311-Gardening Science: How Your Garden Really Grows

| Grow, Podcast

Among gardeners, there is a lot of advice out there that sounds clever and wise, but when you begin to look into it, you learn there is no concrete evidence to back it up. My guest this week, Dr. Stuart Farrimond, has authored a book on gardening science to explain what the research says about what we do in the garden — both the things that work and the things that just aren’t effective.

Stu is a science and medical writer, presenter and educator based in Wiltshire, England. He is trained as a medical doctor and teacher and is passionate about science communication to inspire and engage others about topics that are all too often seen as stuffy and irrelevant. He’s been the food scientist for BBC’s “Inside the Factory” and has authored the international best-selling books “The Science of Cooking,” “The Science of Spice” and “Live Your Best Life.” His new book is “The Science of Gardening: Discover How Your Garden Really Grows.”


Dr Stuart Farrimond

Science communicator Dr. Stuart Farrimond’s new book is “The Science of Gardening: Discover How Your Garden Really Grows.”
(Photo Courtesy of Dr. Stuart Farrimond)


Stu shares that coming out of a round of the COVID lockdowns, he rediscovered the joy of nature and being outside. However, he’s not a gardener — it’s his wife who’s the gardener. She’s a florist, and the garden is her domain. But he thought to himself, “Well, why can’t I learn how to garden?” 

As an outsider to gardening, he was completely befuddled and bamboozled by all the terms and the terminology, he says. He didn’t know what a herbaceous perennial is, what all the Latin words are or even how to sow a seed.

“Now that sounds really ridiculous, but nobody teaches you — you don’t learn it at school,” Stu says of seed starting. “Do I just throw it on there? Do I have to plant it down a certain depth? Am I doing this right?”

He says he considered whether he can write a book that will explain to people who have been gardening for a long time that a lot of the things they do perhaps don’t really work — they’re not science-based; they’re myths — and also reveal the science behind the things that gardeners do that do work. 

For example, gardeners say not to water in the middle of the day because it scorches the plants. Somehow, it became conventional wisdom. But Stu thought, well, that doesn’t even make sense. Gardeners just do things they were told to do, he says.

He wanted to take a fresh look at gardening in a way that is underpinned by science and evidence that avoids jargon and is accessible for new and experienced gardeners, debunking myths along the way. He based his book on the research that has already been done and the consensus that scientific papers have reached.

Writing ‘The Science of Gardening’

Rather than write based on the progression of the seasons, as many gardening books do, Stu wrote about the life cycle of plants, starting as seeds and going on to death and renewal through composting. He covers soil pH, compost selection, making your own compost, as well as watering and growing in containers.

Realizing that gardening can sometimes be snooty, he said he aimed to pen a book that is not exclusive and recognizes that you don’t need to have money to garden. Anyone can garden in a window box, pots or containers, he says, or even on someone else’s scrubland — with permission or, in the case of guerrilla gardening, no permission.

“It’s something that anybody can get involved with,” Stu says.

I always tell people, if you’re waiting until you feel like you have the perfect opportunity to garden — stop waiting. The only way you’re going to become a gardener and learn how to do it is to do it.

You can dive deep into the research aspect of gardening, but even then, it really comes to life with your hands in the soil. 

Why Some Plants Die in Freezing Temperatures and Some Don’t

The Southeast got a blast of subfreezing temperatures around Christmastime that killed many plants that would normally survive winter just fine. Winter hardiness varies from plant species to plant species, with plants that are native to the tropics generally being the least tolerant of cold, and the plants native to the northernmost and southernmost reaches of the globe being the most tolerant. 

Gardeners love to “push” the hardiness zones of plants and can be quite successful for years or even decades, but when a rare deep freeze comes along to a region that typically stays much warmer in winter, those plants go belly up.

Freeze damage occurs on the cellular level, as Stu explains. Inside the plants, the water turns to sharp, spiky ice crystals that essentially puncture all the cells that make up the plant, he says. Then when the ice thaws, the cells release their contents and die.

Some plants have built-in biological antifreeze, a type of protein within them that lowers the freezing point of water. The starches within the plants are converted into sugar when frost comes, creating that antifreeze effect. Stu points out that when cold arrives gradually, the plants have a better chance of survival because they have had time to get their defenses up. Conversely, a sudden change in temperature will doom plants that otherwise can tolerate frost.

Other hardy plants have different types of defenses to the cold. For example, cushion plants are lower growing so once blanketed with an insulating layer of snow they can survive temperatures that are sub-freezing above the snow cover.


Spinach covered in frost

Crops like spinach can tolerate frost. In fact, they become sweeter after being exposed to frost because they produce sugar as a defense against freezing.


The Importance of Hardening Off

Hardening off is the process of taking plants that were raised indoors from seed and gradually introducing them to full sun and the outdoor environment. Gardeners can understandably be impatient and wish to get their plants into the ground right away, but hardening off is a necessary step, even for sun-loving plants. On their first day outdoors, limit plants to a half hour in the sun, and on the second day, make it a full hour. Then add an hour a day, and by the 10th day, the plants will be all set to survive in full sun.

