Advancing the science of gardening and other stuff since 2009
Author: Linda Chalker-Scott
Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and an Associate Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets.
Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019).
In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award.
"The Garden Professors" Facebook page - www.facebook.com/TheGardenProfessors
"The Garden Professors" Facebook group - www.facebook.com/groups/GardenProfessors
Way back in 2010 (and then again in 2012) I wrote about a bizarre belief that cornmeal could be used to treat fungal diseases, from lawn spot to athlete’s foot. Rather than rehash what’s already been written, I’ll invite readers to read those posts for background. And of course look at the comments, which are…interesting.The weird thing is that this post from 2010 is the single most popular post on the blog. (Our stats are only for the last two years since we migrated the web site – who knows how many there were before May 2017?)
The consistent popularity for the topic spurred me to publish a university fact sheet on the use of cornmeal and corn gluten meal in home landscapes and gardens. This fact sheet reviews the pertinent literature, and makes recommendations that are pretty much the same as those I made almost 10 years ago. Nothing has changed in the research world to support cornmeal as a fungicide.
But wait, there IS something that’s happened since 2010! Now cornmeal is being touted as an insecticide! In fact, if you go to Google and search for “cornmeal” and “insecticide” you’ll find thousands of hits. As you might expect, there’s no research to support this notion: researchers in Maine, for instance, found no effect of cornmeal on fire ants. However, it is used as a bait to deliver actual insecticidal chemicals.
But facts don’t get in the way of home remedies, such as Lifehacker’s eyebrow-raising advice.
By refining the search to only include university websites (use “site:.edu” to do this), and swapping out “ants” for “insecticide,” you’ll find at least one Master Gardener group happily (and illegally) recommending cornmeal as an ant killer. The popular mode of action is either (1) they can’t digest cornmeal and starve or (2) the cornmeal absorbs water in their gut and they explode.
This reminds me of yet another food product – molasses – recommended for killing ants. Since you’re already here, you might as well check out Molasses Malarkey parts 1, 2, and 3 too.
Might I recommend everyone use their cornmeal and molasses to make bread or cookies or pancakes? There are some delicious recipes on the internet.
Today’s blog post is courtesy of Mary Blockberger of Sechelt, BC. As you’ll see, Mary and I go way back. I thought it was important to our ongoing discussion to see how the industry can use the root-washing technique effectively and economically. Here’s Mary:
“Before I began managing the Sunshine Coast Botanical Garden in Sechelt, BC I had a small residential landscaping company. By small, I mean that I was the employee of the month every month of the year! One of our Garden’s mandates is to provide relevant and educational programs for our community. Dr. Linda Chalker-Scott has been one of our most popular speakers several times. One of her presentations dealt with the practise of bare-rooting perennials, shrubs, and trees prior to planting, and the tremendous advantages of following this method.
“In November, 2007 I had a chance to try this technique out. My client wanted a ribbon of Carpinus betulus ‘Fastigiata’ planted that would eventually be pleached into an interesting pattern. [Pleaching is a formal tree training technique.] There was a total of 36 trees to be planted; most were container stock as I recall but there may have included a couple of B&Bs as well. Working with another local landscaper, into a wheelbarrow of water went every single tree one at a time. The dirt was clawed away from the root balls by hand with a final spray from the hose. Honestly, it was a cold and miserable job, and I believe a few curses directed at Linda ensued. However, once the roots were cleaned of all soil planting was a breeze. It’s a lot easier moving trees without moving the soil too.
“Flash forward 12 years, and every single tree has flourished. Bare rooting allowed us to identify and correct any problems before planting, and I’m sure this has a lot to do with the trees’ success. It’s a time consuming and at times messy method, but the reward of a healthy row of trees is well worth the effort, IMHO.”
And let me add to Mary’s account that a ZERO replacement rate is going to pencil out to long term economic success. I was able to see these trees earlier this year – that’s my photo at the top of this post.
