The Garden Professors™

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.

One of the most annoying weeds in garden and landscape beds is horsetail (Equisetum spp.), a genus native throughout North America and most of the rest of the world. They have survived since prehistoric times because they are highly adaptable to their environments and are almost impossible to eradicate. There is great debate among gardeners on whether to pull or cut horsetail. Online you can find statements such as this:  “…each time you break the stem, little portions under the soil regenerate new plants. Essentially, you will be creating more horsetail.” This and many other websites recommend cutting instead.

Unfortunately, this is bad advice. The trick to eradicating any perennial weed without chemicals (or at least bringing them to manageable levels) is to starve them to death. Plants depend on their roots (and rhizomes in the case of horsetail) to survive, so anything that reduces root resources is going to eventually kill the plant. Obviously the more above-ground material you can remove, the less photosynthesis occurs and fewer resources are transported to the roots. Pulling weeds, especially if done with a forked weeder (also used in this post), is going to remove far more material than simply cutting weeds off at the surface.

Once you start a weed removal project, you have to keep after it: once is not enough. There will be rhizomes or roots left underground to support new stem growth, and once they reach the soil surface they will start producing resources to send to the roots. “Constant vigilance” is needed to keep these shoots in check. You can significantly reduce the repeated pulling by adding a thick layer of arborist wood chips to the newly weeded site. This forces the roots to put even more resources into stem growth to reach sunlight, meaning fewer weeds and more successful, desirable plants.

There is one caveat for controlling any weed that spreads underground. If you can’t control the spread from adjacent properties, you will not be able to eradicate the problem. In such cases, you may want to install a root barrier along the edges of your gardens. You simply dig a trench and install the barrier of your choice, making sure there are no gaps between the sections. Treated timbers, concrete pavers, and other materials that are slow to degrade can be used. The depth is going to depend on your soil conditions and the weeds of interest; some preliminary digging to determine the depth where you find weedy rhizomes and roots will help. Keep in mind that root barriers will also interfere with the root spread of your desirable plants.

If root barriers are not an option, the other method you can try is to densely plant low shrubs and perennials along the property line to create a competitive line of defense. The roots will compete for space, water, nutrients, and oxygen; the crowns will create a shaded environment where invading stems struggle for space and sunlight. You will still have to watch for invaders, but the amount of weeding needed will be far less than it was before. And don’t forget the mulch, both for the benefit of your barrier plants and to force invaders to use more resources to get their stems to the surface.

This method works for ALL plants – not just horsetail. (Plant physiology is funny that way.) Bindweed, English ivy, Himalayan blackberry, and Canada thistle are all weeds that I have personally controlled through physical removal and deep mulching with arborist wood chips. If you’ve had success with this method on another aggressive weedy plant, be sure to post a comment!

We are again in the midst of excessive heat events in many parts of the United States. Records were broken for the highest temperatures ever recorded just a few days ago. This is also a time when the days are at their very longest, so high temperatures have large impacts on plants in landscapes.

High temperature can have immediate (acute) and continuing impacts (chronic) on plants. When temperatures get much over 90F photosynthesis becomes less efficient and in some plants may stop all together. As temperatures increase beyond 90F photosynthesis shuts down and transpiration may also stop to avoid breaking the chain of water molecules that plants must have to move water. When this happens heat builds up in the foliage leading to cell death and eventually symptoms (acute response). These may initially show as wilting, loss of color in the leaf and rapidly within days show as yellowing and then necrosis. This is usually seen in the center of the leaf first as the edges of leaves dissipate heat faster and more efficiently than around the mid vein area of leaves.

Chronic effects of heat are related to the poor efficiency of photosynthesis at high temperatures. When plants are hot and the photo systems that capture sunlight energy are impaired, or not working, the plant must still use energy in all its cells for respiration. Stored carbohydrates are not available for growth as cell maintenance (respiration) is the first demand for energy. When temperatures are high for long periods, stored carbohydrates in roots and stems are depleted. Since energy for growth is not available, slowed or stopped growth is the biggest chronic effect of hot days on most plants. This is why even hydrated plants just seem to stop growing in hot weather.

What can be done to mitigate high temperatures? First, never let plants dry out during high heat events. Evenly moist soil (but not saturated) will allow plants to absorb water and cool themselves as much as their physiology will allow. If soils are dry the damage of high heat events is “magnified” many fold and foliar damage will increase. Irrigate late in the day or early to avoid evaporation of applied water. Get your plants ready for high heat by irrigating before it hits. We usually have good weather prediction a few days ahead of high heat events.

Another way to mitigate high heat is to avoid plantings in “high albedo” environments. Albedo is the reflection of sunlight. Low albedo surroundings abosorb sunlight energy, high albedo environments reflect it. Plants exposed to reflected sunlight will be more readily damaged by sunlight during high heat events because they can not transpire enough water to cool their leaves. Reflective soils like decomposed granite, or some kinds of rock will damage young trees during heat events. Cover the soil with arborist wood chips which have a relatively low albedo. Young plantings can be protected by placing shade cloth over their canopies until the high heat subsides. If you don’t have shade cloth, a white sheet will do fine as it will reflect heat away from the canopy.

Ensure that the mulch or soil is moist before the heat of the day starts so humidity increases during the day. This will reduce the demand on transpiration and and the possibility of cavitation (the disruption of water chains in the plant and introduction of air which stops water movement), thus preventing a catastrophic heat death event.

A final word of precaution- Never fertilize during high heat events. Even when watered this changes the osmotic potential of water in soil making it harder for plants to pull water in. Adding fertilizer is like adding salt and this is a big NO during high heat events. Try to ensure that plants have all the mineral elements they need before heat becomes an issue.

You might think that during heat events its a wise idea to prune. This is not the case! Avoid pruning, especially thinning, as the removal of leaves will increase the impact of heat on the remaining canopy. Pruning and removing leaves will decrease the humidity around a plant and the remaining leaves will have to transpire more to cool the plant. This can be a disaster during a high heat event.