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In the quarter century that I’ve been researching, publishing, and educating on the topic of landscape mulches, one thing has become clear: cardboard should never be used as a mulch. This viewpoint has been of great interest to gardeners; in fact, my earlier post has been the most frequently viewed post since it was published in 2015. I occasionally appended new information to the original post as needed, but the topic deserves an update.

Rather than rehashing what’s been written earlier (which can be found here, here, here, here, and here in addition to the link above), I’m providing information in a Q&A format that might be helpful:

Q: Is there research on using cardboard mulch in home landscapes?

A: Not much. To date, the only peer-reviewed research relevant to landscape soil conditions is our own work published in 2019. The abstract explains the importance of the results to landscape soil health as stated in the abstract (below):

“The orders of magnitude differences in diffusion coefficients among the mulch materials, however, could negatively impact a diverse soil environment such as those found in biologically rich landscapes with higher oxygen demands. Among the mulches tested, wood chips are a preferred method of mulching in terms of providing best gas permeability, particularly in landscape conditions.”

Q: Cardboard has been used as a mulch in agricultural production. Why doesn’t that research support using it in landscapes?

A: The goal in agricultural production is to maximize yield of an annual crop. In contrast, the goal in caring for a permanent landscape is to maintain a healthy soil ecosystem that will support plant life long term. The table below explains these differences in more detail.

Q: Okay, I understand that science doesn’t support using cardboard as a landscape mulch, but what about my vegetable garden? Isn’t the research on agricultural crops relevant there?

A: The research on agricultural production mulches is more relevant if maximizing yield is your most important goal. But your goals may include maintaining a healthy soil ecosystem, reducing the use of pesticides and fertilizers, and other criteria. Are you concerned about the established negative impacts that cardboard and other sheet mulches have on soil life? If so, then sheet mulches are not a good choice compared to chunky, three-dimensional mulches.

Q: I like reusing cardboard packaging as part of organic weed control. Isn’t that a good enough reason to use cardboard?

A: In addition to interfering with water and gas movement into the soil environment, corrugated cardboard has chemical contaminants that you really don’t want in your soil or even your compost pile. Corrugated cardboard contains environmental contaminants including dioxin and PFAs or “forever chemicals.” No gardener should want to introduce more of these widespread contaminants into their landscape or garden soils.

As I’ve been recommending for nearly a quarter century now, the very best mulch to use for treed landscapes is arborist wood chips. There is robust, peer-reviewed science establishing the benefits of arborist chip mulches in controlling weeds, enhancing growth and establishment of landscape plants, and maintaining a functional soil ecosystem. In contrast, sheet mulches such as plastic, weed fabric, and cardboard have demonstrated negative impacts on the long-term health of landscape soils. Any resource that says otherwise is not paying attention to the research-based facts.

In this blog I’ve talked several times about El Niño and La Niña and how they affect climate across the Northern Hemisphere as well as their impacts on the rest of the world. We are currently in a strong El Niño with sea surface temperatures in the Eastern Pacific Ocean (EPO) that are much warmer than the long-term average. But underneath the surface the ocean currents are starting to change and the El Niño is expected to swing quickly into the opposite phase, La Niña. That will affect us in North America but also other parts of the world since both El Niño and La Niña are linked to global atmospheric patterns. Since a La Niña Watch was just issued by NOAA this week I will be talking about the changes we can expect to see over the next few months and how those changes will affect gardens and gardeners.

Review—What are El Niño and La Niña?

El Niño and La Niña are opposite phases of a large-scale atmosphere/ocean pattern that is driven by temperatures in the EPO. The pattern affects climate in many places around the globe. It is the biggest driver of seasonal climate in the Southeast and Pacific Northwest as well as in some other countries, especially in Northern Hemisphere (NH) winter when it is usually the strongest.

In September 2023 I discussed the likely impacts of the El Niño that was growing at that time and how it would affect your winter gardens. The conditions I expected have mostly been observed, although there are some local differences that are not surprising considering that each event is unique. Northern states have been incredibly warm with little snow, while in the Southeast we have had a lot of rain and cooler (although not frigid) temperatures due to wet soils and a lot of clouds blocking the sun. California is getting hit by one atmospheric river event after another, so they are also very wet and are even seeing a lake in Death Valley. I imagine they will have quite a bloom of spring flowers when it gets warmer because of the ample moisture. What have you experienced in your area? Did my earlier column get it right?

How is this season different than a typical El Niño winter?

Even though we have had the swings of El Niño and La Niña (collectively called El Nino Southern Oscillation or ENSO) for thousands of years as evidenced by layers in ice sheets in Peru and ocean sediments, there are other changes that are not cyclical. The rise in global temperature over time is showing up as a warming trend in all seasons but especially in winter. That does not mean we don’t see other swings in climate over time because ENSO and other atmospheric cycles are still occurring, but they are superimposed on the slowly rising temperature associated with increases in greenhouse gases so the cold outbreaks aren’t quite as severe and the warm spells last longer.

This year one of the most notable things we are seeing in global climate is the unbelievable warmth in the Atlantic Ocean. Temperatures there now are at values consistent with June or July temperatures! This is the energy that will feed tropical storms later in summer (more on that in a minute). Scientists are still not sure of all of the factors that are contributing to these record-setting conditions, but they may include the eruption of Hunga Tonga, the elimination of sulfur emissions from modern cargo ships, and changes in the global ocean circulation.

