ºF; high of 102ºF ; at PDX 66º – 105º
Aug. 4, low of 67ºF; high of 97ºF; at PDX 64º – 96º
Aug. 5, low of 61ºF; high of 90ºF; at PDX 59º – 89º
Aug. 6, low of 63ºF; high of 86ºF; at PDX 61º – 88º
Aug. 7, low of 64ºF; high of 89ºF; at PDX 62º – 89º
Forecast calls for the next 4 days with highs of 90º+ or better.


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