The Garden Professors™

Earlier this spring, I posted an article about seasonal climate forecasting and noted that we expected to see the development of an El Niño after three years of La Niña conditions ended in March 2023. And sure enough, an El Niño was declared in August 2023 and has been strengthening ever since. It has a 71% chance of becoming a strong event by December or January before starting to weaken, as they usually do in spring or early summer. In today’s post, I will remind you all what El Niño is and how it is expected to affect our climate this (Northern Hemisphere) winter and spring. That will affect how our gardens survive the colder conditions and prepare for next year’s growing season.

Refresher: What is El Niño?

If you are a new reader of this blog, you may be wondering what El Niño is. El Niño and its companion, La Niña, are two opposite phases of an oscillation in atmospheric and oceanic weather patterns linked to the water temperature in the Eastern Pacific Ocean (EPO). When the ocean there is warmer than usual as it is now, rising air over the warm water creates thunderstorms which can affect the movement of global air currents that bring stormy weather to parts of the earth while leaving other areas high and dry. When the ocean there is cooler than normal in the La Niña phase of the pattern those currents shift, changing the expected weather pattern to something quite different resulting in a different pattern of temperature and precipitation than in El Niño . The maps below show how the climate changes globally in an El Niño in the December-February and June-August periods, corresponding to Northern and Southern Hemisphere winters, respectively. The El Niño pattern is linked to a variety of unusual weather phenomena around the globe. The variations in temperature and rainfall are what affect how our gardens grow or rest in preparation for the next growing season.

What is the current state of El Niño and how is it changing?

Right now, ocean temperatures in the EPO are from 1 to 3 degrees C ( 2-5 degrees F) warmer than normal all the way from the west coast of South America all the way west to the International Date Line. This is a large area of very warm conditions that are being heated even more by the trend towards warmer temperatures due to increases in greenhouse gases in the atmosphere. The heated water provides a lot of water vapor to the atmosphere that helps fuel thunderstorms and tropical systems. Normally in an El Niño year the number of tropical cyclones in the Eastern Pacific is larger than the number in the Atlantic because of that pool of warm water. This year the Atlantic has near record-setting sea surface temperatures which are helping to produce one named tropical storm after another (today we are have “Philippe” and “Rina” active in the Atlantic but only through the name “Kenneth” in the Eastern Pacific). Global warming has affected our climate patterns to such an extent that what used to be established El Niño and La Niña patterns are less likely than in previous decades, although there have always been variations from one event to the next.

What will happen from N. H. winter through spring?

According to the predictions of how the current El Niño will evolve, we can expect the current pool of warm water and the associated global weather patterns to last through at least the April-June period. Since El Niño seldom lasts for longer than a year we are likely to go back into neutral conditions after that and neither El Niño nor La Niña will dominate. This means that over the next few months we can expect the southern part of the United States to be cooler and wetter than normal since in El Niño years the jet stream is positioned over that part of North America. As storms are pushed through the region rainy and cloudy conditions keep daytime temperatures cool as precipitation in the form of rain or sometimes snow or ice falls. In northern parts of the United States extending north into Canada, warm and dry conditions are likely to lead to a lack of snow cover and shorter ice coverage on what are usually frozen lakes. Warmer than normal conditions are also likely to occur in most of Southeast Asia stretching from India to Japan. Drier than normal conditions are likely in the Western Pacific Ocean and in southern Africa as well as South America, leading to the possibility of droughts in those areas.

What does this all mean for our gardens the next few months?

In the parts of the world that are under the jet stream, cooler and wetter than normal conditions should lead to high levels of humidity and increases in soil moisture over the winter since evaporation will be low in the cold winter months. That means gardens in those areas should be fairly wet going into spring. That means a spring or summer drought there will be less likely than after a La Niña winter, but it could be muddy working in your garden areas next planting season. Since El Niño is already strong and getting stronger this wet winter pattern may start early this year so don’t dawdle in preparing your fall garden for winter since it might be hard to work in those wet conditions. Soil temperatures may stay cool later in the spring, delaying planting of seeds and vegetables or flowers that require warm ground to germinate and grow.