In addition, in spite of one cold big outbreak this winter across the eastern United States, most areas have been warmer than normal resulting in an early spring that has brought honeysuckle leaves to my yard more than a week early. You can follow the “green wave” north and see when it gets to your area or verify that it’s already there at the National Phenology Network site. I am concerned about the possibility of having another late frost like 2023 that could impact the peach and blueberry growers in the Southeast since our average last spring frost date is early to mid-March for most of the commercial peach region. There has been enough cold weather for most of the fruit-producing plants to have reached their required number of chill hours, which means the warm weather is making them ready to bloom. While I don’t see another cold outbreak on the horizon for the next few weeks, we have had frosts in the Southeast into April before so we are not out of the woods yet.

When will La Niña begin?

Climatologists predict that El Niño will weaken through spring and we will swing back into neutral conditions by the April through June period. From there most models predict we will move into a La Niña by the June through August period. By NH fall (September through November) there is a 77% chance we will be in La Niña conditions. This has implications for the summer and especially for the Atlantic tropical season since in neutral and La Niña years the number of tropical storms that occur in the Gulf and Atlantic is higher than in El Niño years. Last year despite El Niño we had 20 named storms, much more than the average of 14 events. This was in part due to the unusually warm water. Most of those stayed over the Atlantic Ocean rather than make landfall due to the presence of a strong jet stream high in the atmosphere which disrupted the development of storms farther to the west and prevented a lot of damage to us in the United States. In spite of that, we still had Hurricane Idalia and Tropical Storm Ophelia, both of which caused a lot of damage to infrastructure and agriculture.

With La Niña fully in place by fall, there will be little to stop the development of tropical storms in the Gulf and Atlantic Ocean except for Saharan dust and unfavorable weather patterns in the United States that could at least shunt any storms away from land. Some early unofficial predictions are for 25 or more named storms to occur this year, although the official predictions are still a few weeks away.

Next winter, we can expect La Niña to control a lot of our climate. That means warmer and drier conditions across the southern part of the United States while cold and wet conditions return to the northern states. Here in the Southeast, that means soil could be pretty dry in spring 2025 leading to issues with planting although it will be easier to drive heavy equipment into the fields than I expect will happen this year.

What does all of this mean for gardeners in the United States?

Because of the recent warm conditions associated with rising temperatures and enhanced by El Niño in northern parts of the country, spring is coming early to many places. That can be a good thing if you like flowers and don’t like snow, but it does mean that your early flowers will still be susceptible to frost damage if we get another cold outbreak later in March or even into April or May for northern states. So you will need to be prepared to protect the tender plants if a frost or freeze occurs.

The end of El Niño and the eventual rise of La Niña also has implications for areas that are affected by tropical systems. This includes the Gulf and East Coasts and areas downwind of those locations but can also include parts of California and the Southwest which can see impacts from tropical systems in the EPO west of Mexico. Rainfall could be hit or miss in the late summer depending on where the storms go. You should prepare well in advance of June 1, the official start of the season, because the warm ocean water could allow tropical storms to develop in May ahead of the “official” start. That means making sure you remove damaged limbs or other objects that could become wind-borne debris, make sure you have adequate drainage for heavy rain, and keep an inventory of your belongings that could be washed or blown away in a storm. Have a family plan to keep in touch and evacuate if you need to, including pets and livestock. You can find a lot of good information on preparing for and recovering from natural disasters in this University of Georgia handbook, even if you are not in Georgia or the Southeast.

While the current warm weather makes gardeners eager to get out in their plots and get started, it’s probably too early to start in most of the country except the most southern areas. But you can dream and start planning for the warmer weather soon to come!

Over the course of this winter there have been several days when the temperature plummeted after the passage of an Arctic front as strong winds blew frigid air into parts of the United States. It can happen in other parts of the world, too. When this happens, the National Weather Service (NWS) often issues Wind Chill Warnings urging people to bundle up before they head outside into the dangerously cold weather. From time to time I am asked if plants also experience wind chill. This week let’s explore how wind does and does not affect plants.

What is wind chill?

The Oxford Dictionary defines wind chill as “the cooling effect of wind blowing on a surface”. That is partially correct as a general statement of how the wind makes you feel, but it is lacking as a scientific definition. A better one is “a quantity expressing the effective lowering of the air temperature caused by the wind, especially as affecting the rate of heat loss from an object or human body or as perceived by an exposed person.” The wind-chill index is a calculated index that tries to quantify how a strong wind can remove heat from a human or animal body. A low wind chill indicates that heat is being rapidly removed from the body, potentially resulting in the lowering of internal temperature and the chance of frostbite to extremities like fingers, toes, and noses if directly exposed to the frigid wind.

“Wind chill” is used by the NWS as a way to provide a warning message to people who might be working or playing outside to make sure they are well protected from direct contact with the wind. As a calculated index wind chill cannot be measured directly although there have been some lab experiments in temperature-controlled laboratories that have tried. The formula for calculating wind chill has been changed in the past as science has improved our understanding heat transfer by wind.

How does wind chill work?

When wind blows across a surface, it causes a transfer of energy between the wind, which is at one temperature, to or from the surface which is most likely at a different temperature. If the temperature of the wind is lower than the surface, then heat (which is just a measure of the energy of the molecules at the surface) is stripped from the surface and transferred to the wind. That lowers the energy of the surface and cools it off (note that this is different than evaporative cooling, which is cooling due to evaporation of water from a surface). The faster the wind blows, the quicker the energy is stripped away. In summer, when the ambient air temperature is high, this cooling effect from a breeze off a cooler water body like the ocean may feel pretty good. But in winter, when temperatures are already icy, it may cool off the surface (and by connection, the body beneath the surface) to dangerous levels resulting in frozen cells that are the hallmark of frostbite or hypothermia if the core body temperature is affected.

