Preparing your landscape for extreme weather

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.
Photo of the Mullen Fire in Wyoming on September 26, 2020. Justin Hawkins, USFS, Commons Wikimedia.

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.

Edit this at Structured Data on Commons
Fordgate: Flooded Garden, Lewis Clarke , Commons Wikimedia.

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?

Fruit tree branch, Vera Buhl, Commons Wikimedia.

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.

Buildings still smolder days after a wildfire gutted downtown Lahaina, August 11, 2023.
Buildings still smolder days after a wildfire gutted downtown Lahaina, August 11, 2023. © Robert Gauthier – Getty Images

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.
Wind damage, Moore OK, May 23, 2013, SSgt Jonathan Snyder, Commons Wikimedia.

Diagnosing Disasters: The Case of the Mopey Mophead

What happened to my hydrangea???

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.
Damaged hydrangea on the left generally outperforms the smaller, undamaged hydrangea on the right.

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
\Collect all pertinent information, especially recent weather data.

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.

All other landscape plants were able to tolerate the spike in temperatures – just not the hydrangea.

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.

Beneficial Bicarbonate?

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.

An advancing colony of powdery mildew on Poinsettia. Attempted control with bicarbonates at this time will cause some phytotoxicity.

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.


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.

It’s the heat (and the humidity!)

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.

Summer flower. Commons Wikimedia, ForestWander.

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!

How heat domes work. Source:

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.

Smog and haze hangs over the Salt Lake valley on a warm, sunny November Saturday. Commons Wikimedia, Eltiempo10.

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.

Roses wilted after a sudden heat wave with high temperatures for about a week. At Gamla Strandgatan 11, Gamlestan, Lysekil, Sweden. Commons Wikimedia, W.carter.

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.

Mandela Garden, Leeds: Fountain. Commons Wikimedia, Stephen Craven.

Horse(tail) sense or nonsense?

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.

Thin layers of wood chips won’t impede horstail. You’ll need 6 or more inches to keep sunlight out.

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.

Well, howdy neighbor!

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!

Arborist chips help us maintain weed-free ornamental beds.

I’m hot! So are my plants!

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.

In 2020 temperatures reached over 120 degrees in Ojai California. This caused immediate impacts to both native and introduced landscape plants.

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.

The leaves of this cherry were damaged by a high heat event in Ojai, CA. Note burn in center of the leaf.

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.

This oak was planted in a high albedo environment and while native to the area could not withstand the high heat it endured because it was not yet established in the landscape.

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.

Avoid pruning during high heat events.

Smoke gets in your eyes…and in your garden!

Over the last month, I have seen many stories related to smoke from Canadian wildfires drifting down into the eastern United States, causing muted sunsets as well as terrible air quality. Even my mom up in Michigan told me how bad the air is up there this week and friends in Wisconsin have told me that they can’t go outside without donning N95 masks to cut down on breathing in all the smoke particles. Of course, our readers in the western U. S. may be rolling their eyes since they have gone through severe wildfire seasons in past years with little attention from the eastern press, and poor air quality from wildfires and pollution is also a frequent problem in other parts of the world. But since it is in the news, I thought I would address aerosols and their impact on the atmosphere, human health, and our gardens.

Great Smoky Mountains, picture taken from Craggy Gardens Trail near the Blue Ridge Parkway in North Carolina, Amart007, Commons Wikimedia. Note that the blue haze here is caused by emissions of organic compounds from the trees augmented by water vapor.

What is an aerosol?

Aerosols are very small particles that float in the atmosphere. They can be from natural sources like salt from breaking ocean waves or pollen from blooming plants or can produced by humans through burning coal, construction, or poor agricultural practices. Saharan dust, volatile organic compounds emitted by trees, wildfire smoke, and volcanic ash can all add to the dust burden in the atmosphere. Some aerosols attract water vapor, causing them to expand in size and reducing the visibility of the atmosphere even more than the particles alone. Aerosols can be toxic, too, and areas with a lot of atmospheric pollution can cause severe problems for vulnerable people and pets when aerosols get deep into lungs.

