In the soil under our feet, amazing things go on that we don’t get to see. The more we learn about the hidden life of roots, the greater our understanding of how plants obtain the water and nutrients they need to grow. Landscape designer and author Robert Kourik joins me on the podcast to explain what roots and other organisms are up to beneath the soil’s surface.
Robert is a pioneer of edible landscaping and a self-taught garden designer. He began his career in natural landscape design and maintenance in 1974 with one of the country’s first sustainability-oriented organic gardening businesses. He has written two books on roots, “Roots Demystified: Change Your Gardening Habits to Help Roots Thrive” and “Understanding Roots: Discover How to Make Your Garden Flourish.”
Robert Kourik is a garden designer and a pioneer of edible landscaping who has written two books on roots.
You can read on for an abridged recap of my conversation with Robert or see the show notes from the original airing for a more comprehensive write-up of our discussion.
How John Weaver Revealed the Hidden Life of Roots
Robert’s interest in roots traces back to 1978 and the University of California, Berkeley Agricultural Library where he recalls finding a book that had a drawing of roots on every page. The book was by John Weaver, a professor of plant ecology at the University of Nebraska who studied prairie plants and vegetables. At a time when many root illustrations were based on guesses and assumptions, John Weaver (1884-1956) made accurate drawings that were true representations of how plants grow.
Weaver revealed just how large root systems can be. Weaver excavated trenches 6-12 feet long around a plant then dug 5-7 feet deep and 2½ feet wide to reveal the plant’s root system. His research demonstrated that assumptions about how deep roots grow based on the above-ground height of plants are usually wrong. For example, Robert says, horseradish may grow only 4-5 feet above ground while its roots grow 13-14 feet deep. Conversely, a tree can have a root system that is much shallower than the tree is tall. The fact is the foliage does not indicate the width of the root system.
John Weaver in a trench examining roots.
The Drip Line and Root Hairs
It’s often said that the drip line is the best place to fertilize a plant because that’s where the roots end — but they can grow much farther out. As the root system of a tree grows, the oldest roots, closest to the trunk, grow bark. The bark-covered roots do not absorb nutrients.
The absorbing roots are the ones with root hairs — microscopic growths from root tips that take up water and nutrients. To drop some root jargon on you: root hairs grow on the lateral roots that grow off the lead roots.
The drip line is the area under the edge of the canopy of the plant. When water runs off the plant’s foliage, the drip line is where most of it lands. However, the horizontal growth of roots passes the drip line and keeps on going.
Robert recommends that when establishing a tree with the help of a drip emitter, the emitter should be moved farther away from the trunk over time as the roots grow.
Tomato is an example of a vegetable plant with roots that extend horizontally far past the drip line.
Roots Grow Shallower in Clay Soil
Knowing what type of soil you have greatly influences where you put water, mulch and fertilizer, Robert says. In compacted, heavy clay soil, roots will grow closer to the surface to find aerobic zones that are better for their growth, and roots won’t grow much wider than the foliage. In clay, 90% or more of the roots will be found in the top foot of soil. As much as 10-30% are found in the top 4 inches.
The top 4 inches are the most critical because that’s where soil is most aerobic — meaning that’s where the most oxygen is found. The more oxygen in the soil, the easier it is for the soil biota to convert minerals into soluble nutrients that can then be taken up by roots.
Tree roots don’t like to grow in clay, so when they do hit clay they tend to start growing sideways.
Deeper Doesn’t Mean Better Roots
Oak trees, pine trees, nut trees and persimmons are examples of trees with taproots. However, fewer than 5% of trees have a taproot. So how do trees stay standing?
Most trees have a root system that is wide in diameter, like a tripod. That width stabilizes a tree more than a taproot can.
Even for the trees that have taproots, they are not essential. For instance, when an oak tree on a tree farm is dug to be balled and burlapped, the taproot is cut — but the tree survives. In nature, Robert says, as an oak tree grows, its taproot will be squeezed off by the lateral roots.
A study also showed that trees grown alongside grasses will have deeper roots. In the absence of grass, the roots have less competition and they grow closer to the surface.
