270-Plant Science for Gardeners, with Robert Pavlis, Part I

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I’m always encouraging gardeners to learn the “why do” behind the “how to,” because I believe gaining that understanding makes us better at what we do, and to help me do that this week, I’m welcoming back to the podcast Robert Pavlis, a garden myth buster who can break down the plant science behind good gardening.

Robert’s background and training are in chemistry and biochemistry, and he later became a maker of laboratory software. He now applies his science background to his gardening hobby. His private, 6-acre botanical garden in southern Ontario, Canada, is packed with more than 3,000 different species of plants, trees and shrubs. It’s located in USDA plant hardiness zone 5, which means it’s analogous to many northern United States growing regions. While he has a small vegetable garden, his acres are mostly dedicated to ornamentals. He says his main focus is on really interesting plants.


Robert Pavlis

Robert Pavlis has experience as a chemist and biochemist and he applies that background to gardening. He busts some of the most persistent garden myths. (Courtesy of Robert Pavlis)


“What I really love is a plant I’ve never grown before or one that I’ve never seen before,” Robert says. “So that’s really my passion.”

Robert is one of my top science go-to guys. I love that he really gets down to the science of everything and explains it in a way that a layperson can easily understand. His newest book, aptly named “Plant Science for Gardeners: Essentials for Growing Better Plants,” is chock-full of information and has lots of neat stuff to help gardeners up their games.

He believes that when gardeners learn a little more about science, they’ll understand what they’re doing and will be able to better interpret what they see and hear. He’s been on the podcast twice before, discussing houseplant myths and garden products you don’t need

“Social media is great for getting gardening information, but how do you decide which piece of information is right and which one should you really follow?” Robert asks. “That’s the tricky part gardeners have today.”

Often people who like to type offer answers to gardening questions on social media despite not having the requisite experience, Robert points out. 

He is analytical, so when he hears information about gardening, he doesn’t take it at face value. He asks lots of questions to get down to whether that info is really true, and he debunks bad gardening advice at and in books. In six years, his originally short list of garden myths has grown into nearly 500 articles and 165 more myths that he has started debunking but hasn’t published yet.

“I have enough for the next 20 years,” Robert says. “There’s so many myths out there.”


Plant Science for Gardeners Book

Robert’s new book “Plant Science for Gardeners” explains how healthy soil and plants function in a way that is easy for the lay person to understand. (Courtesy of Robert Pavlis)


Before going any further with my discussion with Robert on the science behind gardening, let me take a moment to remind you that I have a book of my own coming out in September. The title is “The Vegetable Gardening Book: Your complete guide to growing an edible organic garden from seed to harvest,” and it’s available for pre-order now. It’s chock full of insider tips and new-to-you information that will help you step up your gardening game and tackle challenges. 

And on tap for 2023 is my new Online Gardening Academy™ premium course, Organic Vegetable Gardening. Sign up for the waitlist here.

The Rules Vs. The Science

In gardening, there are many, many rules: when to plant, when to prune, and so on. Learning and remembering the rules is one way to garden, but it’s not the way that Robert and I subscribe to. In his book, Robert advocates learning the science behind gardening, because once you understand that, you don’t have to remember all the rules. It’s easier to just follow the science.

“I think the rules are easy for beginning gardeners, but as you grow more and more plants and you try different things, there’s just too many rules — and you can’t remember them all,” Robert says.  “And then as your gardening education progresses, you reach a point where you suddenly realize: ‘I don’t need those rules.’”

Robert says he can grow a plant he’s never seen before because he understands how plants grow. He doesn’t need to know the specific rules for that unfamiliar plant. “I can grow it, I know how to prune it and I know how to take care of it, but I don’t go and find a rule for that new plant,” he says.

Foliar Feeding Myths

Foliar feeding, the practice of fertilizing plants on their leaves rather than applying fertilizer to the soil, is misunderstood. In most cases, it’s a waste of time and resources, though there are narrow circumstances in which it’s beneficial. 

Plants get most of their nutrients through the roots, and if we look at a plant out in the field, there’s very little that it’s going to get through the leaves,” Robert says.

Leaves have pores called stomata, which are critical to how plants function because they allow excess water to come out of the leaves. The idea of foliar feeding is that the roots and stems can be bypassed, and that nutrients meant for the leaves can go directly on the leaves and be absorbed through the pores. But does this shortcut work? Robert says the answer is “sort of.”

Stomata, due to their chemistry, don’t let things in very easily, he says. However, there are other holes in leaves that nutrients can get in.

“Foliar feeding should work and it does work,” Robert says, “The problem is the amount of nutrients that get in through leaves is very tiny compared to what a plant can absorb through roots.”

In the case of a micronutrient deficiency, such as a plant with not enough zinc, foliar feedings can yield results within hours. A discolored leaf can quickly change color in response to the added zinc.