Plants do not generate internal heat and so are generally the same temperature as the air. Because of this, there is no transfer of heat energy between the air and the plants and so the plants would not experience “wind chill.” However, anything that has an internal source of energy, including humans, animals or running engines, could experience a chilling effect as heat is stripped away from the surface by the wind especially if the warm surface is exposed directly to the cold wind. That is why it is important to wear layers to provide protection from the wind when the wind chill is expected to be extreme since it keeps heat from being removed from the skin. Ranchers help protect their livestock in blizzards by creating wind breaks that reduce the wind speed and so limit the impacts of wind chill on their cattle. Newborn calves may be especially susceptible to wind chill on their ears and may be outfitted with earmuffs to protect them from frostbite.

What other impacts does wind have on plants?

Even though plants do not experience wind chill, that does not mean that there are no impacts from the wind. In addition to transferring heat, wind can also transfer moisture from the plant to the air, desiccating the plants if the humidity of the air is low. This can happen at any temperature as long as there is not much water vapor in the air, but we generally think of it happening at high temperatures because the difference between the water content of the plants is much higher than the water vapor in the air. The difference in humidity, like the difference in temperatures, leads to transfer of water out of the plant and into the air blowing over it. It’s not all bad, though–wind blowing through the plant canopy can be a good thing if it keeps humidity levels near the plant low enough to prevent the development of fungal diseases that thrive on very moist conditions, so gardeners should consider wind conditions in their garden when they determine where to place their plants and what to grow.

Winds can also cause damage to trees and garden plants. I recently read an article that indicated nearly all tree trunks break at the same wind speed of around 90 mph (42 m/s), regardless of species or size. According to the authors, “In a strong wind, a tree may break through one of three mechanisms. Uprooting can occur in rain-moistened ground, or if the tree’s roots are rotten. Alternatively, if the roots can hold, then it becomes the tree trunk that is at risk from breakage – either through torsion or, more commonly, bending.” Of course, damage to tree limbs can cause breakage at weak spots as well at lower wind speeds.

Wind also has beneficial impacts on plants. The gusty nature of wind causes variable forces on tree trunks and plant stems that increase their strength by forcing them to resist the wind. That is one reason why you might not want to stake young trees too tightly, because they need to be able to move in the wind to form strong tapered trunks. Wind that blows all the time in the same direction causes trees and plants to grow in distorted (and sometimes beautiful) ways as their shapes are formed by those constant winds. Wind also provides a way to spread pollen from one tree to another, helping to spread genetic material through a wide area for reproduction.

If you are interested in reading more about how wind affects gardens, check out my earlier blog post at https://gardenprofessors.com/who-has-seen-the-wind/.

I’ve been a gardening mythbuster for almost a quarter century. You’d think the quality of information would slowly be improving, given the increased sophistication of many gardeners regarding their information source. But every day my news feed connects me with articles that I’m sure some AI entity thinks will be enlightening. One recent story getting lots of eyeballs is entitled “The Benefits of Girdling a Tree Vs. Cutting It Down.” It makes for a good application of the CRAP analysis. While the link to the publication I’ve provided will go into more detail on CRAP analysis, all you need to know for this post is we’re going to assess Credibility, Relevance, Accuracy, and Purpose of the information.

First, let’s consider the Credibility of the source. According to HouseDigest.com, the author is “a plant mom…intrigued by nature and plant life which she exhibits by caring for and doting on her succulents.” Her college degree was in applied biochemistry, which has no substantial connection to applied plant and soil sciences. Bottom line, the author is not an expert in the science of tree care.

Let’s look at Accuracy next (we’ll get to relevance later). The author’s premise that girdling is “another great option” for tree management is grossly inaccurate. The article contains no links to any published research supporting her opinions, and demonstrates a lack of understanding woody plant physiology. The author states that girdling “would prevent erosion from occurring” in contrast to cutting the tree down which apparently removes “the tree roots acting as a protective cover for the soil.” Cutting a tree down removes its crown, but leaves the root system undisturbed. The roots stop transporting water aboveground (there’s no demand for it any longer) thought they can continue to grow as long as they have stored resources. Eventually they will die and their woody structure slowly decomposes.

Girdling the tree, on the other hand, does not prevent root uptake and transport of water through the xylem to the crown of the tree. It does prevent phloem movement of sugars and other resources from the crown to the roots. In other words, roots remain active in transporting water and nutrients but slowly starve to death without phloem-transported sugars and other resources. A good article on the topic of tree girdling goes into more physiological detail on the process that causes trees to decline “before entering an irreversible state of desiccation caused by definitive root death.”

Another possibility is that the girdled tree might send up new shoots below the girdling, leading to the formation of a new crown. Without constant vigilance in removing these new branches, the tree will survive and presumably continue to cause whatever problems that led to the original mismanagement.

The author also suggests that girdling is useful in preventing disease spread: “A sick tree would need to be killed in order to prevent the disease from spreading out to other trees and vegetation.” Or, if your trees “are hoarders by nature,” girdling renders them “incapable of taking in nutrients and being a burden to the environment.” Neither of these statements is accurate.

While the information in this article is somewhat Relevant to homeowners, it does wander into agricultural advice. We’re advised if we have “a danger tree or one that is resting on arable farmland with crops on it, it’s strongly advised that you chop it down and not girdle it. You don’t want an unpredictable girdled tree falling on your harvest, house, or worse — on someone — out of nowhere.”

At this point it’s worth noting that deliberately killing a tree by girdling also opens the property owner up to legal action should the tree fall and damage property or injure someone. This alone should be enough to dissuade property owners from taking advice from this article. And given the number of years it can take for a girdled tree to die (and eventually fall), is this really a useful process if you need to have a tree removed for some reason?