Anyone who plants or cares for woody plants eventually hears the term “root flare” (or root crown). It’s easy to describe a root flare (it’s the region where stem or trunk morphs into roots). What’s sometimes difficult or even impossible is finding it in improperly planted trees and shrubs.
One of the primary causes of tree and shrub failure is improper planting depth. This is not a problem with bare-root plants, as you can easily see the region of transition. During planting you should make sure that the root flare is at grade, so that the roots are underground and the stem/trunk is above ground. The only mistake you can make with bare-root plants is to plant them upside down.
The problem really started with the advent of containerized and balled-in-burlap (B&B) plants. This technology is less than 100 years old, and before it existed everything was either planted from seed or from bare-root stock. It’s possible to use containers and B&B properly for temporarily housing trees and shrubs, but increasingly automated production methods with unskilled workers and undereducated supervisors means increasing numbers of poorly planted woody plants entering the retail market.
I’ve written earlier posts about how to select plants at the nursery. As you’ll note, finding the root flare can often be impossible without removing container media or B&B burlap. Because so many people are unaware of the problem or unwilling to disturb the root ball, these plants are then installed with the root flare still buried.
Why does it matter if part of the trunk is underground? For some species, it really doesn’t matter. Wetland species, for instance, can tolerate low soil oxygen levels and submerged trunks. But most of us are not planting wetland species, and many ornamentals are not tolerant of this treatment. Roots that are buried too deeply don’t receive enough oxygen to survive, and the plants respond by trying to create a new root system. These adventitious roots are unable to supply enough water to the growing crown, however, meaning shrubs and trees suffer chronic drought stress when the rate of evaporation exceeds the ability of these substandard root systems to supply water.
There are other problems, too. Stem and trunk tissues of non-wetland species are not adapted to being buried. The excessive moisture and lack of oxygen contribute to the attack of opportunistic pests and diseases, both of which can cause irreversible damage and eventual death. You can even see this happening to plants in the nursery.
Finally, consider this landscape evidence of the impact of buried root flares. These magnolias are all planted on the campus at Princeton University. The one of the left is significantly smaller than the other three. A close up of the trunks explains why.
If you have newly planted trees that look more like telephone poles than trees, the best thing you can do is dig them up and plant them correctly.
One of the “advantages” of being in the same office suite as me is hearing (a) that arborist wood chips are about as close to a miracle product as you can get and (b) that landscape fabric is hell on (the) earth. So my office mate Liz, either because she was convinced of the above or just wanted to shut me up, decided to rip out the landscape fabric in her ornamental bed and replace it with wood chips. She even made it a family project, somehow convincing her two young daughters that this was “fun.” Here’s her pictorial essay of the process.
Before the switch
Why on earth does anyone still believe that “weed block” fabric actually does anything remotely related to controlling weeds? It provides a great substrate for all those weed seeds blowing around, which find themselves the recipients of any rainfall or irrigation. They germinate and grow like crazy – because they are WEEDS. It’s what they do.
Worse yet for the soil – all of those pores in the fabric that supposedly allow water and oxygen to move through are soon filled with bits of soil. The resulting mat is anything but permeable. But weeds love it!
First Liz had to score some woodchips, which as you can see pretty much filled her driveway. The girls, however, thought they were a great addition.
Next, all of that fabric had to come out. This is not an easy process, because the surface of the fabric was completely colonized by weeds. A mattock is a great tool for getting this done.
Now, let’s look at the soil underneath the fabric. You can see how dry it is. That’s because even during our rainy springs in the Pacific Northwest all of the rainfall stays on top of the fabric, allowing lush weed growth. The roots of all of the landscape plants get virtually none of this, and in the summer that’s a source of chronic drought stress.
Fortunately, the soil underneath, while dry, looks pretty good. Once the shrubs and perennials are able to take advantage of the increased water and oxygen they will thrive.
Maybe you don’t like the look of arborist wood chips, but it’s certainly better than the weedy mess that used to be there. Plus, the soil benefits from the increased water and oxygen, the beneficial microbes in the wood chips, and the slow feed of nutrients as those chips slowly decompose.