Northeast smoke as seen from NOAA satellite, June 6, 2023

Impacts depend on where they are in the atmosphere

The impacts that aerosols have on humans and the environment near the ground depends on how high up the aerosols are concentrated. If the particles were lifted above the surface due to the heat from burning forests or trash, the main effects that the aerosols might have are optical, reducing the amount of incoming sunlight but not significantly affecting the air we breathe near the ground. Some acidic particles that attract water vapor might also contribute to acid rain that falls to earth. But if the dirty air is mixed down to the ground or is produced locally, the aerosols can cause significant issues for human and animal health because of their irritating effects on lungs and sometimes skin and eyes. They can also provide hazards to transportation if visibility gets too low. Acidic particles can also cause damage to plant tissues or change the pH of the soil if they affect an area over a long time period.

How do aerosols affect climate?

Aerosols affect climate by reducing incoming solar radiation. Volcanic ash and sulfuric acid droplets from volcanic eruptions can cut enough sunlight to reduce global temperatures for several years after a large volcanic eruption, especially if they occur in the tropics. This year’s unusually warm Atlantic Ocean temperatures can be linked in part to a lack of the usual plume of Saharan dust blowing off the west coast of Africa, which has allowed more sunlight to warm the surface water. The so-called “warming hole” in the Southeast has been linked to aerosol emissions from power plants upwind in the Midwest and Western U. S., which caused reductions in sunlight over the Southeast until the passage of the Clean Air Act of 1970 reversed that effect. Since then, the temperature in the Southeast has risen in concert with rising temperatures across the rest of the world. Aerosols contribute to the development of clouds, too, and that has the potential for affecting climate at larger spatial scales.

Saharan dust, NASA-NOAA, 20 June 2020.

How do aerosols affect health?

Aerosols affect human and animal health when they are inhaled into the lungs, irritating tissues and causing swelling and producing fluid as the lungs try to clear the aerosols out. According to estimates from the World Health Organization (WHO), particle pollution contributes to approximately 7 million premature deaths each year, making it one of the leading causes of worldwide mortality. Fine particles that are smaller than 2.5 micrometers (called PM2.5) are the most damaging because they are so small that they can make it deep into the lungs where they are deposited on the lung tissue. Because of this, gardeners and others who spend a lot of time outside need to be aware of the current air quality measurements and minimize time outside when the air quality is bad. You can find current air quality information in the United States at AirNow. Many state health agencies also post air quality information and the National Weather Service also puts out alerts on days with bad air quality. When the plumes of smoke from the Canadian wildfires moved over the Midwest and the Northeast, some U.S. cities had the worst air quality of any metropolitan areas in the world while the smoke was present.

Dusty leaves at Kaukaukapapa, Kahoolawe, Hawaii. December 20, Forest and Kim Starr, Commons Wikimedia

How do aerosols affect gardens?

Aerosols have several impacts on plants and gardens. Aerosols provide benefits for gardeners since clouds and rain form from water that is collected into water droplets on aerosol particles known as Cloud Condensation Nuclei (CCN). No doubt if you collect rain or snow water, you have seen the dirt that remains after the water is gone. But aerosols also have detrimental effects. Aerosols aloft can reduce incoming sunlight, leading to slower plant growth, especially for plants like corn that are sensitive to the amount of sunlight they receive. Aerosols at ground level can cover the plants with a layer of dust that decreases photosynthesis by blocking incoming sunlight and clogging pores. If the aerosols are acidic or contain toxins, they can damage the plants or increase the acidity of the soil, especially over long time periods. In the case of smoke from wildfires, the smoke particles can also affect the taste of grapes or other food products they interact with. Smoke taint on wine grapes, caused by compounds from aerosols that are absorbed by the grapes, can impart an ashy flavor to the wine made from those grapes, making it unsellable, as producers in California and Europe have found in recent years.

If you are experiencing air quality issues in your community, we encourage you to monitor the weather forecasts closely and stay inside when the aerosol count gets too high, especially if you have asthma or other lung conditions that may be made worse by poor air quality. If you have noticed other impacts of the wildfire smoke or other air quality issues on your garden plants, please feel free to share them in the comments.