But for the macronutrients — nitrogen, phosphorus, potassium — foliar feeding doesn’t work. “Because the plant needs just so much of that, you’ll never be able to get it through the leaves,” Robert says.

Foliar feeding is not a good long-term strategy and is difficult to do right, so he recommends gardeners simply ignore it. 


Leaves as mulch under around plants

Plant leaves have pores called stomata that are designed to release excess moisture in a plant. Stomata are not designed to let anything in, so foliar feeding is not a replacement for adding fertilizer to the soil, where roots can take it up.


Blossom End Rot Myths

Blossom end rot is a nuisance that all tomato growers encounter from time to time. It’s also the subject of a persistent gardening myth.

The notion is that blossom end rot is caused by a calcium deficiency and that the way to fix it is to fertilize with calcium. The reality is quite different.

Robert says the research has shown that blossom end rot occurs even when plants and the soil they are growing in contain plenty of calcium. Popular but incorrect advice calls for adding Tums (calcium carbonate) or egg shells to the soil to prevent blossom end rot.

“There’s at least a dozen myths about this, and they’re all wrong because it’s not a calcium problem,” Robert says. “It’s a watering issue.”

Calcium is a nonmobile nutrient. Robert explains this means that calcium contained in one leaf can’t move to another. Meanwhile, nitrogen is a mobile nutrient that can move to where it is needed. This is why lower leaves on a plant go yellow — the plant is recovering nitrogen and redeploying it to the growing stem. “But it can’t do that with calcium,” he says. “The chemistry isn’t there.”

The only way to get calcium into a fruit is through the roots because calcium only moves up the plant, never across the plant. Calcium that’s in a leaf will never move into a fruit.

The real way to fix blossom end rot is to water consistently. A tomato plant — or pepper plant or cucumber plant — can avert blossom end rot if it has water readily available to help move calcium. 


Blossom End Rot

Blossom end rot is a symptom of calcium deficiency due to inconsistent water, not due to a lack of calcium in the soil. Foliar feeding with calcium will not move calcium into the fruit because calcium can only move from the roots up; it’s can’t move from leaves into the fruit.


Nitrogen Deficiency Is the Likely Culprit

Nitrogen levels can change quickly. “By the time you get your soil sample to the lab, it’s changed,” Robert says. 

He tells gardeners they can assume that nitrogen is always the limiting factor in their soil, more likely than any other nutrient issue. Because nitrogen is so soluble when it rains, nitrogen is moved out of the soil, so the plants experience a deficiency, he says. 

“The plants, the fungi, the bacteria — everything is looking for nitrogen. It’s the holy grail in that soil,” he says. “They all need it to live on and they’re all fighting for it. So there’s very rarely excess nitrogen laying around in the soil for plants to get.” 

No matter what the growing instructions state on a seed packet, Robert’s advice is to start fertilizing right away once a seedling develops its first true leaves.


Tomato seeding

After the first pair of leaves (the cotyledon leaves) that come up with a tomato sprout, the true leaves follow. (Courtesy of Robert Pavlis)


How Do Plants Grow?

Here’s a word that needs to be preceded with a geek alert: meristematic.

Meristematic cells are undifferentiated cells that are capable of cell division, compared to differentiated plant cells that can’t divide and can’t produce cells of another type.

Cells that can produce other types of cells help answer a simple but important question: How do plants grow?

“How do they get taller? How do roots get longer? And I know for most gardeners, they don’t think about those questions. But geeky people like me, we do,” Robert says.

In his book, he keeps his explanation of the science of meristematic cells light and easy to understand. Once again, he does a great job weaving the science in without readers’ eyes glazing over.

He explains that undifferentiated cells can be thought of as blobs; they don’t look like anything. But differentiated cells each have a certain function: Think skin cells, muscle cells, red blood cells, etc.

“They look different, they function differently, but once they are those cells, they cannot change in most cases,” Robert says. “So we have a muscle cell — it’s a muscle cell for life.”

He says the same thing happens in plants: Once a cell is a leaf cell, it can’t become a root cell. And differentiated cells can’t multiply.

But an undifferentiated cell, or meristematic cell, doesn’t yet know what it’s going to become. It may become a root, a stem, a leaf, a seed — whatever the plant needs.

Undifferentiated cells are found in special spots in plants, like the tip of the root. The meristematic cells there reproduce like crazy, and over time some become root cells and some become root cap cells. “As that process happens, the root actually gets longer,’ Robert says.

The same thing happens at the shoot end, the top of the plant. These cells can also be found where branches meet a trunk, at leaf petioles (where leaves touch a plant) and along roots. And because these cells are hidden in plants, when we prune, new shoots can grow, and when we dig up a plant, new roots can grow.