If you are ready to switch from “fabric fail” to “wood-chip win,” you can start with this fact sheet which will guide you through the process.
Over the last couple of weeks I’ve been in London having some unforgettable garden experiences. Thanks to the generosity of my UK colleagues Glynn Percival and Jon Banks I was treated to Kew Gardens, RHS Wisley Gardens, and Windsor Castle. I hope to construct several blog posts from these visits, but today’s post is an homage to the English garden meadow. Instead of monocultural turf lawns, mowed and sprayed into submission, why not consider a more biodiverse and visually pleasing approach to groundcover?
As the title of this post suggests, this is not a new topic in our blog. (You can read other related posts here, here, and here.) What was so stunning about these garden meadows (meadow gardens?) was the scale and effortless beauty. For instance, consider this tree-lined parkway at Kew, covered with English daisies.
I saw my first honest-to God cowslip in a meadow garden at the British Museum of Natural History.
How about these adorable tiny daffodils and checker lilies?
And here they are en masse.
This isn’t to say that the formal lawn isn’t a thing in England, It is.
But unless you have a castle, a baseball diamond, or a putting green to manage, why not consider something more appealing, not only to the eye but to your beneficial wildlife?
In my educational seminars I’ve long shared a version of the CRAAP test (currency, relevance, authority, accuracy, and purpose) for analyzing information related to gardens and landscapes. My version is CRAP (credibility, relevance, accuracy, purpose), and we’ve published an Extension Manual that explains in detail how to apply it. This past week I was at the Philadelphia Flower Show participating in Bartlett’s Tree Care Update panel. Given that the theme of the show was “Flower Power,” I figured that a talk on Magical Mystery Cures was in order. And the 1960’s was the decade where the late Jerry Baker gained prominence as a garden authority – and whose presence is still widely felt nearly 60 years later.
Now, I could spend the rest of the year discussing all of Jerry’s advice, tips, and tonics for gardens – but it’s more useful to determine whether he is a credible source of reliable information. So let’s apply the CRAP test.
C = credibility. What are Jerry’s credentials as a garden expert? It’s easy to find this information from the internet, including the Jerry Baker website. He had no academic training in plant or soil sciences but started his career as an undercover cop who often posed as a landscaper. His books are all popular publications, meaning they have not gone through critical review by experts before publication.
R = relevance. For our purposes, his information is relevant to our focus of managing gardens and landscapes (as opposed to production agriculture, for instance).
A = accuracy. Jerry’s advice is not based on any scientific source. He relies on common-sense approaches, folklore, and his grandmother’s advice. In fact, many of his assertions are at odds with published scientific evidence. Now, science evolves, and older scientific publications are sometimes found to be inaccurate after new information comes to light. If Jerry’s books were meant to be accurate sources of information, they would be updated with new findings as subsequent editions were published. This is what happens with textbooks, for example.
P = purpose. What is Jerry’s ultimate purpose? It’s sales. There’s no way around this conclusion. Over twenty million copies of his books have been sold, and during his career he became the spokesperson for several gardening products. Probably the most well-known of these was the Garden Weasel (which parenthetically is a great way to destroy fine roots and soil structure). There’s no doubt he was a brilliant self-promoter and marketer. But he was not a reliable resource, and many of his “tips and tonics” are extraordinarily harmful to plants, pets, and the environment.
While I was wrapping up my research on Jerry Baker I was particularly taken by a chapter in one of his books (one of his Back to Nature Almanacs) called “The Tree Quacks.” I thought some of these quotes were particularly ironic:
Imagine my surprise when I discovered that these quotes were actually not his own. In fact, the entire chapter was plagiarized from a 1964 article by John Haller in Popular Science, which is online. This action is uncomfortably similar to his 1985 trademarking of the phrase “America’s Master Gardener,” 12 years after the Master Gardener program was formed (but not trademarked) at Washington State University.
