Most people know that roots take up nutrients and water from plants, but there are so many other incredible and unseen things roots do that are exciting to learn about. To share his enthusiasm for roots and other organisms beneath the earth's surface, my guest this week is landscape artist and author Robert Kourik to demystify roots and the amazing invisible things that roots do.
Robert is a self-taught garden designer and edible landscaping pioneer living in the San Francisco Bay Area. He began his career in natural landscaping and maintenance in 1974 at one of the country's first sustainable organic horticulture companies. He has written two books on roots, "Roots Demystified: Change Your Gardening Habits to Help Roots Thrive"e"Understanding the roots: Discover how to make your garden bloom🇧🇷 And in October her last book will be published, "Sustainable gardens: myths and solutions.“
Robert Kourik is a garden designer and pioneer of edible landscaping who has written two books on roots.
How Robert Kourik met Roots
Robert's interest in roots dates back to 1978 and the Agricultural Library at the University of California, Berkeley. He remembers pulling a book from the shelf with a root drawing on each page. It comes from John Weaver, a professor of plant ecology at the University of Nebraska, who studied prairie plants and vegetables.
At a time when many illustrations of roots were based on guesswork and guesswork, John Weaver (1884-1956) made accurate drawings that were true representations of plant growth. He would dig a trench 6 to 12 feet long around a plant, then he would dig 5 to 7 feet deep and 2½ feet wide to expose the plant's root system.
Weaver clarified how large root systems can be. For example, says Robert, horseradish can only grow 4-5 feet above the ground while its roots grow 13-14 feet deep. On the other hand, a tree can have a root system that is much shallower than the height of the tree. The fact is that the foliage does not indicate the width of the root system.
"I always tell people that what you see above ground has nothing to do with what's going on below ground," says Robert. "There are very few tree roots that are 18 feet deep."
John Weaver in a ditch examining roots.
Roots go beyond the drip line
As gardeners, we often hear that the best place to fertilize a plant is the drip line because that's where the roots end—but they can go much, much further.
The drip line is the area below the edge of the plant canopy. When water drains from the leaves of the plant, most of it falls into the drip line. However, horizontal root growth crosses the drip line and continues.
Robert recommends that when establishing a tree using a drip jet, over time, move the jet further away from the trunk as the roots grow.
Tomato is an example of a vegetable with stalks that extend horizontally beyond the drip line.
Soil type affects root growth
In loamy soil, the roots can be wider than the foliage above ground, says Robert. In sandy soil they usually grow three times as long but can be up to five, six or even seven times as wide as the foliage.
Knowing what type of soil you have has a big impact on where you apply water, mulch and fertilizer, says Robert. In loamy, compacted soils, roots grow closer to the surface to find aerobic zones best suited for their growth. In fact, in clay, 90% or more of the roots are in the top 12 inches of soil, including up to 10-30% in the top 4 inches.
The top 4 inches is the most critical as this is where the soil is most aerobic, meaning this is where the most oxygen is found. The more oxygen in the soil, the easier it is for soil organisms (microorganisms, animals and plants that live in the soil) to convert minerals into soluble nutrients that can be absorbed by the roots.
The top 4 inches has twice as much biological activity as the next 4 inches below, says Robert.
Robert recommends digging a trench to see how deep or shallow the topsoil is and where the clay starts underneath. Tree roots don't like growing in clay, so when they encounter clay they tend to grow sideways.
Horseradish roots can reach great depths, but the type of soil is important. The roots tend to grow horizontally instead of going deeper when encountering clay soil.
Sucking Roots vs. Non-Absorptive Roots
As a tree's root system grows, the oldest roots closest to the trunk develop bark. These bark-covered roots do not absorb nutrients.
Absorbent roots are those with root hairs — microscopic growths from the root tips that absorb water and nutrients. To drop some root lingo on you: root hairs grow on lateral roots, which grow on lead roots.
The roots of older trees tend to form bark and do not soak up at this point. The most extensive and newest are the suction roots with root hairs.
Deep roots don't mean better roots
Oaks, pines, walnuts and persimmons are examples of trees with taproots. However, less than 5% of trees have a taproot. How do trees stay upright?
Robert explains that most trees have a root system with a diameter similar to a tripod: this width stabilizes a tree more than a taproot. He also points out that 50% of a tree's roots are found in the top 18 centimeters of the ground.
Even for trees that have them, taproots are not essential. For example, when an oak tree is dug up on a tree farm to be rolled and rolled, the taproot is cut - but the tree survives. In nature, says Robert, as an oak tree grows, the taproot is squeezed by the lateral roots.