The Rhizosphere

The thin layer of soil surrounding roots is an area known as the rhizosphere. “It’s millimeters thick, and that rhizosphere is completely different than anywhere else in the soil,” Robert says.

This is where the roots actually influence the soil around them by interacting with microbes. They do so by releasing exudates, “a general term for hundreds, if not thousands of different chemicals that the plant makes,” he says. “And we have to understand that what plants are, are fantastic, chemical manufacturing machines.”

Exudates serve various roles and can be designed to be helpful to the plant or harmful to bacteria. They are mostly sugars, with lots of carbon in them. They are produced in the leaves and then sent down to the root and squeezed out into the soil. 

The sugars (carbohydrates) that roots put into the rhizosphere attract bacteria and fungi, which love to eat carbs. In exchange for the sugars, the microbes provide the roots with nutrients and protection from pathogens. This relationship is critical for plants. 

“These microbes are going off into parts of the soil that the root can’t get to,” Robert says. “So on a microscopic level, plant roots are actually pretty big things compared to, say, fungi, and the fungi go off into all these nooks and crannies that the roots can’t get to, pick up nutrients and bring it back to the root.”

Plants can only absorb very small molecules, like nitrates, phosphates and some amino acids, he points out. They can’t dissolve rock into a form that they can absorb, but some fungi can.

“So the plant creates these sugar exudates and hands it over to the fungi who are working all day at breaking up rock for them and bringing back phosphate,” he says.

The bacteria and fungi also attract protozoa and nematodes, which eat them. All of these creatures excrete nutrients in a plant-available form. 

If we apply soluble nitrogen fertilizers, a plant won’t need to rely on bacteria and fungi for its nutrients, so it will stop expending its energy on exuding carbohydrates. 

Plants can condition the rhizosphere so that the pH is in line with what they want, which is important because the pH affects which nutrients a plant can absorb through its roots. The pH level is recorded on a scale of 0-14, with lower numbers indicating more acidic and higher numbers indicating more alkaline. A plant can change the pH in its rhizosphere by as much as two pH units, according to Robert. 

“My soil is 7.5, but the rhizosphere, the part that touches the root, could be a pH 6, which is perfect for plant growth,” he says. 

The difference between a pH of 7 and a pH of 6 is a factor of 10, Robert points out. That means a 6 is 10 times as acidic as a 7. 



Roots influence the soil in their direct proximity. This area is called the rhizosphere.


How Compost Helps Plants Absorb Nutrients

Compost itself is low in nutrients, so why do we add it to our gardens to help plants grow? Robert explains that it is a misperception that compost feeds plants. 

“Compost is made up of large molecules that are absolutely useless for the plant,” he says. “So you can actually have a plant sitting in a whole bunch of compost that’s starving because it can’t get the small nitrate molecules it needs. Something has to come along and degrade that compost and turn it into nitrate molecules, and that something is microbes.”

It’s a process that takes a long time. In fact, “finished” compost will continue decomposing for five years. Robert says the compost we apply today is almost useless for feeding a plant today. 

I think of adding compost as feeding soil. Then the soil feeds the plants. 

Compost adds billions of fungi and bacteria that build better soil and bring wanted nutrients to roots. It’s not the nutrients in soil themselves that are making a big difference — the nutrient ratio of compost is very low — but it’s the biological activity that compost adds to soil that helps plants succeed. 

“It’s all about building long-term soil,” Robert says. “So if you keep adding it every year, five years from now, you’ll have great soil and there’ll be lots of organic matter in there and lots of microbes and your plants will be growing really well and you won’t need fertilizer.”


filling up raised beds

Compost is low in nutrients compared to fertilizer and it can take years for all of those nutrients to become available to plants. The immediate benefit of amending soil with compost is that it’s full of microbial life that helps feed the plants.

The Magic of Root Hairs

Roots themselves don’t absorb nutrients and water to a great extent. What are really absorbing resources are the root hairs, Robert says. 

Root hairs grow near the root tips and last about two weeks, though if the soil dries up they will fall off right away. “It varies depending on plants, but they’re not long lived,” he says. “So a plant is continually making new root hairs and it’s the root hairs that are doing all this absorbing.”

Root hairs greatly increase the surface area of roots, which is better for absorbing nutrients and water. 

Another myth is that plants grow their roots toward water. Plants don’t know where the water is, Robert says, but they can tell which side of the root and wetter and which side is drier. It tends to grow toward the moist side and where the root tips are receiving the most nutrients.

“It sounds as if these roots are really smart, but there’s no intelligence here,” he says. “When you look at the details of how this works, it’s just simple chemistry.” 

When removing a root ball from a pot to plant it, there are a few ways to free the pot-bound roots and to encourage growth in all directions. Four vertical slices is a method that works well, or you could cut the bottom off. What Robert finds works best is cutting the root system into a cube. This method severs the root tips from the plant. Before the plant can absorb any more water, it will require a couple of days to develop new root tips and new root hairs.