Smoke from wildfire on Angel Island blankets Downtown San FranciscoBay BridgeSan Francisco Bay and the rising sun, Brocken Inaglory, Commons Wikimedia

A Toast to Agaves

Agaves, those bat pollinated, succulent, strong leaved, slow-growing, xeric- and heat-loving Western Hemisphere plants, are literally the heart of the tequila and mezcal industry. As fascinating as the bat pollinator aspect is we’re going to focus on the how agaves are used to produce liquor.

Image by Jesus Cervantes/Shutterstock

Let’s start with the differences between mezcal and tequila. These include region of origin, plants used and production methods.

We’ll start with regions and plants.

The name “mezcal” comes from the Nahuatl word “mexcalli” which means “oven-cooked agave.” Although mezcal can be made from any agave species, production focuses on roughly 30 agave species, varieties, and sub-varieties. While mezcal’s history centers around the region of Oaxaca, Mexico, it’s now produced throughout the country. As mezcal can be made with any agave species the name has become a general one for most agave liquors in Mexico. It often implies an artisanal aspect to the drink whether it’s deserved or not. In 1994 the name mezcal was recognized as an Appellation of Origin  (AO, DO). There is also a Geographical Indication (GI), originally limited to the states of Durango, Guerrero, Oaxaca, Puebla, San Luis Potosí, and Zacatecas. Similar products are made in Guanajuato, Jalisco, Michoacán, and Tamaulipas but these have not been included in the mezcal DO.

(Patricia Zavala Gutiérrez/Global Press Journal)

While both mezcal and tequila are made with agave, only one species is legally allowed for tequila production, the blue agave.
Tequila production is located primarily in the area surrounding the city of Tequila, which is northwest of Guadalajara, and in the Jaliscan Highlands of the central western Mexican state of Jalisco.  Tequila is also recognized as an Appellation of Origin (AO, DO). It can be produced only in the state of Jalisco and limited municipalities in the states of Guanajuato, Michoacan, Nayarit, and Tamaulipas.

Blue agave field
Photo by Christian Heeb

Now let’s take a look at production methods. Harvesting agave for mezcal and tequila production starts out the same.

Seven to ten years after planting the plants are mature enough to harvest. They are manually harvest by “jimadors,” highly skilled people trained in the art of agave harvesting. It’s hard, labor-intensive work.

Using machetes or a “coa de jima”, a specialized agave cutter, the jimadors cut off the long agave leaves to get to the core of the plant called the piña.

The piñas are collected and taken for roasting. Roasting method is where mezcal and tequila production methods differ.

Pit roasting the piñas is traditional for mezcal production.

Agave piña roasting pit for making Mezcal
The rocks in the pit are first heated with charcoal
When the the temperature is correct, the piñas are added.
Alternating layers of piñas and chopped agave leaves are added until the pit is full.

The entire thing is covered and left to smoke for 2-7 days depending desired smokiness of the final product.
Roasted piñas.

Cooking piñas for tequila is a much simpler process. They’re actually baked.

Traditional brick ovens can be used.
Or large metal ones such as these.
The end result is the same.

After roasting or baking the piñas receive the same treatment regardless of the final product, mezcal or tequila. They’re crushed or shredded to extract the juice which is then fermented for a period of time. The fermented product is then distilled twice and then usually aged. Some mezcal is not and is sold a “joven” or young. Aging can last from one month to as long as 12 years. After aging the liquor is usually stored in stainless steel tanks to reduce evaporation.

And yes, I hear you there in the back row, “But what about the worm?!”

Gusano de Maguey in a bottle, waiting to be added to finished mezcal.

The worms are only found in mezcal, never tequila, and not all bottles have one. Bottles of mezcal which have a worm (called gusano) are labeled “con gusano,” meaning “with worm.” The worm is actually a caterpillar of the moth Comadia redtenbacheri which can infest agaves. If a “worm” is to be included it’s added at bottling. Doesn’t that sound like a fun job.

There are various stories as to why a “worm” would be added. Some claim it’s a marketing ploy. Others say it’s there to prove that the mezcal is fit to drink…OK. Others believe that it brings good fortune and strength to the lucky person who finds it in their glass. If you’re fortunate to find one in your glass be sure to swallow it whole, don’t chew it. And some claim it’s there to impart flavor. Yummy.