A study also showed that trees growing alongside grasses have deeper roots. In the absence of grass, the roots have less competition and grow closer to the surface.
The depth of the bacteria is one of the reasons why the deepest roots are not necessarily the best roots. Soil bacteria help roots absorb nutrients, and bacteria are concentrated near the surface.
Sinking roots and root grafting for added stability
Fruit trees are given additional stability by so-called "sunken roots". Along the length of the side roots, these sinkers grow vertically.
Other trees, like redwoods, use root grafting for added stability. In redwood groves, the roots of one tree grow into the roots of another. That means a redwood tree alone is more likely to be blown over by the wind than a redwood tree in a grove, says Robert.
Root grafting occurs naturally between trees of the same genus, Robert explains. It happens when two roots grow with a gap between them, and a root from another tree grows in the gap. As the roots grow in diameter, they form a graft.
Although the roots held together over a long period of time, researchers doubt that this is the way they transfer nutrients from one tree to another. Instead, mycorrhizal networks in the soil are more likely to transport nutrients.
The amazing depth of plant roots
Robert recalls being amazed to learn from John Weaver's book in 1978 that there are vegetables with roots that grow 4, 5, or 6 feet deep. Still, roots are concentrated in the first foot, even in vegetables that grow much deeper. For this reason, Robert recommends that raised beds should be at least 30cm deep so that 60% of the potential root system is available for nutrition. Even better is 2 meters deep, for up to 90%. The roots that go deeper—up to 8 feet deep for kohlrabi—are not the critical roots.
John Weaver's drawings of sugar beet roots illustrate their surprising depth.
How nitrogen-fixing bacteria act on legume roots
Legumes like peas and beans work with a nitrogen-fixing bacterium called Rhizobium to convert nitrogen gas into a solid form. This nitrogen is stored in white nodules on the roots of legumes. If you uproot a bean plant and don't see those tubers, your soil needs to be inoculated with Rhizobium before planting the next bean, says Robert. These white nodules should be pink inside when they open. If they are brown, inoculant is needed.
The nitrogen stored in these nodules eventually travels up the plant to become leaves. When the plant produces seed pods, it moves nitrogen from the leaves to the seed pods. When the plant is mature, there isn't much nitrogen left in these tubers.
In order for a legume to share its nitrogen with other plants—called “tuber detachment”—the plant must be killed in one way or another. Once the plant is trampled, cut, burned, or processed, the tubers release their nitrogen into the soil. However, this must be done before the plants have sown so that the tubers are still filled with nitrogen.
Green beans are an example of a legume that benefits from nitrogen-fixing bacteria.
One way to break up compacted soil is to plant a deep-rooted plant. Daikon radishes are a crop used for this practice, known as "biodrilling."
Radishes can die and rot in place to loosen the soil. In cold climates, radishes will die if they freeze, but in warmer climates, a gardener may need to snip off the tops of radishes to kill them.
Hydraulic redistribution, formerly known as hydraulic lift, is referred to when deep roots absorb moisture from the deepest zone and accumulate near the surface overnight. During the day, when it's hot and dry, the plant can capture this water and put it on its foliage, where it evaporates.
Chaparral plants - drought tolerant plants in thickets - are known to use hydraulic redistribution. One of the most fascinating things is that the water brought closer to the surface is not only used by the chaparral plants themselves, but also by neighboring plants.
Why the rhizosphere is so important
The beneficial organisms in the soil – bacteria, blue-green algae, fungi – prefer to live close to the roots. This area is called the rhizosphere and is the place where roots can affect microorganisms in the soil.
The microorganisms provide the roots with nourishment by breaking down minerals into nutrients available to the plants, and the roots in turn release carbohydrates, proteins and fats to feed the living creatures. The roots can also provide phosphorus and other compounds to help dissolve minerals.
In a forest, healthy roots come not only from the narrow rhizosphere, but from vast mycorrhizal networks. Robert says a mycorrhizal mesh can increase the equivalent surface area of tree roots by 1,000 percent.
Phytoremediation removes or neutralizes soil contaminants. Robert says another word for it is hyperaccumulation, since the idea is that the roots of some plants absorb large amounts of certain minerals and chemicals in the soil. Some plants are even effective at absorbing radioactivity through their roots.
For example, in an area near a mining operation where zinc toxicity is present, plants that readily absorb zinc are grown. When the top of the plant is cut and transported, the zinc has also been removed from the site. It's a lot cheaper than hauling the contaminated dirt to a toxic waste dump.