Finally, what is the Purpose of this article? It’s hard to know exactly why the author promotes girdling, and the language she uses in describing tree care is odd. Statements such as “sometimes circumstances call for trees to be put down and killed” and “trees need to be put down for all kinds of reasons” seem to equate trees with stray animals or dangerous wildlife. It creates an antagonistic situation where none exists.

Tree management, especially when it come to discussions about removal, needs to involve a certified arborist who can assess potential risks associated with leaving, as well as removing, any tree.

Like it or not, the use of AI (Artificial Intelligence) has become a part of our daily lives. While you might not use AI directly (or you don’t know that you do) it is now a common part of society, especially in the online world. Many people, sites, can companies use it to create content. It is part of the “smart” gadgets that we use at home. Map software (like Google Maps), search engines, ride share apps, and even the spam filter on your email all use AI. You’re even more likely to encounter AI on social media and even standard media these days, with it being used to write articles and text, create ads, and images.

We know that there’s no getting around it these days. You’ve probably heard stories about how AI provides incorrect information, steals content, or might help the robots overthrow humanity today. While it seems to be the wild, wild west, there are a few (voluntary) safeguards in place now to prevent the overthrow of humanity (I hope).  Where the real damage is right now is the use of AI to mislead people outright. There’s also some danger from just lazy use of AI where whoever is using it to create content just copies and pastes it verbatim without checking what it actually says.

Gardening misinformation on the internet is nothing new. Gardening misinformation before the internet is nothing new either. But the risk that AI poses is the amplification and multiplication of that information. It is now easier than ever for someone to create online content at the click of a button. And the way that AI works is it scours the internet for existing information to learn how to respond. This new(ish) generation of AI is generative, meaning that it can actually put together information to form something new. Previously, if you did an internet search you would just get a list of websites to read for information. Now AI can use those sites as source material and write the information in a new way – however you prompt it to do. Search engines like Microsoft’s Bing (the much-loathed replacement for the archaic Internet “Exploder”) now have AI built in as a feature. AI is only as smart as what it can learn on the internet, and we all know that while there is good information on the web it is also full of a lot of manure.

Identifying Artificial Gardening Information

First, how can you tell if it is AI generated?  At first glance you might not be able to tell. It is just like any writing. There are tools you can use to try to figure it out, but you’ll often find that writing from AI doesn’t have personality. It is often very dry. You can tell it to add humor or any other style to its writing, but it often misses the mark. When I asked ChatGPT (one of the common AI platforms) to add humor to something it sounded like I was trying to be cutesy, like an employee at the famous park in the swamps of Orlando run by a mouse and not my usual wit, sarcasm, and cynicism. AI often fails to see context and answers are often very black and white. Which doesn’t work in dynamic systems like gardening and the environment. AI therefore sticks a lot with generalities and doesn’t often provide a lot of specific information unless you ask for it.

As an example, I prompted ChatGPT to write “three sentences about seed starting”. Here’s what it spit out:

Nothing alarming there, but it is pretty dry and isn’t very informative. It isn’t what I would write. Now, I prompted it to rewrite those sentences, but to add “humor and wit”. This is what it vomited out:

Vomit, indeed. Though still nothing too alarming. Definitely too cute and not enough snark.

Where AI generated text falls short is that it incorporates some of that incorrect manure from the web in answers. It doesn’t know that information on the web is incorrect. A few times it told me to put rocks in the bottom of pots for drainage (something we’ve fiercely disproven time and time again).

It told me to practice companion planting (another fallacy we’ve taken on) but it never gave me a lot of details about how to do it.

I did ask it directly about biodynamic gardening, which is the pinnacle of garden misinformation. It gave an amazingly nuanced and diplomatic response, which is much closer to what I’d actually say and much nicer than what GP founder Dr. Linda Chalker-Scott would say. (Don’t tell her I said that).

So, nothing too earth shattering in text, but where I think the real risk lies is in AI generated images and videos. It is easier than ever to create images of things that aren’t possible or incorrect and pass them off as real. People often do this to drive traffic – by making wild claims that people must check out or by “rage baiting” people who just have to respond to tell people how wrong something is (it still drives engagement and earns money). 

Fake images are nothing new in the gardening world. I can’t tell you how many ads I’ve seen for magical rainbow-colored rose seeds, trees that grow 10 kinds of fruits, and more all before the advent of AI. But now it is easier than ever to create those images at the click of a button.

For an example, I turned to DALL-E, which is a common AI Image generator. I tried to think of things that wouldn’t be possible. My first prompt was “monarch butterfly on a snow-covered flower”. Something that isn’t possible, but that someone might create to make a social media post about something amazing or miraculous that people have to see to believe.

The results look realistic(is) enough, though improbable. But you’d have to know that to not believe it.

The second test, not so much: “realistic looking tree that has 15 different types of fruits and veggies growing on it”. I had to add the “realistic” because the first results were cartoon-y. It didn’t help much. So, I guess my magical 15 fruit and veggie tree won’t be coming to an online scam shop any time soon.

So, I moved on and created “a grape vine covered with scary looking bugs”.

At first glance, the result can look terrifying. But if you inspect it closely, you’ll see that those bugs have all kinds of legs coming from all over their bodies. Scary, yes, but realistic – no. But could someone do something like this to scare people about an invading insect? Absolutely!

Cutting through the Artificial CRAP

GP Founder Dr. Linda C-S has written about using the CRAP test to identify if a source of information is trustworthy. She used it to talk about Jerry Baker, the self-appointed “America’s Master Gardener” who peddled misinformation and garden snake-oil for decades through books and tv shows to earn big bucks. The same principles can be applied now to digital content created by AI to help figure out if the information is reliable. Here are the steps:

C = credibility. What are the credentials of the person or organization presenting the information? Are they actual experts? Or is it a random account that doesn’t have ties to a credible source? Does the source have academic training, or even practical knowledge?