Mmmm, pickled ‘pillar!

And lastly, I’m sure some of you have seen “worm suckers” at shopping emporiums which carry a certain type of tourist stuff with a (supposedly) south-of-the-border flavor. Yes, I’m talking about the famous, or infamous, tequila-flavored worm sucker.

Also available in different colors and flavors. Look for them at finer tourist traps across the Southwest USA.

Don’t fall for this! As educated and discerning Garden Professors blog post readers you now know that #1: Tequila never contains a worm and #2: the “worm” is actually a caterpillar and the above critters encased in sugar are actually the larvae of the darkling beetle, commonly known as mealworms. Be a savvy consumer, hold out for the real thing.

Buying lady beetles and mantids for your home gardens is probably not the best pest control strategy

Biological control is the use of natural enemies such as predators, parasites/parasitoids, and pathogens of pests in order to suppress or control them. This is a great tool for pest control and we hear about biological control a lot, especially when we talk about IPM (Integrated Pest Management). It usually comes with the classic imagery of a hungry lady beetle (often incorrectly referred to as the lady ”bug”) munching on aphids.

Cartoon of lady beetles munching on aphids by Sara Zimmerman (

Yes, many lady beetle species are great predators of pest insects…so much so, that the multicolored Asian lady beetle (Harmonia axyridis) was intentionally imported and released in North America in 1916 as a more ‘natural’ way to control common pests. Species of North America’s native convergent lady beetle (Hippodamia convergens) were also collected from their habitat (around 1924) and relocated to agricultural locations within California for aphid control, which showed high success rates.

Another popular insect that comes to mind when we think about biological control is the mighty and charismatic praying mantid (aka praying mantis). These ferocious predators, in the family Mantidae, are beautiful and captivating creatures that even grab the attention of the non-entomologically-inclined. With their large eyes and raptorial front legs, you can’t help but be fascinated by them. Although there are some native species of mantids in North America, the ones you are most likely to come across in your yards and gardens include the European mantid (Mantis religiosa) and the Chinese mantid (Tenodera sinensis). Like their names suggest, these are not native to North America, though they have been here for over a century being both accidentally and intentionally introduced overtime. The Carolina mantis (Stagmomantis carolina) is another mantis that you might come across, especially in the southeastern United States, and this one is native to the Americas, from the southern US to Brazil.

Adult European mantid eating a grasshopper
(Photo: Whitney Cranshaw, Colorado State University, )

The predatory nature and biocontrol successes of some of these insects have given rise to their popularity as a commercial pest control product and resulted in an increased interest in purchasing them. These are widely available online, in nurseries, garden centers, and in several other retail outlets. Often marketed as a “good alternative to pesticides” the intention behind this practice is a positive one: reducing unnecessary pesticide use by incorporating beneficial insects that will help manage pests in the landscape. That being said, like many other simple and catchy solutions to common issues, this may not be the most responsible or effective option for home gardeners to reduce pest populations while still being good stewards of their yard and garden ecosystems.

What are the issues associated with releasing purchased beneficial insects in home gardens?

Introducing populations of species into new ecosystems can have several unintended consequences. This applies to non-native and native species alike.  A Washington State University Extension publication by our very own Dr. Linda Chalker Scott and Dr. Michael Bush from the Washington State Department of Agriculture does a great job of summarizing some of the issues. Whether or not they are native or widespread throughout the country and/or continent, not all regions and/or ecosystems may have high numbers of these insects and their introduction could result in competition with other common predatory arthropods and further unintended ecosystem impacts. These insects can also consume beneficial organisms, especially in the case of praying mantids, who are just as likely to feed on any insect they catch including other predators, pests infested by parasitoid wasps, and even pollinators. In some of these insects, cannibalism is also a common survival strategy, especially if resources are scarce.  

Adult convergent lady beetle
(Photo: Kansas Department of Agriculture , )

Introducing these insects into new locations can also introduce their pests, including potential parasites and diseases, which could impact previously unaffected populations and even other species of beneficial insects in our home landscapes. This doesn’t even account for the ethics of sourcing some of these insects and the impacts of removing large quantities from their natural habitat.