If you are in northern parts of the United States and up into Canada, you can expect warmer and drier conditions than usual. That could mean a lack of snow cover and loss of some plants that need insulation provided by the snow to survive the winter. Even though temperatures will be overall warmer than in non-El Niño years, there are still going to be cold outbreaks that can cause damage to plants that are over-wintering. The lack of precipitation could also lead to dry soil conditions in spring that could require increased irrigation or hinder the growth of seeds or new seedlings you might plant. The lack of soil moisture could also contribute to the development of drought later in the growing season.

Of course, even though El Niño and La Niña are the most reliable predictors for climate several months ahead, there are always other factors that can affect climate patterns too. There is no guarantee we will see these exact patterns this winter and there are sure to be some surprises that we don’t expect.

I’ve been doing horticulture myth-busting for almost 25 years now – and what I’ve learned is that myths are zombies. Not only do myths not stay dead, but new zombie myths are also continually created. One of the newest bright-n-shiny distractions is electroculture. It’s EVERYWHERE.

What is electroculture, you might ask? Well, Jaccard (1939) described it as “the stimulation of growth in plants by means of electricity passed into the atmosphere surrounding them or into the soil in which they are growing.”

There was a surge in research in the late 1800s through the early 1900s, partially due to earlier observations which tied electrical storms to improved plant growth. (Further research determined that lightning fixes atmospheric nitrogen into a solid form (nitrate), which dissolves in raindrops and enters the soil system. This was undoubtedly responsible for the reported improvement in plant growth after electrical storms.)

Scientific interest in electroculture tapered off with advances in plant physiology and the development of commercial fertilizers. Furthermore, the few scientific publications that came from early studies showed no consistent benefit from electroculture:

• “Favourable results in increased growth and yield have been obtained from time to time, but they are uncertain and largely dependent on weather conditions.”
• “Plants on poor soil are little influenced, since electricity…does not provide either nutrients or energy.”
• “A current of 10 milliamps inhibited growth in 5 plots, but in 10 others yields increased.”
• “Electroculture experiments produced no differences in the treated trees in growth or yield.”

In the 21^st century, a desire to use fewer chemical fertilizers has spurred renewed interest in electroculture. There are vast numbers of websites proclaiming improved plant growth from sticking copper wires in the ground. None of these are backed with any reliable evidence, but proponents argue that new research (since 2000) supports this practice.

I did a search of the scientific literature through AGRICOLA, CABI, and Web of Science/BIOSIS. There were zero publications after 1968. However, Google Scholar lists several. Google Scholar searches for publications in any form on the internet that have been authored by scientists. Here’s an example of what you will find:

• A 2021 conference report for the IEEE 13th International Conference on Humanoid, Nanotechnology, Information Technology, Communication and Control, Environment, and Management (HNICEM).”
• A 2021 Research Centre Lab report from the Late Ramesh Warpudkar ACS College Sonpeth, India.

These are not peer-reviewed, scientific journal publications. Even earlier ones from the 1980s are in irrelevant journals such as Journal of Biological Physics. Plant scientists publish in plant science-related journals. Researchers outside the field of plant sciences really have no clue how plants function, nor do they have the expertise to design experiments which consider the variables that affect research in applied plant sciences.

Yet, proponents of permaculture push this concept without evidence, using anecdotes as equivalents to scientific data. Conspiracy theories abound, with accusations that chemical companies have forced scientists to cease electroculture research.

Proponents of electroculture “rely on theories of geobiology and use light devices, antennas, or magnets intended to act on the cosmo-telluric electromagnetic fields of the universe, as in dowsing (Wikipedia).”

Worldwide, scientists consider electroculture to be a pseudoscience, particularly because it does not propose any plausible scientific mechanism to explain how electricity would stimulate plant growth.

If and when this changes – when recognized plant science experts publish positive results that are confirmed by other plant researchers – those results will be in bona fide plant science journals and be worth discussing. Right now, don’t waste your time with another horticulture myth that refuses to die.