R = relevant. Is the information relevant for home gardeners? Or does it try to use information other than home gardening, like production agriculture, to answer the questions. For AI, especially images, I could also say that R= realistic. Is it something that could actually be true, or is it a monarch butterfly covered in snow?

A = accuracy. This could lend itself to the realistic assertion, but I see this as more in accuracy of the source of information. Does it site sources, like journal articles, extension publications, USDA reports, etc.? And does the information follow along with trusted information from other sources?

P = purpose. Why is someone presenting this information? In the Jerry Baker example, he was raking in money with books, TV shows, and product promotions. But what benefit does someone get from posting incorrect info on the web? Also, money. Whether you give them a dime, most social media sites and websites generate income by the number of clicks or viewers they have. How do you think people get rich and famous from TikTok? People aren’t paying them to watch them, but they generate income from engagement and interaction. So, creating content that is fanciful to get people to check it out, or even wrong for people to interact with it to rail against it, creates income.

Is all AI bad?

Not necessarily. I mean, the technology is applied in so many ways to solve so many problems. Sure, there is a risk and people do misuse it. But AI can be a powerful and useful tool when used appropriately, when information is checked, and when it isn’t copied and pasted directly. For example. Over most of 2023 I wrote a series of GP articles about plant diseases. No, I didn’t have AI write the article. That would have been wrong. But I did ask my friend ChatGPT to create lists of common diseases for each type of disease to write about. Instead of me having to dig through social media to see what people were asking about, the platform searched to see what the most common diseases that people talked or asked about were, or which ones were most likely to show up on websites. But I took that list, added to it, subtracted from it, and then wrote the article myself. But the more unethical (and lazy) users of AI just copy what it says verbatim without even reading or editing for accuracy. Or even have automated systems that just crank out AI-generated content with no oversight.

In the end, AI isn’t going away. So as savvy gardeners we just have to know what to look for to “spot the bot”.  And always be ready with a shovel to scoop away the CRAP.

January is here with its resolutions, cold long nights and not that warm days. Winter is a season of rest and survival. The cats and horses have long furry coats, the resident song birds eagerly clean out the feeder every day and the garden beckons. For me Winter is a special season when I can do a lot of fruit tree pruning, especially enjoyed with my daughter. Father-daughter pruning bonding is not to be missed if it’s an option for you. Gardens are tuned to winter as period of rest but the promise of longer days that will initiate the changes that happen in Spring will soon be upon us. In this post I’ll reflect on how plants survive winter and what we can do to help them.

Winter is actually a very dry time of the year in many places and the winter cold that freezes soil leads to dehydration. Plants installed just before winter will not emerge in spring alive w/o moisture in their systems. Mulch is an essential and natural part of winterization for many North American temperate plants. Protecting the root ball of a newly planted perennial is a must do for winter survival. In nature this is accommodated by the deciduous habit of many trees and shrubs, falling leaves are a big part of winterization. In our gardens we can do this with mulch.

Deciduosity

I know deciduosity is not often used but I like to use unusual words so here we go. The deciduous habits of many north American temperate trees enable them and other plants to survive cold, dry, freezing winters. Environmental cues (photoperiod and cooling temperatures) signal trees to drop their leaves (Fadon et al., 2020). Cold temperatures are also required by temperate perennials to invigorate buds and make starch into soluble sugars for strong spring growth. Deciduosity also leads to abundant mulch on the forest (or garden) floor. This protects soil and surface root systems, seeds, perennial herbaceous plants and bulbs and provides an insulating layer under snow, if snow is a thing where you are. When warm temps arrive in Spring the leaves quickly break down as growth under them emerges.

Solutes

Deciduosity brings certain challenges to woody perennials that donate their canopy to the soil each year. Trees in spring have no photosynthetic organs to supply the energy of growth. That energy has to be stored in the wood and roots as carbohydrates, mostly as starch, at the end of the growing season and before leaf fall. In spring at the end of dormancy when buds grow, these stored carbohydrates convert to soluble sugars and fuel the rebirth of a a new canopy. Having all that stored sugar in cells throughout the plant also reduces the freezing point of water in the cells so that subzero temperatures do not lead to ice crystal formation (and cell death) of the dormant plant.

Seeds

Another way plants survive Winter is by forming seeds. The strategy of annual plants is to “go to sleep” as seeds and “wake up” by germinating. To ensure that seeds don’t germinate too early, they often have inhibitors that need to be washed away by water (Spring thaw), burned by fire (usually summer time), or by scarification (tumbling in the creek etc). Many seeds germinate better after a cold winter than if they were sown without cold chilling. Not all seeds will germinate at the same time as inhibitors delay germination. This ensures that conditions will be right for some of the seeds and thus the species will survive, even thrive in the right place.

Roots

While the above ground part of gardens can be in a dormant state in January, the situation underground is different. Roots respire (break down sugars to get energy for growth) during winter and may grow continuously depending on climate, depth and soil coverage conditions. Roots, just like buds, utilize stored carbohydrates to fuel their growth. If temperatures remain more moderate under the soil they can continue to respire well into winter months. Soils freeze when they lack snow cover or mulch, Reinmann and Templer (2016) propose that roots in frozen soils are less active. Leaf mulches help protect soils from hard freezes.

Am I crazy or What?