The same UVB rays that cause us sunburn will damage plants — unless those plants have been given an opportunity to build up their defenses by producing molecules that act as natural sunscreen.

Wind is also a consideration when hardening off. Plants raised in the still environment of a home or a greenhouse can become tall and spindly because they have not had to deal with any buffeting by winds. If they are suddenly exposed to a full day outdoors with wind they have never experienced before, they may not have the resilience to remain standing.

The same goes for trees that have been staked, Stu says.  When a young tree is held up by stakes and straps, it doesn’t have to put any resources into building steady supports around the trunk.

One trick many seed starters use is to brush over their seedlings once a day to stimulate the plants to produce more tough cellulose. The scientific term for this is thigmomorphogenesis, which is a great vocabulary word to have in your repertoire.


Shade cloth over seedling trays

Hardening off under shade cloth protects plants from sunburn.


Understanding Hardiness Zones

In the United States, we get a sense of what plants will overwinter in the region where we live based on U.S. Department of Agriculture hardiness zones. It is based on the average minimum temperature over the past three decades or so. These zones also guide the planting time for our vegetables and annuals, but these zones don’t give us a full picture. For example, I live in Zone 7b in Georgia, but my growing environment here is far different from 7b in California or the Northeast.

We need more information, such as our average frost-free date of spring and our first frost date of fall. 

Sunset, a magazine founded in 1898 focused on the Western United States, has its own zones for gardening on or near the West Coast of North America. It has many more tiers than the USDA growing zones and is much more in-depth, taking in many more criteria to determine the true hardiness of a plant.

In writing his book, Stu endeavored to make it applicable to growers everywhere, because the climate is different everywhere. He didn’t want to limit his book to only users of USDA or Royal Horticultural Society hardiness zones.

No matter what zone system you use, remember that a zone is only a guide. “Nature throws us curveballs,” Stu says.

Why Plants Bolt

Bolting is when a plant shoots up its flower stalk because it perceives that its lifecycle is over and it must reproduce by making seed. When a vegetable plant such as lettuce or cabbage bolts before it can be harvested, it becomes bitter and no longer desirable to eat. 

Stu spends time in his book explaining what bolting is and how to prevent it. Every plant wants to produce seeds, even grass, but because we mow our lawns, the grass doesn’t get the chance. The problem with vegetable crops bolting, he explains, is that when a vegetable turns its focus toward producing seed, it puts its resources into that next generation, and the plant itself has less starch and sugar that give it the taste we are after.

If you plant a vegetable in spring, you will be in a race to harvest as much as you can before its natural flowering time, Stu says. Alternatively, you can plant it after its flowering time and race to harvest it before frost kills it. 

Plants will bolt even earlier when they are stressed. They take that stress as a sign that their life is almost over, and they try to produce seed so their genes live on.

Drought and heat can stress plants, so staying on top of watering and using shade cloth can delay bolting. But most important is planting at the right time, mindful of when summer heat will set in, in the case of spring planting, or when the first frost will come, in the case of late-summer planting in anticipation of a fall harvest.


Cilantro flower

Bolted cilantro. The leaves will taste bitter after the plant bolts — but you can let is go to seed and collect the seeds for next year’s crop. (photo: Amy Prentice)


Getting Watering Right

When we’re in the garden watering, we’re not really watering the plants, Stu says, we’re watering the soil. The plants drink from the soil.

To know how much to water, it’s important to understand the makeup of the soil. Does the soil hold on to water well or not? Sandy soil drains readily while heavy clay soil retains water and loamy soil rich in organic matter holds just the right amount of moisture to keep plants satiated between waterings.

More plants are killed by overwatering rather than underwatering, especially potted plants, Stu says. His advice is to let plants feel a little bit of thirst. If plants never experience soil that gets a little dry, they won’t spread their roots. Then the first time there’s a drought or the gardener goes on vacation, the plants can’t tolerate being without water. It’s better to water deep but infrequently, so roots grow in search of water and nutrients, and the plants build up a tolerance for dry conditions. 

Roots are opportunistic. If they are mollycoddled in constantly moist soil, they won’t grow deeper, Stu explains. But when the water table is lower, the roots will put resources into going down further.

Stu practices the finger test. Just stick your finger in the soil and observe whether it comes out clean and dry or wet and dirty. “If it’s dry, then water it,” he says. “If the plant is showing signs of overheat stress, then give it water.”

He aims for soil that is dry to about an inch before watering again. If it’s dry below that point, you’ll begin to see signs of stress such as dull foliage and wilting. 

Stu doesn’t believe there is a wrong time of day to water. If the plant needs a drink, give it a drink, he says.