If you are interested in honing your BS detector, please take advantage of this peer-reviewed Extension Bulletin.
My thanks to Sylvia Hacker for finding great vintage photos to help illustrate this post.

Last week there was much ballyhooing over a new article on the benefits of native plants in supporting insect populations. I’ve posted on the fallacy of native plant superiority before, pointing out that landscape biodiversity not plant provenance, is most important for supporting all types of beneficial wildlife. Despite robust, published evidence to the contrary, more people and governing bodies believe that native plants are the magic bullet for urban landscapes. (Never mind the fact that there are no plants native to urban environments.)

Using CRAP analysis to assess information:

• C = credibility. Are the authors experts in the field of interest?
• R = relevance. Is the information applicable to the field of interest (in this case, management of plants in urban landscapes)?
• A = accuracy. Is the information grounded in current, relevant science?
• P = purpose. What is the underlying reason that the information is being shared?

This most recent paper warrants a careful dissection as it has gone viral online. For me, the first red flag is that there are no plant or soil scientists on this team. The first two authors, who were responsible for developing the main ideas and designing methodologies, are both ecologists by training. The other authors are involved in insect collection and identification as well as ecological modeling. Not having plant and soil scientists on the team to ensure science-based practices are followed during landscape modification is a serious oversight.

The methods section regarding the study site is astonishingly vague, given this is essentially a landscape plant installation and management project (i.e., applied plant science, not ecology). A well-designed experimental project would include control plots, replication of treatments, site analyses (including soil type and texture as well as soil testing for nutrients and organic matter), and detailed explanations of how the site was prepared, how plants were selected, prepared, and installed, and what site management occurred post-installation.

Here is the section on how this “experiment” was designed:

“In mid- April 2016, 80% of the site was substantially transformed through weeding, the addition of new topsoil, soil decompaction and fertilisation, organic mulching and the addition of 12 indigenous plant species…Selected plant species met the criteria of being locally indigenous to the City of Melbourne and represented a range of growth forms— including graminoids, lilioids, forbs and trees— requiring no ongoing management such as watering and fertilisation.”

The purpose of the methods section is to provide detailed explanations on how the study was conducted so that it could be replicated by other scientists elsewhere in the world. There is no way to replicate this study properly, as the methods are vague and very possibly not based on applied plant and soil sciences:

1. “Addition of new topsoil” As we’ve pointed out in this blog numerous times over the past 14 years, you don’t add new topsoil to landscapes.
2. “Soil decompaction” What is this? Does it involve tllling, which would directly affect the health of the two existing trees?
3. “Fertilization” What is the fertilizer? When was it applied in the process? At what concentration and based on what data was it applied? You need to know baseline levels of nutrients before you can rationally add any fertilizer.
4. “Organic mulching” What material? Compost? Cardboard? Bark? How deeply was it applied?
5. “Addition of…plant species” Were these bare-rooted or simply popped out of the pot and dropped in the new topsoil?
6. “Requiring no ongoing management such as watering” News flash: newly installed plants REQUIRE irrigation during the establishment period regardless of their nativity. And this site now contains substantially more plants than before, meaning increased competition for water and other resources.

The problems with this nonscience-based approach to landscape plant management can be see by comparing the two spotted gum trees that were on site before this project began. Corymbia maculata is a threatened native species in Australia and the continued health of these trees should have been paramount before any site work was initiated.

Unfortunately, these sorts of projects, conducted by teams with no soil or plant scientists and published in journals that are not specific to urban plant and soil sciences, are neither well-designed nor useful. The mindset of many researchers outside the applied plant and soil sciences is that there’s no real science to preparing soil, installing plants, and maintaining the site. This current paper does not even meet the standard of being experimental: it is merely a report on what happens to insect populations when a landscape is altered. There is no basis for comparisons. Any conclusions drawn are anecdotal.

It’s bad science.