I know that a leaf dump on the garden every year is not what many gardeners want to deal with. That is what leaf blowers are for right? Some municipalities even have line items in their budget for disposing of fallen leaves which are some of the most disposed of green waste. Leaves that accumulate on hardscape can be a pollution source accounting for up to 80% of phosphorus pollution in one study (Bratt et al., 2017). It’s best to utilize leaves around perennials and keep them away from streets, gutters and sidewalks.
Trees evolved to drop their leaves on the ground and for them to stay there. Finding ways to accommodate this in gardens will lead to a healthier garden and less waste in landfills. Leaves can be mown on turf areas and the biomass will be incorporated into the turf sward (Nektarios et al., 1999) without loss of turfgrass quality. In gardens they can become part of the surface mulch. If you are really crazy, you can grind them in a shredder to make really high quality micro mulch to be used around certain plants or vegetables (we do this with coast live oak leaves of which we have an abundance in California). Stavi, (2020) encourages us to think of fallen leaves as a resource not a waste product. Your garden will benefit.

For more information on leaves please see the other blogs at this site:

• https://gardenprofessors.com/noraking/
  
• https://gardenprofessors.com/leaves-for-lawn-fertilizer/
  
• https://gardenprofessors.com/one-trees-leaves-over-400-kinds-of-bacteria/

References

A. R. Bratt, J.C. Finlay, S. E. Hobbie, B. D. Janke, A. C. Worm, and K.L. Kemmitt 2017. Contribution of Leaf Litter to Nutrient Export during Winter Months in an Urban Residential Watershed. Environ. Sci. & Technol. 6: 3138-3147
https://pubmed.ncbi.nlm.nih.gov/28215078/

Fadon, E. E. Fernandez, H. Behn, and E. Luedeling. 2020. A conceptual Framework for Winter Dormancy in Deciduous trees. Agronomy 10(2), 241; https://doi.org/10.3390/agronomy10020241

P. Nektarios, A.M. Petrovic and D. Sender 1999. Tree Leaf Deposition Effects on Kentucky Bluegrass (Poa pratenses L.), J. of Turfgrass Man., 3:(1) 69-74. DOI: 10.1300/J099v03n01_06

Reinmann AB, Templer PH. 2016. Reduced winter snowpack and greater soil frost reduce live root biomass and stimulate radial growth and stem respiration of red maple (Acer rubrum) trees in a mixed-hardwood forest. Ecosystems. 19:129- 141.
https://www.jstor.org/stable/48719251

Stavi, I. 2020. On-Site Use of Plant Litter and Yard Waste as Mulch in Gardening and Landscaping Systems. Sustainability 12(18), 7521; https://doi.org/10.3390/su12187521

This year is almost certain to be the warmest on record for the earth as a whole, although there are still a few days in December that could slightly affect the final numbers. As we close out 2023 I want to spend a few minutes reviewing the weather and climate of the past year, both the average conditions and some of the extremes we saw. While this is skewed towards the United States, I did include some events happening in other parts of the world for our non-US readers. I will also take a peek at what is likely to happen in 2024.

What were the average climate conditions in 2023?

Since the year is not quite over I can’t provide a final average for temperature or precipitation for the complete 365 days, but there are some websites that allow me to look at all but the last few days. The images below are from the High Plains Regional Climate Center for January 1 through December 27. They show the temperature departure from normal and the percent of normal precipitation for the continental United States. (You can see the global temperature statistics for January through November 2023 at the National Centers for Environmental Information.) In most parts of the U.S. the temperature was warmer than the 1991-2020 normal; the exception was the western mountains, where temperatures were colder than normal. Keep in mind that the normal period being used for comparison (1991-2020) was a period that was quite a bit warmer than the long-term temperature average in the United States, so this map underestimates how warm this year was compared to most of the 20^th century.

Precipitation was more variable than temperature, as it usually is. The driest areas this year were in the southwestern Gulf of Mexico states, particularly Louisiana and Texas, and in the Pacific Northwest.  It’s not surprising that these were also areas with significant droughts, including a lot of the Corn Belt which also saw very dry conditions during the growing season. By comparison, California and New England experienced multiple storms bringing significant rain to those areas, including Tropical Storm Hilary (the first tropical storm to hit California in 84 years) in mid-August. In the Southeast, Hurricane Idalia did almost $5 billion in damage in late August from heavy wind and rain, half of that in Georgia alone. But that did not stop a flash drought from developing there in fall with the almost complete cessation of rainfall for up to 60 days.

What extremes did we see in 2023?

The averages show the overall conditions that occurred this year but don’t begin to capture the extremes in temperature and precipitation that occurred. In the United States alone there have been 25 billion-dollar weather disasters so far this year, including the tropical systems mentioned before along with numerous rounds of severe weather across the country and the devastating firestorm in Maui in August. In other parts of the world, many regions experienced their warmest September-November period since records began in 1880. Significant heat waves occurred in Texas and Mexico as well as Europe, Chile, and Canada, where widespread forest fires that flared up blanketed Canada and many parts of the eastern United States with poor air quality and low visibility in the summer.

Floods and droughts occurred around the world this year. The Mississippi River dropped to record-low water levels for the second year in a row due to the drought in the Midwest. On the other extreme, notable flood events occurred around the world, include floods in Ghana, the Horn of Africa, Pakistan, Bangladesh, and Chile. Storm Daniel brought unprecedented rain to Libya, breaking dams and causing tremendous damage in September. Nine separate atmospheric river events caused tremendous flooding in California early in the year, significantly reducing drought conditions there and contributing to the reduction in drought area in the United States in the first half of 2023.