Plants lose most of their moisture through their leaves, so the more leaves a plant has, the more water it will need. Also, pay attention to the air temperature and the humidity. In hot, dry conditions, plants will need water more frequently, but in high humidity, plants can go longer between waterings even when the temperature is warm.

Wind also has a drying effect and can be very damaging. In fact, if you see trees in a field that appear lopsided, it’s the drying effect of wind, rather than the force that caused those trees to bend, Stu explains. Trees naturally delegate their resources to the side that’s not constantly dried out by prevailing winds.


The soil moisture finger test.

If you stick your finger into garden soil and it comes out dirty like this, the garden is adequately watered. But if your finger comes out clean, the dirt is not clinging to your finger because it is too dry.


Bringing in Native Pollinators

We’re all trying to do a better job of bringing in native pollinators, and at the same time, we like a pretty garden. Breeders come up with doubles and really elaborate multi-layered petals, and although they are indeed pretty, they’re not as functional in many cases for our pollinators. 

We need to be thinking more about our plant choices that serve the benefits of the pollinators and what they can get to and how they can access the pollen and the nectar.

Stu says the flowers considered the most beautiful are not natural at all because they have been bred to have an abnormal number of petals. The cost of this is they have fewer stamens, which contain pollen. So we find the plants more attractive, but the pollinators don’t find the nectar they need to survive. 

“Avoid those ones that are most pretty,” Stu says. “Go for the ones that don’t have all those extra petals.”

The sunflower family, Asteraceae, gives you the most bang for your buck, he says. The central part of a sunflower or daisy, the disc, that looks like the middle of a flower head is, in fact, host to numerous individual flowers. Asteraceae flowers are packed with nectar and protein for pollinators.

“You’ve got hundreds of little meals there within the head,” Stu says.

He adds that native plants are very useful to pollinators, but a mix of native plants and non-natives in a garden hosts the widest number of pollinators.

“Your garden can serve a really important part of preserving the natural pollinator population that we have,” he says. “They’re better than agricultural land. They’re better than lots of wild land because we bring this curated sample of all the best from around the world into this little place. It’s like a feast for pollinators, which includes moths, beetles, flies, wasps, as well as bees. There’s lots of really important insect life that relies on our gardens.”

Not to mention the predatory insects that provide natural, biological pest control.


Monarch butterfly on aster

Native asters are an important source of nectar for pollinators, including monarch butterflies. Cultivars of asters are not as useful as natives because they contain less pollen and nectar. (Photo Credit: Amy Prentice)


Making More Plants From Cuttings

Depending on the species, a leaf, stem or root can be grown into a new plant,

“It’s like taking somebody’s hand and a whole new person growing out of it,” Stu says. “The same thing can happen with cuttings. It’s mind-boggling that you can do this.”

Rooting cuttings is a bit hit or miss, even when using rooting compound. Stu recommends just giving it a try. Put a cutting in a bit of compost or potting soil and see if it takes.

Within all plants are cells called meristem cells. Think of “stem cells” found in humans. These are basically primitive cells that can turn into any other type of cell, Stu says. While humans lose most of their stem cells after they are born, plants hold bundles of stem cells so they can rebuild any part of their body should it be damaged.

Maristems are like utility players that can play any position on a field depending on where the coach sends them. 

Auxin, a plant hormone, controls what grows and doesn’t grow in a plant. It’s produced in the growing tip and signals side shoots not to grow so resources can be dedicated to growing up toward the light. When a growing tip is damaged or pruned off, the side shoots starts to grow in the absence of auxin.


Rooted cutting of basil

Herb cuttings can be propagated by rooting. These propagations can be overwintered indoors and planted out come spring. 
(photo: Amy Prentice)


If you haven’t already listened to my conversation with Dr. Stuart Farrimond on gardening science, you can do so now by scrolling to the top of the page and clicking the Play icon in the green bar under the page title.

How has gardening science influenced your practices in the garden? Let us know in the comments below.

Links & Resources

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

Episode 011: Plant Propagation Basics – With Brie Arthur

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

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

Episode 124: Using Compost the Charles Dowding Way: More Than Just a Great Soil Amendment

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

Episode 270: Plant Science for Gardeners, with Robert Pavlis, Part I

Episode 272: Plant Science for Gardeners, Part II, with Robert Pavlis

Episode 283: A Soil Chemistry Primer: How Protons and Electrons Influence Soil Moisture and Fertility

Episode 305: Compost Science for Gardeners, with Robert Pavlis

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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 Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.

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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The Science of Gardening: Discover How Your Garden Really Grows” by Dr. Stuart Farrimond

The Science of Spice: Understand Flavor Connections and Revolutionize Your Cooking” by Dr. Stuart Farrimond

The Science of Cooking: Every Question Answered to Perfect Your Cooking” by Dr. Stuart Farrimond

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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: Corona Tools, Milorganite, Soil3, Greenhouse Megastore, Territorial Seed Company, Earth’s Ally, Proven Winners ColorChoice and Dramm. 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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