I hope this post has helped you learn to analyze the credibility of information and information sources. If so, you can claim the of America’s Master CRAPper ™!
Last week I discussed the mechanics of how cold hardy plants can survive temperatures far below freezing. Today we’ll consider the practical implications of this phenomenon and what, if anything, you can do to help your plants through cold snaps.
What happens when temperatures change at unusually high rates?
Remember, supercooling occurs when temperatures drop slowly, allowing water to leave living cells and freeze in the dead spaces between cells. When rates drop quickly, which can happen on sunny winter days once the sun goes down, water can freeze inside the cells before it has time to migrate into the extracellular space. When that happens, those cells die when ice crystals pierce the cell membrane. Sometimes this damage will be visible right away – you’ll see water-soaked areas in leaves, for instance, where the contents of the cells have leaked into the extracellular spaces.
In other cases you may not see damage until spring, especially in buds that have frozen. The scales prevent you from seeing what’s happened to the tissues in the bud, but once warmer temperatures arrive you will see brown or black leaf and flower buds. These are NOT diseased buds, though they are often colonized by opportunistic pathogens.
What about wind chill?
The wind chill question is an interesting one. Despite the way it feels to you, wind chill does NOT lower the temperature below the ambient air temperature. It just cools things off faster than they would without the wind. For cold hardy plants, this has two important effects:
The rate of temperature decrease around the plant speeds up – so ice can form faster than normal. This can result in freeze damage to the plant as described above.
The wind itself is dehydrating, pulling away water from plant tissues and causing freeze-induced dehydration (as discussed last week). This also causes damage to susceptible tissues and is often called winter burn.
So even though the temperature itself is not lowered by wind, the rate at which it decreases and the additional dehydration stress means that plants can be damaged at temperatures they would normally survive in the absence of wind.
What can we do to help plants survive?
Before cold temperatures are expected, it is critical to mulch the soil well with a thick layer of coarse, woody mulch. This insulates the soil and roots, which are the least cold tolerant of all plant tissues. Roots never go dormant, so they are generally unable to supercool much more than a few degrees below freezing. Oh, and be sure your soil is moist (but not waterlogged). Moist soil is a better heat sink than dry soil.
Next, be sure insulate freeze-susceptible plants. This can be done by constructing a cage of chicken wire around small trees and shrubs, filling it with leaves, and then wrapping it in burlap. Containers should be moved to the leeward side of the house or other building and grouped together. The containers need to be protected from freezing at all costs.
Speaking of insulation, snow is a great insulator. But it’s not always best to leave it in place. If temperatures are cold and snow is dry and light, leave it in place to insulated tissues. But if temperatures are near freezing and the snow is wet and heavy, remove it as much as possible. Its insulative value is marginal and the damage that heavy snow can do to trees and shrubs is permanent.
With record low temperatures in some parts of the country, gardeners are understandably worried about the ability of their perennial and woody plants to survive the cold. What today’s post will do is give you some context for understanding how plants can survive temperatures far below freezing.
Why ice floats and how this damages cells
Everyone knows that ice floats, whether it’s an iceberg in the ocean or cubes in your favorite chilled beverage. Ice is lighter than water because its molecular structure is different: there is more space between water molecules in ice. When water freezes naturally, the molecules organize into hexagons, forming a crystalline lattice (which helps explain why snowflakes look the way they do). This hexagonal shape forces water molecules farther away from each other, resulting in a porous material that’s lighter than liquid water.
As ice crystals grow, they take up more space than the water did in liquid form. You know this if you have ever left a filled can or bottle in a freezer. The pressure can blow off the lid or split the container – and the same thing happens to animal cells: the membranes are distended until they burst. But plant cells are different: there are cell walls outside the membrane which are rigid and prevent membrane rupture. However, ice crystals are sharp and can lacerate membranes, including those in plant cells.