Since my last post, the news has been full of one weather disaster after another. Wildfires in Maui. The remains of Hurricane Hilary moving north into California and other parts of the western USA with moisture even streaming as far east as Wisconsin. Record-breaking heat and humidity across most of the continental USA and severe weather outbreaks in the Midwest and Northeast. This does not even include the typhoons, floods, droughts, and heat waves occurring in other parts of the world at the same time. And today I am watching the development of a tropical system in the northwestern Caribbean that is likely to become Tropical Storm or Hurricane Idalia (pronounced ee-DAL-ya) by early next week, bringing rain and strong winds to parts of Florida and southeastern Georgia.

Each of these weather events can impact our gardens and properties. Not all the impacts are bad since tropical rainfall is an important source of summer moisture in many areas. In the Southeast as much as 40% of our summer precipitation comes from tropical systems and if we don’t get that rain, we can go into a drought quickly in our hot summertime temperatures. The rain from the remains of Hurricane Hilary helped provide some needed rain to help increase reservoir levels in the desert Southwest USA, which desperately needs the water. But if we get too much very strong rain or winds the damage to our homes and yards can be severe. This week I want to discuss some ways you can prepare your gardens and landscaping for the severe weather that will inevitably occur in your area at some point in the future (and that future may be nearer than you think).

Awareness is key to proper preparation

To properly prepare your gardens and homes for severe weather, you need to know what kinds of weather to expect and how it will impact your plants and buildings. The types of weather you are likely to experience will drive how you prepare. If you live in the Pacific Northwest, you are not too likely to experience hurricanes, but you certainly can experience extreme rain storms in winter and wildfires in summer especially if you have a heat wave like you did last year. If you live in the Southeast then hurricanes and tropical storms are much more likely but damage from straight-line winds can be just as important, as I found out in late July when strong thunderstorms knocked down so many trees in my neighborhood that we lost power for 44 hours. A friend of mine lost the roof of her house to two pine trees that toppled over in the strong winds. We can even experience ice and snow storms here in the South from time to time; this means you should not just prepare for the most common disasters but for any extreme event that could occur there. Of course, getting ready for the most common natural hazards is the best way to save your homes and gardens because those are the most likely to occur where you live. But you should also think about rare events like floods even if you are not in a flood plain because the consequences of an event are so severe.

Once you have identified the natural hazards that affect your area, then you need to think about what kinds of weather conditions are likely to occur in each of those events. In a hurricane or a strong winter storm on the West Coast, heavy rain and high winds are both likely weather conditions your garden will experience. In a heat wave, high evaporation rates and dangerous outdoor working conditions are likely to be the major dangers. Those are the impacts you will need to consider when protecting gardens and gardeners.

Look at your landscape and home to identify potential problems

Once you have determined what hazards are likely to affect your property, you need to do an assessment of where your risks are. Take a walk around your garden and look at the trees and plants you have. Are there dead trees that could fall over or broken limbs that could snap off in high winds from hurricanes or thunderstorms and become wind-borne missiles? Do you have garden decorations like garden gnomes or mirror balls that could also blow into the sides of cars or buildings? Do you live in an area that is prone to frequent flooding? How will you keep that water away from your house and out of your garden plots? Are there a lot of plants close to your home where they could spread wildfire in a drought under gusty winds?

After you have determined the risks take steps to minimize or remove those risks where you can. There are a number of ways that you can storm-proof your house and garden, many of which should be done anyway to maintain your garden’s health.

I was interested to read the story of the lone house in Lahaina, Hawaii, that survived the recent fires that destroyed much of the town because they had a large area around the foundation covered by river rock, which did not burn and helped keep fire from igniting the house (although I am sure there was some luck involved there too). One of my favorite books, The Control of Nature by John McPhee, also describes the increase in debris flows in western landscapes that occur due to fires that burn waxy plants. This creates perfect conditions for rapid land movement downhill after rain events following the fires, often right through people’s yards and houses.

After the storm is over and you are safe, then it is time to assess your garden and house for damage and take care of your plants, lawns, and buildings. Be careful since there may be downed power lines and dangling tree limbs that could be hazardous. You may need professional help to prune or remove trees or clean out contaminated soil after flooding. Once the immediate hazards are taken care of, then the longer-term work of repairing drainage, eliminating the effects of erosion, rebuilding beds, and other work can begin.