John Weaver’s drawings of sugar beet roots illustrate their surprising depths.
Nitrogen-Fixing Bacteria on Legume Roots
Legumes, such as peas and beans, work with a nitrogen-fixing bacteria called rhizobium to turn nitrogen gas into a solid form. That nitrogen is stored in white nodules on legume roots. If you pull up a bean plant and don’t see those nodules, your soil needs to be inoculated with rhizobium before your next bean planting, Robert says. Those white nodules, if broken open, should be pink on the inside. If they are brown, an inoculant is needed.
Nitrogen stored in those nodules eventually makes its way up the plant to become foliage. When the plant makes seed pods, it moves nitrogen from the foliage to the seed pods. By the time the plant is done growing, there is not that much nitrogen left in those nodules.
For a legume to share its nitrogen with other plants, the plant must be killed one way or the other. Once the plant is stepped on, mowed, burned or tilled in, the nodules will release their nitrogen into the soil. This should be done before the plants have gone to seed so the nodules are still packed with nitrogen.
Green beans are one example of legumes, with store nitrogen with the help of nitrogen-fixing bacteria on their roots.
The Rhizosphere
The beneficial microbes in soil — bacteria, blue-green algae, fungi — prefer to live near roots. This area is known as the rhizosphere.
The microbes give roots nutrition by dissolving minerals into plant-available nutrients, and the roots, in turn, exude carbohydrates, proteins and fats to feed the microbes. The roots can also give out phosphorus and other compounds to help dissolve minerals.
In a forest, healthy roots come not just from the rhizosphere, which is narrow, but from vast mycorrhizal networks. Robert says a mycorrhizal network can increase the equivalent surface area of tree roots by 1,000 percent.
How Roots Fight Disease
Roots affected by disease can secrete malic acid to attract the bacteria Bacillus subtilis to increase the plant’s immunity. Roots colonized by Bacillus subtilis gain protection from fungal infections.
Mycorrhizal networks help fight diseases and pest attacks as well. Robert explains that when one plant sends out a signal that it’s being attacked by aphids, another plant will begin to produce chemicals that aphids don’t like.
The oldest tree roots tend to grow bark and are non-absorbing at this point. The farther reaching and newest roots are the absorbing roots, with root hairs.
I hope you enjoyed my conversation with Robert Kourik on the hidden life of roots. If you haven’t listened yet, 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.
Have you discovered anything interesting when excavating 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: Gardening Myths BUSTED, Pt. 1 with Linda Chalker-Scott
Episode 035: Gardening Myths BUSTED, Pt. 2 with Linda Chalker-Scott
Episode 048: The Simple Science Behind Great Gardening, with Lee Reich
Episode 058: Biochar Basics and More: Digging Deeper Into the Science of Soil
Episode 116: Understanding the Soil Food Web, with Dr. Elaine Ingham
Episode 153: The Science Behind Great Soil
Episode 194: Easy No-Dig Gardening, with Charlie Nardozzi
Episode 215: Roots Demystified: The Amazing Unseen Things Roots Do
Episode 245: The Hidden Life of Trees, with Peter Wohlleben
Episode 270: Plant Science for Gardeners, with Robert Pavlis, Part I
Episode 282: The Vital Role of Soil Bacteria in the Garden, with Jeff Lowenfels
joegardener Online Gardening Academy™: Popular courses on gardening fundamentals; managing pests, diseases & weeds; seed starting and more.
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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.
GGW Episode 207: Dispelling Garden Myths – Linda Chalker-Scott
“Sustainable Food Gardens: Myths and Solutions” by Robert Kourik
“Understanding Roots: Discover How to Make Your Garden Flourish” by Robert Kourik
“Roots Demystified: Change Your Gardening Habits to Help Roots Thrive” by Robert Kourik
Milorganite® – Our podcast episode sponsor and Brand Partner of joegardener.com
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 was 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, Territorial Seed Company, Earth’s Ally, Proven Winners ColorChoice, Farmer’s Defense, Heirloom Roses and Dramm. These companies are either Brand Partners of joegardener.com 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.