Robert says there is a misconception among gardeners that they can detoxify the soil by growing plants. But in reality, the right plant must be selected, cultivated for years, and the top growth discarded. The toxic material is concentrated in the top crop, so if that top crop is composted, the toxicity is still there.
How roots fight disease
Roots affected by the disease can secrete malic acid to attract Bacillus subtilis bacteria and improve plant immunity. Roots colonized by Bacillus subtilis receive protection against fungal infections.
Mycorrhizal nets also help control disease and pest infestations. Robert explains that when one plant sends out a signal that aphids are attacking it, another plant starts making chemicals that aphids don't like. Robert's book uses broad beans as an example, and researchers continue to learn about others.
How street trees find water
How are street trees surrounded by areas with little permeability irrigated? Roberts says that in addition to the water falling on the limited permeable surface, there is water moving horizontally through the ground by capillary action.
After heavy rains, the pavement serves as a mulch for the softened soil underneath. This soil serves as a reservoir that the street trees can later use as water.
Limited access to water is suitable for smaller trees. A street tree will never be as happy as a tree planted in a field, Robert notes, but he says a tree with an 8-inch diameter trunk can survive in limited soil volume, as illustrated in his book.
How trees cling to rocks
Have you ever noticed conifers hanging precariously from jagged rock surfaces while hiking? I know I marveled at the sight.
Robert says there are a lot more cracks in the rocks than meets the eye. Microscopic root hairs can penetrate even the smallest cracks. Exposed roots will eventually develop bark, but absorbing roots will continue to work their way deeper into the cracks.
Considerations when mulching trees
To suppress weeds and protect tree roots, Robert recommends 6 to 8 inches of arborist woodchips as a mulch. If you don't have enough wood chips to get it that deep, an alternative solution is to use cardboard in conjunction with just a few inches of wood chips or compost. However, until the cardboard rots, it slows down gas exchange between air and soil, which negatively affects root growth.
Mulching suppresses weeds and protects tree roots. A tree does not want to compete with grass and weeds for water and nutrients, especially when it is establishing itself.
I hope you enjoyed my conversation with Robert Kourik. If you haven't already done so, you can do so now by scrolling to the top of the page and clicking the play icon in the green bar below the page title.
What Amazing Facts Do You Know About Roots? Let us know in the comments below.
Links & Resources
Some product links in this guide are affiliate links. See full disclosure below.
Episode 021: How Plants Work with Linda Chalker-Scott (The Science Behind Plants)
Episode 034: Garden Myths BUSTED, Pt. 1 with Linda Chalker Scott
Episode 035: Garden Myths BUSTED, Pt. 2 with Linda Chalker Scott
Episode 048: The Simple Science Behind Great Gardening, starring Lee Reich
Episode 058: Biochar basics and more: delve deep into soil science
Episode 116: Understanding the Soil Food Web with Dr. Elaine Ingham
Episode 153: The Science Behind the Great Solo
Episode 194: Simple gardening without digging with Charlie Nardozzi
joegardener Online Gardening Academy™: Popular Gardening Basics Courses; pest, disease and weed management; seed starters and much more.
joegardener Online Gardening Academy Perfect Soil Recipe Master Class: Learn how to create the perfect soil environment for thriving plants.
joegardener Online Gardening Academy Beginner Gardener Essentials: Basic principles you should know to create a thriving garden.
joegardener Online Gardening Academy Growing Epic Tomatoes: tomato specialistCraig LeHoullier is joining me to teach this course on how to grow healthier, more productive tomato plants and how to overcome tomato growing challenges.
„Sustainable gardens: myths and solutions" by Robert Kourik
„Understanding the roots: Discover how to make your garden bloom" by Robert Kourik
„Roots Demystified: Change Your Gardening Habits to Help Roots Thrive" by Robert Kourik
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Disclosure: Some product links in this guide are affiliate links, which means we get a commission if you buy. However, none of the prices for these features have been increased to compensate us, and compensation plays no role in their inclusion here. The selection of all items featured in this post and podcast was based solely on merit and was in no way influenced by any related or financial incentives or contractual relationships. As of the writing of this article, Joe Lamp'l has professional relationships with the following companies that may include products in this post and podcast: Rain Bird, Corona Tools, Milorganite, Soil3, Exmark, Greenhouse Megastore, High Mowing Organic Seeds, Territorial Seed Company, Wild Alaskan Seafood Box and TerraThrive. These companies are brand partners ofjoegardener.comand/or advertise on our website. However, we do not receive any additional compensation for selling or promoting your products through this guide. The inclusion of all products mentioned in this post is completely independent and excludes any relationship.