How cold hardy plants avoid freeze damage
Woody plants have evolved a mechanism to survive winters that allows ice formation in certain areas and prevents it in others. This process takes advantage of the fact that plant cells have walls, and that the area between the cells – called the extracellular space – is not alive. Extracellular space is filled with gases and liquids – including water. Water can freeze in these spaces without causing damage because there are no membranes in extracellular spaces, only cell walls. As ice freezes in these “dead” spaces, more liquid water is drawn into them by diffusion from the adjoining cells. There are two outcomes of this: one is that ice only forms in the dead space, not the cells themselves, and two is that the liquid inside the cells becomes more concentrated.
Water that is full of dissolved substances (like sugars and salts) is less able to form ice crystals because there are relatively fewer water molecules in concentrated solutions. We can see this when we add deicers to frozen walkways and roads. The ability of water to stay in liquid form at temperatures below freezing is called supercooling. Plants that are cold hardy are able to tolerate ice formation in dead tissues and avoid ice formation in living tissues by supercooling.
Supercooling is different than flash freezing
We need to discard any comparison of supercooling to flash freezing, a process used for cryopreservation. Flash freezing rapidly lowers the temperature of the tissue or organism being preserved at rates far faster than what happens in nature. The water molecules don’t arrange themselves in a crystalline lattice as they freeze. Instead they form small crystals in an unstructured form, which don’t take up more space than liquid water. This means that ice doesn’t damage the cells, which are still viable once thawed.
Supercooling is a process that occurs under natural conditions, which usually mean slow decreases in temperature. This allows water to continue to move out of the cells into the extracellular space where it freezes. (There are exceptions to this naturally slow rate, and I’ll discuss those in a follow up post.)
There is a limit to supercooling
Unfortunately for plants (and gardeners) there are limits to supercooling. These limits vary with species but even the most cold hardy plants will eventually experience injury and death. The reason this happens, however, isn’t from the freezing itself, but from drought stress. Let’s look at what’s happening inside the cells during supercooling.
As water continues to diffuse into the extracellular spaces, the cell becomes less turgid; this is called freeze-induced dehydration. Without water forcing the cell membranes against the walls, the membranes start to pull away as water is lost. Eventually the membranes and plasmodesmata (which connect living cells to one another) are stretched and break. These cells are now dead – they are isolated from the rest of the plant and the torn membranes allow liquid to seep out. So cells, tissues, and entire plants that die from low temperature stress are usually killed by drought stress!
In my follow up post, I’ll discuss the practical significance of this phenomenon, including rapid temperature changes in natural and the influence of wind. And, of course, some suggestions on how to help plants survive these stressful conditions.
Welcome to 2019! In keeping with the tradition of a new year, I’m hoping you will join me in resolving to promote good gardening science among your friends, relatives, colleagues, and customers. One of the most important tools you’ll need is a collection of resources that are not only science-based, but are relevant to gardens and landscapes (not agricultural production). With that in mind, here’s my list of authors and institutions who are credible resources.
First off, of course, I’ll have to start with the Garden Professor faculty. While this blog is a great archive of information from all of us, some of us have also published books and articles, recorded podcasts, webinars, and DVDs.
These are popular publications rather than peer-reviewed journal articles. But the authors have solid credentials and years of experience in teaching and research. That makes them reliable sources of information, and while no one is infallible, these authors are active learners and educators. You can be sure that they present the information in their disciplines as accurately and objectively as possible.
Print and digital media – university Extension publications
Ideally, university Extension publications undergo stringent peer review and are updated regularly. In reality, not all Extension publications are equal in quality. I’m on the faculty at Washington State University and one of my jobs is to keep our Home Garden series of articles current (http://gardening.wsu.edu/). I can confidently say that the fact sheets and manuals on our site have been through peer review and are as accurate as possible. Some are getting near the end of their shelf life (five years at WSU) and need to be revised or removed.
Are there other universities that have peer-reviewed, current, and relevant Extension publications for gardens and landscapes? If so, please add them to the comments and I will check them out. (To save time and aggravation, please check these out yourself first. Don’t just list them and wait for me to go through them with a critical eye.)