Does it actually work for controlling yard and garden pests?

One of the first things that happen when you release these purchased insects into your home gardens is that many will simply disperse. That is, if they survive the harsh conditions of sitting on a store shelf in hot temperatures. In fact, to have the most success in releasing them in your gardens, you need to take special care and pay attention to factors including time of day/temperature and the number and type of pest insects available for them to eat. For more detailed information on lady beetle release best practices, see this publication from UCANR.

Commercially available convergent lady beetles (H. convergens) are harvested as adults in a dormant state from their overwintering sites. They have a migratory behavior where they will disperse before they feed and lay eggs. As mentioned in this publication from Cornell University, some commercial insectaries will feed these adult beetles a special diet to reduce this migratory behavior. If you do still plan on purchasing lady beetles, these could be a better option. Even if these beetles don’t disperse once you have released them, you need enough pest insects to make it worthwhile for them to stick around for a little while. Although H. convergens are considered generalist predators that feed on aphids, scales, thrips, other soft-bodied insects, and even pollen and nectar when prey are scarce, their preferred diet is aphids. Unless you have heavy aphid infestations in small areas, it’s probably a waste of money (and lady beetles) to introduce them to your landscape. If you do however have a very heavy infestation of aphids, you need to make sure you have enough lady beetles to do the job properly. Even if you do everything correctly and have ample aphids for them to eat most lady beetles will still fly away after a couple of days. They are unlikely to lay eggs on the plants that they are released on thus requiring subsequent releases to continue managing a concentration of pests.

A group of adult convergent lady beetles
(Photo: Scott Bauer, USDA Agricultural Research Service, )

Mantids, on the other hand, are released as egg cases (ootheca) or newly hatched nymphs from those egg cases. You will often see mantid egg cases available for sale, and if you don’t release them within a day or two of hatching, most of these nymphs will cannibalize each other. You can try to spread them out around your garden, but they will still likely eat any arthropod that they come across and catch (including other beneficial insects). They are also unlikely to stay localized around a specific pest issue, so they’re not really effective pest control agents. More information on mantis releases can be found in this publication from University of New Hampshire.

European mantid egg case (ootheca)
(Photo: Whitney Cranshaw, Colorado State University,

What is a better alternative to purchasing insects for home gardens?

Encouraging the natural enemies that are already in your yard and garden landscapes (also known as conservation biological control) is the best way to incorporate long-term and effective biocontrol for home gardens. These natural enemies include predatory beetles, lacewings, parasitoid wasps, spiders, and countless others!

Tomato hornworm caterpillar, parasitized by braconid wasps
(Photo: Gerald Holmes, Strawberry Center, Cal Poly San Luis Obispo, )

Sustaining these beneficial critters also means providing a diversity of habitat, including food and shelter for them. Include a variety of flowering plants all season long because these natural enemies will also feed on nectar and pollen in addition to their prey. Let your landscapes be a little ‘wild’ by keeping some leaf litter, rotting wood, dead perennials, and ornamental grasses which provide shelter for overwintering. More information on encouraging insects for biocontrol in home landscapes can be found here.

Another important factor for maintaining beneficial insects in home gardens is to utilize IPM strategies when pest outbreaks do occur and to minimize unnecessary pesticide use, especially pesticides that are broad spectrum, or persist in the environment for long periods. Utilizing cultural controls, barriers, and tolerating a little bit of pest damage is all going to contribute to the long-term health of your home garden ecosystem.

People and Plants

In this edition of P&P we’ll be exploring the life of the “Father of Texas Botany”, Ferdinand Jacob Lindheimer.

On May 21, 1801, Herr and Frau Lindheimer of Frankfurt, Germany welcomed little blue-eyed Ferdinand to the family. After schooling at the Frankfurt Gymnasium and a Berlin prep school, Ferdinand spent the next 30 years studying at universities in Bonn, Jena, and Wiesbaden.

In 1833, for political reasons, Ferdinand decided it was best for him to leave Germany. By 1834 he was in Belleville, Illinois. Not finding Belleville to his liking, he caught a boat down the Mississippi to New Orleans, LA.