Take care of your family and pets too

Of course, all this planning for your garden and property should not take the place of emergency planning for your own family. If extreme weather does occur, you need to have a plan already in place to determine where to shelter in your home if you can and how to evacuate safely if you can’t stay. Every minute saved makes a difference although in the worst cases even that may not be enough time. FEMA has a good website that provides a lot of information on how to plan for both natural hazards and other emergencies like chemical spills. Many states also have excellent resources for dealing with emergencies, such as the Resident’s Handbook To Prepare for Natural Hazards in Georgia, which covers all kinds of severe weather and how to prepare for it in Georgia and beyond.

This past week I was out of town at a conference, and since the week was supposed to be a scorcher I made sure my husband was going to water the container plants daily. And indeed, temperatures were in the 90s, dropping to the mid-60s at night. But the container plants looked great when I got home and I didn’t think much more about it until the next day. My husband called me into the living room, pointing at our massive mophead hydrangea which looked like it had been torched. Leaves and blossoms were wilted and browning. Every single stem was affected. Since our landscape is on an automated sprinkler system, what the heck happened?

This is when caution and objectivity are important. I wasn’t going to go cut the whole thing down, even though it looked terrible. Instead, I made observations of the site (not just the plant):

• The site is on the north side of the house, where plant only receive direct sunlight in the morning and late afternoon during the summer.
• No other plants were affected – not even the smaller hydrangea to the west of the damaged plant.
• The irrigation system had been working normally.

When diagnosing plant problems, it’s also important to consider the history of the plant and the landscape:

• Hydrangea is at least 55 years old.
• No soil disruption or other site disturbance
• No pesticide or fertilizer use
• Mulched with arborist wood chips

Given that no other plants were affected, the problem was with the hydrangea itself. Hydrangeas use a lot of water to support their large, thin leaves and massive flower heads. When the weather suddenly turned hotter and temperatures stayed abnormally high in the evenings, the plant could not recover its water loss overnight. Many flowers and leaves experienced terminal wilt – that means they lost too much water and tissues turned brown. Other flowers and leaves were able to recover as day and night temperatures returned to normal.

What could we have done to prevent this? Had we seen the wilt occurring during the day, we could have turned on the sprinklers manually in that part of the landscape. Hydrangeas are a good indicator of low soil water. In future summers, as we continue to experience hotter and drier conditions, we will keep an eye on our hydrangea and use additional irrigation if necessary.

The hot weather that stimulated the last blog is still with us! Keep up the mulch and occasional watering to help shade trees. Today I want to cover a topic that seems like a garden myth but actually has considerable science behind it. Bicarbonate! The miracle cure for all garden pests? No. My wife came across an article in her news feed about a ‘garden guru’ who touted baking soda as a miracle cure for powdery mildew and other “blight” diseases. In this blog I will review the efficacy of the bicarbonate anion in disease control.

Bicarbonate as a disease control agent is not a new concept.  Many studies going back a few decades documented the efficacy of this molecule in controlling foliar diseases of plants.  Most studies are on powdery mildew of many crops and ornamental plants as well.  Bicarbonate is typically available with three cations: ammonium, sodium and potassium.  Most of the studies and efficacy are with the potassium salt of bicarbonate (not baking soda which is the sodium salt).  All the salts are more or less efficacious against powdery mildew and a few other fungi like apple scab. 

The mechanism of action of bicarbonate control of fungi is not clear, however Monterey Chemical that manufactures a Bicarb. product (Bi Carb Old Fashioned Fungicide) indicates on its label that the mechanism is the disruption of the potassium ion balance in the fungus cell, causing the cell walls to collapse 

One frustrating thing with internet-based articles by garden “gurus” is that everything is some kind of “garden hack” Like we are getting something done on the sly or with common household materials. Using chemistry to control diseases is using pesticides. Companies test and approve these materials based on efficacy data as shown in the references at the end of this article. The benefit of using a registered product is that there are instructions that indicate how to spray, what the target organism is and the environmental conditions under which the product will work. Much of this is never mentioned in the “hack” articles.