Social media, including blogs and Facebook
I first got into social media with the construction of my Informed Gardener web page. The white papers, podcasts and other materials housed here are all science based, but they have not been through peer review. Many of them have been adapted into peer-reviewed Extension fact sheets but all of them represent a collection of relevant information that remains accurate despite being dated. Hey, there’s only so much I can do…
And yes, I’ve probably left someone or something out
By now you’re probably saying “What about Dr. X’s Facebook page or Professor Y’s blog?” This post is admittedly narrow, because I only know the people that I know. I’d like to expand the recommendations in this post to include other discipline experts who have information directly relevant to the mission of the Garden Professors. (This means we are NOT including information more relevant to farming or other types of agricultural production.)
So feel free to add your suggestions as comments, keeping in mind the criteria I mentioned above. Hopefully what we can create together is a really nice resource list for all us to use.
As many of you know, the Garden Professors host a Facebook group dedicated to the discussion of science-based practices for gardens and landscapes. (Side note – if you haven’t joined us please do!) Recently we’ve had a spate of “what’s wrong with my plant” posts, usually focusing on some leaf issue and little other information. And far too often an eager group member will jump in with a fertilizer recommendation. So today’s blog post has two objectives: explaining why you can’t reliably diagnose problems from a picture of suffering leaves and why blanket fertilizer recommendations should be avoided.
To illustrate the problem with armchair diagnosis, consider this photo below.
Now there are two ways to ask a question here: the first is “what’s wrong with these leaves” and the second is “what’s wrong with my plant.” We can easily answer the first one: there is nutrient deficiency in the leaves, most probably iron or manganese. But that does NOT mean there is a deficiency in the soil. So we can’t address “what’s wrong with my plant” because we don’t have enough information.
How can we determine what’s wrong? My first question to the poster is invariably “have you had a soil test?” Soil test results will indicate whether the element in question is actually deficient, and will provide levels of other nutrients that could interfere with root uptake. If there’s no deficiency of the nutrient in question, then adding fertilizer is not going to help! And adding fertilizer unnecessarily can create further soil nutrient imbalances and contribute to environmental pollution.
Once we have the soil test results, we can then begin to address “what’s wrong with my plant.” But not from the original picture. (If you are curious about what else could be causing this problem, check out this blog post from 2011.)
Let’s try another. Consider the leaves in this photo:
We now know to ask “what’s wrong with these leaves?” Ignore for now the deficiencies in the older leaves and look at the size of youngest ones compared to the older. The answer is fairly straightforward here: there was too little water available when the newly emerging leaves were expanding. Leaf expansion depends on turgor pressure – the higher the turgor pressure, the larger the leaves get. Once expansion stops, protective plant biochemicals are laid down which prevent further expansion. By comparing the youngest leaves to the leaves from previous years, you can see that they are significantly smaller. But why?
Again, we need more information before we can answer “what’s wrong with my plant.” Was there too little available soil water during leaf expansion? It’s possible, but this example is from western Washington State, a climatic region with wet springs. Most likely there is an issue with the roots. My first question with these cases is “can you easily move the plant in the ground?” This is my “wiggle test” – a way to determine if roots are established. In this case – and in nearly every case like this that I’ve seen personally – the roots are NOT established. Often this is because the plant (1) was not bare-rooted at planting and/or (2) was planted too deeply. Without decent root establishment there is not enough water uptake to support full turgor in expanding leaves.
Lack of an established root system also account for the interveinal chlorosis you can see in the oldest leaves. These leaves are fully expanded, probably because the plant was still at the nursery when these leaves emerged. But their color is off. A root system that doesn’t supply sufficient water for leaf expansion is by default not going to provide sufficient nutrients, either. Adding fertilizer to this plant is not going to help! It needs to be dug up and replanted correctly or replaced. It is never going to thrive under the current conditions.
Armchair diagnosis can be accurate and fun if you follow a set of guidelines to extract more information. But simply recommending a fertilizer based on leaf appearance is neither science-based nor environmentally responsible.