“Port City of New Orleans” by Adrien Persac.

After some time he and a few companions tried to go to Texas. But the Texas revolution was heating up and they wound up being sidetracked to Mexico, eventually winding up in Veracruz. There he worked on a banana plantation for over a year all the while becoming infatuated with the regional flora and fauna. But he still wanted to go to Texas and left Mexico just as the hostilities in Texas were escalating. In an effort to reach Texas he tried joining the Texas revolutionaries but alas, it was not to be. He wound up ship-wrecked on the Alabama coast near Mobile.

So close and yet, so far.

Being the headstrong German that he was, he tried once again to reach Texas and finally arrived at San Jacinto (pronounced Hah-seen-toe) the day AFTER the final battle of the Texas Revolution on April 22, 1836. Despite missing most of the action he joined the army of the new Republic of Texas and served 19 months. During this time and after his discharge in 1837 he spent any free time exploring the flora of his new home.

An old friend from Frankfurt, Georg Engelmann, invited Lindheimer to spend the winters of 1839–40 and 1842–43 with him in St. Louis. (Englemann had immigrated to America in 1832 and dabbled in botany as a hobby.) Lindheimer brought preserved Texas plant samples with him on these visits. Via their friendship Lindheimer’s collections came to the attention of professor Asa Gray, founder of the Gray Herbarium at Harvard University and author of the original Gray’s Manual of the Botany of the Northern United States. The plants from the Republic of Texas generated quite a bit of excitement in old Harvard Yard.

In 1843 arrangements were made for Lindheimer to collect plant specimens for Engelmann and Gray. He spent the next nine years collecting from a variety of Texas areas, including Chocolate Bayou, Cat Springs, Matagorda Bay, Indianola, and Comanche Springs. 

Over the next thirteen years, Lindheimer collected over fifteen hundred species in central and south Texas for Engelmann, Gray and others who were building collections. The samples had to be pressed and dried with multiple changes of blotting paper, then mounted and shipped. The collection date, location and habitat were logged for each specimen. Lindheimer earned $8 for each hundred specimens submitted. Occasionally he sent seeds or cuttings so Gray could try propagating the plants at Harvard. Using his own knowledge and whatever reference materials he could find, Lindheimer could place most plants in the appropriate family and make a good guess at the genus. But official classification was left to the scholars who received his samples.

Ipomea lindheimeri 
Photo by Greg Goodwin

In 1844 Lindheimer was granted land on the Comal River in the new community of New Braunfels, TX. and remained in the area for the rest of his life. He kept collecting, started a botanical garden, and in 1852 was elected the editor for the town newspaper, Neu Braunfelser Zeitung, one of the earliest newspapers in Texas. He was associated with the paper for the next 20 years, eventually becoming the publisher. Legend is that it never missed an issue, not even during the Civil War when newsprint was not to be had. Lindheimer printed on butcher paper, wrapping paper, and leftover paper from his plant-preserving supplies.

Neu-Braunfelser Zeitung (New Braunfels, Tex.), Vol. 1, No. 16, Ed. 1 Friday, February 25, 1853

In 1872 Lindheimer ended his association with the paper to devote more time to his work as a naturalist. He is credited with discovering several hundred plant species and his name is used to designate forty eight species and subspecies of plants and one species of snake. ( I really wanted to put a picture of the snake here but was advised that some people don’t like reptiles as much as I do. Sigh)

In 1879 his essays and memoirs were published under the title Aufsätze und Abhandlungen.

Lindheimer died on December 2, 1879, and was buried in New Braunfels. His grave is registered on The Historical Marker Database and his house on Comal Street in New Braunfels, is a museum, a Registered Texas Historic Landmark and is on the National Register of Historic Places.

Lindheimer’s plant collections can be found in at least twenty institutions, including the Missouri Botanical Gardens, the British Museum, the Durand Herbarium and Museum of Natural History in Paris, the Harvard University Herbaria, the Smithsonian Institution, and the Komarov Botanic Institute in St. Petersburg

Want to learn more about Ferdinand Lindheimer?,shows%20up%20in%20people%27s%20houses.