While El Niño usually means that the Atlantic tropical season is quiet, this year was unusually active with 20 tropical storms and hurricanes. This is in spite of the presence of a jet stream aloft due to El Niño that usually keeps storms from developing. Most of those storms stayed out to sea, so impacts on the United States were limited (except for Idalia and Hilary out west). In other countries, Hurricane Otis hit western Mexico near Acapulco in October, bringing catastrophic damage to an area that almost never gets hits by tropical storms. Cyclone Lola devastated the northern part of Vanuatu in late October as well.

The global temperature will set a new record for warmth in 2023

The New York Times provided a sobering look at monthly temperatures for each month going back to 1850 (above). It shows that 2023 had several months that were the warmest on record for those months, due to the expansive area of warm ocean water associated with El Niño in the Eastern Pacific Ocean along with record-setting sea surface temperatures in the Atlantic that contributed to a very active tropical season in spite of being an El Niño year. The impacts of this warmth are being seen in dropping sea ice coverage, more and stronger heat waves, and increases in wildfires in forested areas. Some people argue that the warming trend appears to be accelerating in recent years, a concern that urges us to consider how we can reduce greenhouse gas emissions and slow down the increasing temperature trend.

What do we know about 2024 so far?

The current El Niño is expected to continue through the next few months before it weakens and turns back to neutral conditions around the April-June period. A La Niña could occur later in 2024, which means that next year’s winter could be warm and dry in southern parts of the Northern Hemisphere and colder and wetter along the northern border of the US and up into Canada. Pending on how long the El Niño lasts, the warm ocean temperatures could contribute to another record-setting warm year in 2024 although it’s too early to be sure. It also depends on shorter-term weather events like more frequent occurrence of cold weather due to a shift in the weather pattern in January to more variable conditions later this winter, as many forecasting models think is likely. Meanwhile, neutral conditions or La Niña conditions later in the year could mean that Atlantic tropical activity increases to an even more active level than last year.

Thank you, gardeners, for another great year!

Most of the United States as well as the rest of the world experienced a warmer climate again in 2023, so gardeners will continue to need to choose plants that are appropriate for their warming climate zones. Extreme conditions, including devastation by individual storms as well as natural climate variability, will continue to affect home gardens through water stress caused by drought and extreme heat as well as damage caused by floods, high winds, and freezing temperatures. Building a resilient garden that can withstand these extremes will allow your garden to thrive through whatever conditions the atmosphere throws at it.

I want to end this year by thanking you all again for your loyal readership and your thoughtful questions and comments on many topics. I encourage you to share your 2023 garden challenges (weather or otherwise) in the comments along with your plans for how you plan to address them in 2024 and beyond. I look forward to reading them! We will see you again in the New Year.

Some lists of top weather and climate events for 2023 (mostly videos):

Weather Nation: Looking Back at the Top 10 Weather Events of 2023

Pattrn: 2023: Year of Extremes

NBC News: The biggest climate stories of 2023

Climate.gov: Climate Highlights of 2023

Atmos Earth: Your 2023 Climate Wins, Wrapped

In this late fall edition of People and Plants we’ll take a look at an early American female botanist, Martha Daniell Logan.

She was born in 1704 in St. Thomas Parish, South Carolina, the second child of Robert Daniell and his second wife Martha Wainwright. After her father died in 1718 she inherited his land along the Wando River. In 1719, Martha married George Logan, Jr. and they lived on the Wando River, ten miles from Charleston, where both the Daniell and the Logan families owned extensive property.  Over the next sixteen years, she gave birth to eight children, six surviving to adulthood.  In 1750 the family moved to a plantation near Charleston. Needing to enhance the family income she advertised her services as a teacher but her attention gradually shifted to horticulture. She began her botanical career collecting in the woods near her home.

Martha soon gained the reputation of a skilled gardener and maintained a well-known garden “on the Green, near Trott’s Point in Charles Town.” Gardening became her focus and occupation and she embarked on a career as a “purveyor of botanical goods,” selling seeds and plants from her home. 
In addition to native plants, she dealt in imported specimens. Gardening, especially landscaping with rare plants, had become a favored pastime among wealthy locals and Martha was quick to capitalize on this. An advertisement published in the Gazette on November 12, 1753, announced the availability of “a parcel of very good seeds, flower roots, and fruit stones of several kinds” that were “just imported from London.”

She exchanged seeds, roots, and plants, like gardeners do, with other botanical enthusiasts including the naturalist John Bartram. His visit in 1760 initiated a three year correspondence and trade of specimens. They swapped lists of available plants and used silk bags to send seeds to each other. They also exchanged lists of plants that each desired from the other’s geographical area. Logan enthusiastically sent Bartram plants from Carolina which “may be New to you” and “be an adision [addition] to yr Collection.” In return, she asked him to send bulbs and double-flowering plants that her London contacts had failed to procure or took too long to send. She shipped and received tubs of cuttings and roots on ships traveling between Charleston and Philadelphia, where Bartram lived. Bartram praised her in a letter to a London friend and wrote, “Mrs. Logan’s garden is her delight and she has a fine one.”

With the popularity of urban gardening on the rise Martha realized that many people needed help and guidance with their horticultural endeavors. In 1752 her first advice column titled “Gardners Kalander [sic], done by a Lady of this Province, and esteemed a very good one.” appeared in the South Carolina Almanack. Her first publication was so successful she continued to publish her calendar, updating and enlarging it each year.

Martha continued her business, what we nowadays would call a garden center, for the rest of her life. She even wrote a treatise on gardening at the age of seventy. In 1809 the early Charleston historian David Ramsay described her as “a great florist, and uncommonly fond of a garden,” and claimed she “reduced the knowledge she had acquired by long experience, and observation, to a regular system which . . . to this day regulates the practice of gardens in and around Charleston.”

