The Garden Professors™

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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.

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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.

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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.

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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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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?!”

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.

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.

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.

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.

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.

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.

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?

https://biodiversity.utexas.edu/news/entry/the-father-of-texas-botany#:~:text=Many%20species%20in%20central%20Texas,shows%20up%20in%20people%27s%20houses.

https://www.tamupress.com/book/9781623498764/the-writings-of-ferdinand-lindheimer/

https://www.tamupress.com/book/9781585440214/life-among-the-texas-flora/

https://archive.org/details/mobot31753003757678

I saw an article describing an atmospheric phenomenon called the “pneumonia front” this week and it made me start thinking about local kinds of wind and their names. No matter where you live, in the United States or elsewhere in the world, you have wind patterns that are related to your local geography. These winds can affect gardens, especially if they are persistent over time, but I enjoy hearing about the different names for wind too.

What causes the wind to blow?

Wind is the movement of air from one place to another. The air movement is driven by differences in air pressure from one place to another—the atmosphere tries to even out the pressure so air molecules are always moving from areas with higher density and pressure to areas with lower density and pressure. Since density and pressure are related to temperature (remember your ideal gas law from high school chemistry?) and temperature frequently changes as the sun moves across the sky or lakes and oceans warm and cool, the air is nearly always moving except where there is locally no variation in pressure such as the center of a high-pressure area.

Two common types of local winds

Winds are often linked to specific geographic features. For example, sea or lake breezes are located along the shores of large water bodies and are driven by pressure differences related to the relative temperatures of the land and water. When the water is colder than the land (for example, on a hot summer day), air pressure over the hot land is lower than over the cold water due to rising air over land (you can often see clouds where this is occurring). Air from over the water blows onshore in response to the lower pressure on land, leading to a cool breeze flowing over the hot land, cooling things off. At night when the land cools off more quickly than the water, the flow reverses and becomes a land breeze. Monsoons like the ones in India, the Southwest US, and other places are the largest-scale version of a sea breeze over thousands of miles and develop over weeks instead of hours.

Another geography-linked local wind is the katabatic wind. Katabatic winds are related to differences in elevation that cause temperature variations that result in density differences in the air. In a katabatic wind, air at upper elevations cools off at night, creating a pool of very dense air that rushes down the sides of the mountains to pool in the valleys, creating pockets of very cold air. Vineyard owners know this and plant vines on the sides of hills so that the vines are not exposed to the coldest air (and to take advantage of sunlight, too). The recent frost in New England caused severe losses of apple blossoms in the bottom of valleys while orchards in higher elevations were less affected. In your gardens, this occurs on a small scale with frost pockets that can form in the lowest-lying areas of your yards and garden plots. Antarctica has some of the strongest katabatic winds, with shallow winds that can reach up to 200 mph due to extreme temperature and elevation differences in that continent.

Other local wind names

There are many other location-specific winds and weather patterns linked to wind that occur in other parts of the world. Some are driven by elevation differences, with wind blowing through gaps in mountain ranges (the mistral in France and the tehuantepecer in Mexico, for example). Others blow in specific directions where mountains prevent air movement in some directions, funneling the air into channels that bring characteristic weather to the local area. In northeast Georgia, for example, we have frequent incursions of cold air from the northeast, with air pushed south due to high pressure in northern latitudes that is prevented from spreading to the west by the Southern Appalachian Mountains. We call that phenomenon “the Wedge” due to the shallow and dense wedge of surface air that is pushed by the wind flow into our region numerous times a year. Areas with very persistent topography-driven winds often have trees with most of their limbs on the downwind side of the trunk.

How does wind affect gardens?

Wind causes many effects on gardens. It can blow frigid air into a region from the poles towards the equator, leading to advective frost which causes damage to fruit blossoms in spring. If the humidity of the wind is low, it can quickly remove soil moisture and desiccate plants where irrigation is limited or unavailable. When strong, it can rustle leaves, break limbs, and even topple entire trees, especially where wet ground weakens the anchoring of tree roots. In fact, one measure of wind speed, the Beaufort Scale, uses an empirical scale related to the appearance of waves (on the sea) and tree movement (on land) to categorize wind strength. Some wind is a good thing for many plants because it provides stresses that help strengthen the stems and trunks, but too much can cause a lot of damage from wind-blown debris or direct force on the plants.

What local winds do you see and what impacts do they have on your gardens?

This blog has reached 194 different countries with many thousands of unique visitors a year, so the variety of local winds you experience must be amazing. Some of them are variations on the winds described above, either topography-driven winds like katabatic or anabatic (the opposite of katabatic, with up-valley winds during the day) or foehn winds. Others may develop due to unique geographic features of your area such as the Columbia River Gorge with winds so strong it is a haven for windsurfers. We’d love to see a comment on your local winds and how they affect your gardens!

I close by quoting the famous poem from Christina Rossetti that provided our title for this blog post, one of my favorites:

Who Has Seen the Wind?

Who has seen the wind?
Neither I nor you:
But when the leaves hang trembling,
The wind is passing through.

Who has seen the wind?
Neither you nor I:
But when the trees bow down their heads,
The wind is passing by.