“That’s why you shouldn’t be doing this in the middle of summer and you do it in spring and fall when it’s cooler,” Robert says.

Disturbance to the root system is the main cause of transplant shock, so don’t add to the plant’s stress by transplanting it in the heat of summer. Lower temperatures and shade will reduce how much moisture a plant loses through it’s leaves during the day.

I’ve transplanted some large plants that took too full seasons before they recovered from transplant shock. If you move a plant and it doesn’t look great, it may just need more time. 


shopping for plants - rootbound

Rootbound plants like the one on the right need to have their roots freed before transplanting. Cutting off the root tips means the plants will suffer transplant shock for a couple of days, but soon they will grow new root tips and root hairs for absorbing moisture and nutrients.


I hope you enjoyed my conversation with Robert Pavlis and come back for part two in a couple of weeks. If you haven’t listened yet, you can hear this episode now by scrolling to the top of the page and clicking the Play icon in the green bar under the page title. 

What garden myth do you find most irksome?  Let us know in the comments below.

Links & Resources

Some product links in this guide are affiliate links. See full disclosure below.

Episode 048: The Simple Science Behind Great Gardening, with Lee Reich

Episode 083: Gardening Indoors: The Science of Light, with Leslie Halleck

Episode 116: Understanding the Soil Food Web, with Dr. Elaine Ingham

Episode 139: Houseplant Myths: The Facts Behind Caring for Indoor Plants

Episode 143: Gardening Products You Don’t Need, and Why

Episode 153: The Science Behind Great Soil

Episode 179: Plant Partners: The Science-based Benefits of Companion Planting, with Jessica Walliser

Episode 223: How Soil Microbes Make Good Soil Great

joegardenerTV YouTube: How to Get the Best Drainage for Your Container – Why What You’ve Been Taught Is all Wrong

joegardener Online Gardening Academy™: Popular courses on gardening fundamentals; managing pests, diseases & weeds; seed starting and more.

joegardener Online Gardening Academy Organic Vegetable Gardening: My new premium online course membership opens in 2023. Sign up for the wait list here.

joegardener Online Gardening Academy Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.

joegardener Online Gardening Academy Master Seed Starting: Everything you need to know to start your own plants from seed — indoors and out. 

joegardener Online Gardening Academy Growing Epic Tomatoes: Learn how to grow epic tomatoes with Joe Lamp’l and Craig LeHoullier. 

joegardener Online Gardening Academy Master Pests, Diseases & Weeds: Learn the proactive steps to take to manage pests, diseases and weeds for a more successful garden with a lot less frustration. Just $47 for lifetime access!

joegardener Online Gardening Academy Perfect Soil Recipe Master Class: Learn how to create the perfect soil environment for thriving plants.

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Garden Fundamentals on YouTube

Garden Fundamentals Facebook group

Building Natural Ponds Facebook group

Plant Science for Gardeners: Essentials for Growing Better Plants” by Robert Pavlis

Soil Science for Gardeners: Working with Nature to Build Soil Health” by Robert Pavlis

Building Natural Ponds: Create a Clean, Algae-free Pond without Pumps, Filters, or Chemicals”  by Robert Pavlis

Best Garden Plants for Connoisseurs: The best plants from my collection of over 3,000 garden gems” by Robert Pavlis

Garden Myths: Book 1” by Robert Pavlis

Garden Myths: Book 2” by Robert Pavlis

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Disclosure: Some product links in this guide are affiliate links, which means we get a commission if you purchase. However, none of the prices of these resources have been increased to compensate us, and compensation is not an influencing factor on their inclusion here. The selection of all items featured in this post and podcast were based solely on merit and in no way influenced by any affiliate or financial incentive, or contractual relationship. At the time of this writing, Joe Lamp’l has professional relationships with the following companies who may have products included in this post and podcast: Rain Bird, Corona Tools, Milorganite, Soil3, Greenhouse Megastore, PittMoss, Territorial Seed Company, Earth’s Ally and TerraThrive. These companies are either Brand Partners of and/or advertise on our website. However, we receive no additional compensation from the sales or promotion of their product through this guide. The inclusion of any products mentioned within this post is entirely independent and exclusive of any relationship.

About Joe Lamp'l

Joe Lamp’l is the creator and “joe” behind joe gardener®. His lifetime passion and devotion to all things horticulture has led him to a long-time career as one of the country’s most recognized and trusted personalities in organic gardening and sustainability. That is most evident in his role as host and creator of Emmy Award-winning Growing a Greener World®, a national green-living lifestyle series on PBS currently broadcasting in its tenth season. When he’s not working in his large, raised bed vegetable garden, he’s likely planting or digging something up, or spending time with his family on their organic farm just north of Atlanta, GA.

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