Bicarbonate anion is an effective control of powdery mildew often as good as commercial fungicides with very different chemistries and methods of action. There does not appear to be resistance to bicarbonate. It is a contact fungicide (not systemic), the material must contact the fungus in order for it to work. In order to get good contact usually a wetting agent such as an ultrafine oil is combined with the tank spray solution to increase control.

It is best if bicarbonate is applied early in the disease cycle. Powdery mildew organisms are obligate biotrophic fungi. This means that the mildew fungus must grow in living plant cells. So when the mildew is killed the living plant cell is also killed. Powdery mildew is a disease of the epidermal tissues of plants so cell death is superficial but if late stage mildew is control by bicarbonates there can be phytotoxicity (plant damage) as most of the epidermal cells of leaves and flowers will collapse and die.

Powdery mildew fungi are in the Ascoymcete group of fungi and most have two spore stages. The first infections are caused by ascospores (sexual spores) that are released in the springtime. The primary infections develop and then asexual spores develop on plant surfaces that we recognize as powdery mildew. Stopping the primary infection by applying control early in the season will slow powdery mildew down on sensitive plants. Since many powdery mildews have broad host ranges they form on weeds an other plants and can move into protected plants later in the season. Frequent sprays will be required if conditions for the fungi are optimal and the spores are present. Bicarbonates do not have long-term protective effects since they work only when the solutions contact living fungus.

References

Zivand, O. and A. Hagiladi. 1993. Controlling powdery mildew in Euonymus with polymer coatings and bicarbonate solutions. HortSceince 28:134-126

Moyer, C., & Peres, N. A. (2008, December). Evaluation of biofungicides for control of powdery mildew of gerbera daisy. In Proceedings of the Florida State Horticultural Society 121: 389-394.

Holb, I.J. and S. Kunz. 2016. Integrated Control of Apple Scab and Powdery Mildew in an Organic Apple Orchard by Combining Potassium Carbonates with Wettable Sulfur, Pruning, and Cultivar Susceptibility. Plant Disease, 100(9), 1894-1905

El-Nogoumy, B. A., Salem, M. A., El-Kot, G. A., Hamden, S., Sehsah, M. D., Makhlouf, A. H., & Nehela, Y. 2022. Evaluation of the Impacts of Potassium Bicarbonate, Moringa oleifera Seed Extract, and Bacillus subtilis on Sugar Beet Powdery Mildew. Plants, 11(23), 3258.

Türkkan, M., Erper, İ., Eser, Ü., & Baltacı, A. 2018. Evaluation of inhibitory effect of some bicarbonate salts and fungicides against hazelnut powdery mildew. Gesunde Pflanzen, 70(1), 39-44.

My news feed has been filled in recent weeks with many stories about the unusual heat that has affected many parts of North America, Europe, and the Atlantic Ocean. High temperature records are being broken at an amazing rate, and while we don’t expect every day or season to break a new heat record, the trend towards warmer global temperatures overall from greenhouse warming makes periods of extreme heat more likely.

Today I want to talk about the term “heat dome” and explain what they are, how they affect gardeners (and other humans) as well as the plants in our gardens and agricultural fields. We had a great introduction to the physiological impacts of heat a few weeks ago in Jim Downer’s post “I’m hot! So are my plants!” on how high temperatures affect plant growth and respiration. If you have not already done so please read that for more details about heat’s impacts on garden plants.

What is a “heat dome?”

A “heat dome” is an expansive stationary area of high pressure that is associated with unusually hot temperatures. While slow-moving high-pressure areas are often seen in summer months, the size and unmoving nature of a heat dome coupled with the extremely high temperatures in the area beneath them make “heat domes” especially dangerous for humans and animals. They can also cause detrimental effects on many garden and agricultural plants because they are also associated with long dry spells along with high rates of evapotranspiration. The term “heat dome” is a phrase that has been popularized by the news media as a way to explain extreme heat events across large regions and is more jargon than a true scientific term, but it is certainly descriptive!