Martha died in Charleston on June 28, 1779, and was buried in St. Philip’s Churchyard. She is considered one of the founding gardeners of South Carolina.

Read back through the pages of her 1757 Gardener’s Kalendar shared above. In your opinion, how much of it is still applicable? One bit of her advice that is always appreciated: “What was neglected last month may be successfully done in this.”

This month has been an exciting one for climatologists around the United States with the November 14 release of the Fifth National Climate Assessment (NCA5), a massive project that is undertaken every four years to capture our current understanding of climate change based on recent research. I was a chapter author for the Southeast and spent the last two years working with over 700 authors around the United States to gather and document how the climate is changing and how it is affecting all of us. This week I will explain how NCA5 was put together, what it says about climate, and what gardeners can do to help reduce the future impacts of global warming and other climate changes. But this month was also exciting because USDA just released an updated Plant Hardiness Zone map, just a few weeks after my post in October about how the 2012 map was outdated. I guess they were listening (just kidding!). I will discuss that briefly at the end of this post, too.

What is the National Climate Assessment?

The National Climate Assessment (NCA) is a report mandated by Congress to compile the latest scientific findings on how climate is changing so that we can respond to reduce its future impacts. It is published every four years, and the last one (the 4^th NCA) was released on the day after Thanksgiving in 2017. While the underlying message has not changed, each assessment focuses on the newest scientific research that has been published since the last assessment was done. The document is divided into chapters so that the authors of each chapter could concentrate on that topic.

NCA5 starts with a review of the general scientific principles of how the climate is changing. That is followed by seventeen chapters focused on national topics such as agriculture, water, energy, and transportation as well as specific groups that are being especially affected by climate change such as indigenous peoples. Following the national topics, chapters address changes that are happening in ten different regions of the country . These address how we need to reduce future greenhouse gas emissions that are driving the warming of the earth as well as how we can adapt to the changes that are already happening now and may get worse in the future.

How was NCA5 produced?

There is a long process involved in producing a national climate assessment. Teams of scientists from an array of disciplines were chosen as authors for each chapter to write the initial text of the document. To keep the authors on task and within tight word limits, there were lead chapter authors and technical advisors who moderated group meetings where the key messages for our chapter were identified. Initial figures to include in each chapter were drafted by a graphics team or requested from scientific journals. After the first draft was complete it was first reviewed by federal agencies to make sure that their concerns were addressed and then by the public, who provided many additional comments. All of these comments were provided to the chapter authors so they could refine their text and figures for the next draft. In all, the document when through six different reviews and all comments were addressed.

Where can I read NCA5 and learn more about what it says?

The NCA contains a vast amount of information in its 32 chapters, five appendices, and special topics, so it is hard to summarize. I encourage you to explore the document online to see what it says about your region and special topics of interest like agriculture, land, and ecosystems. A good starting place is the introductory website https://www.globalchange.gov/our-work/fifth-national-climate-assessment, which explains how the report was written and provides links to read the report, attend a webinar on an individual chapter, and see where the figures came from. I also encourage you to explore the excellent interactive atlas developed in conjunction with the report. Many other resources such as podcasts are available, too.

What are some steps that gardeners can take to respond to climate change?

There are two approaches that gardeners (and all of us) need to take to respond to the challenges of a changing climate. We are already dealing with the consequences of trends towards warmer temperatures and more extreme swings in the water cycle such as increases in floods and droughts. Gardeners are adapting to these changes in climate by planting different plants that are better suited to the warmer climate and changing how they manage their gardens using rain gardens, drip irrigation, and other techniques. Adaptation is a key approach that gardeners will continue to need to follow as the climate continues to get warmer and more variable.

In addition all of us, including gardeners, have a responsibility to cut the emissions of additional fossil fuels which are driving most of the warming (mitigation is reducing the inputs to prevent future harm). This will reduce the impacts that our world will have to navigate in the future. Even a small decrease in the emission of greenhouse gases now can prevent the worst outcomes. A prime target for gardeners is the elimination of gasoline-powered equipment like blowers, mowers, and trimmers. These small tools have highly inefficient engines that emit a lot of greenhouse gases as well as air pollutants (and a lot of noise, too). Switching to electric tools and vehicles, composting, adding solar power to houses and businesses, and conserving energy and water (which often uses energy to purify it) through carefully chosen plantings as well as through other methods can also help reduce future warming.

What about the new USDA Plant Hardiness Zone map?

I was surprised last week that the USDA had just produced an update to the 2012 map that I discussed last month. In that post, I noted that the 2012 map was already outdated due to the increasing temperatures we have seen in the 21^st century. The new 2023 map uses data from 1991-2020, the current 30-year normal period, to identify the current plant hardiness zones for the United States. You can see the new map and zoom to your city at https://planthardiness.ars.usda.gov/. By comparing it to the 2012 map, you will see that more than half the country has increased by half a zone, which correlates to about a 5-degree F increase in the average lowest minimum temperature a location experiences each year. I asked USDA for a map that showed the changes of zone and was provided one by Chris Daly of the PRISM group that put together the 2023 map (below). Areas in tan experienced a half-zone change since the 2012 map. (There are a few areas in the Mountain West where the zones got colder, as shown in green, but these are mostly linked to new datasets that were available for the analysis rather than any changes to the local climate there.)

Science has made it clear that the earth’s climate is changing and that most of the warming we are experiencing is due to burning of fossil fuels. We must learn to adapt to these changes and make sure that all groups can be protected from the worst impacts of the more extreme weather we are likely to experience. But we can also make changes now to reduce those future impacts, and I know gardeners will be part of that solution.