Any high-pressure center in the atmosphere is characterized by sinking air, light winds, and relatively cloud-free skies. The sinking air heats up as it compresses near the surface, and the rising temperatures are enhanced by the lack of clouds, which lead to more incoming sunlight and even warmer air in summer. Pavement also absorbs sunlight and radiates it out at night, leading to overnight low temperatures that can be in the 90s in extreme cases.

When the center of high pressure is locked in place over one area over a long time period, it tends to divert cooler, moister air away. That makes it appear as though there is a glass dome overlying the area blocking rain from falling. The hot and dry conditions tend to get worse quickly, leading to temperatures that in the worst cases can break records. This week many daily high temperature records were broken in the western United States as well as in parts of Europe. If the air is already warmer than average to begin with, that makes it even easier to reach record high temperatures. Often, a very wavy but stagnant pattern in the upper-level winds causes a series of heat domes to form across the globe, leading to heat waves at several locations at the same time.

What danger does a heat dome cause to gardeners?

Air in the center of the high pressure that makes up the heat dome tends to trap pollutants in a shallow layer near the surface of the earth. This can lead to episodes of smog and high surface ozone that can cause health effects on people, especially children and people with poor lung function. It can also trap smoke from wildfires, as I discussed last month. The trapped pollution makes it hard to breathe and can cause lung damage as well as leaf damage on plants. Temperatures that stay in the 80s or higher overnight do not allow human and animal bodies to cool down to their normal temperature, resulting in health issues that build up over several days as the heat wave continues. Phoenix has experienced every day this July with a maximum temperature of 110 F or higher, and has seen deaths due to heat increase, especially in homeless populations that have little access to cooling. Deaths from heat spells increase after several days of extreme temperatures when the body is not able to cool itself down and the heat causes physiological changes that can lead to severe impacts. While drinking a lot of water is important for hydration, it is not enough to fend off the impacts of the high temperatures. You must find a way to cool off or your health will suffer.

Humidity is also a factor in heat-related illnesses. Humans cool their bodies off by sweating. The moisture is evaporated from the skin, leading to an energy transfer that makes the skin cool off as the sweat is changed to water vapor. But if the humidity is too high, evaporation is so slow that it can not provided the needed cooling, and the body stays hot. That can eventually lead to death. In fact, it is not just humans and terrestrial animals to suffer. The water temperature near southern Florida this week was near 100 F, leading to predictions that the coral reefs in the ocean there would soon die because it is too hot for them to survive, much less thrive. This is also happening in other ocean locations around the world. Since coral reefs provide food and shelter for many marine species like fish, it is likely to cause dire consequences for the oceanic food chain, including those of us who eat shrimp, fish, and other seafood.

What do gardeners need to do when a heat dome is forecast?

When a heat wave or heat dome is predicted, gardeners and farmers should recognize that a protracted period of extremely hot, oppressive conditions is likely to occur. The first action they should take is to make sure they are protected from the effects of the heat. That means working outside early in the day when it is cooler, drinking plenty of water, wearing light-colored clothing to reflect sunlight, and taking plenty of breaks in the shade or air conditioning. They should take care of pets and livestock as well and consider keeping them inside or in shady places with plenty of water for drinking. They should monitor their plants carefully for signs of heat stress and water regularly to reduce the impacts of the hot, dry conditions. Trees should also be watered, especially if they are not well established or if the dry period is especially lengthy. If the humidity is high, watering incorrectly can increase the likelihood of fungal diseases, so read Linda Chalker-Scott’s article Water: Garden Friend….and Foe? – Water, Relative Humidity, and Plant Diseases – The Garden Professors™ to learn the best ways to water your plants and preserve soil moisture. Saving Your Trees From Drought! – The Garden Professors™ also provides information about watering trees during dry periods.

Eventually, all heat domes and dry spells do pass, but it is always good after one is over to assess how your garden did and to plan ahead to protect soil moisture using some of the techniques mentioned in the links above.