Most people know that roots are how plants take up nutrients and water, but there are so many other amazing, unseen things that roots do that are exciting to learn about. To share his enthusiasm for roots and the other organisms beneath the soil’s surface, my guest this week is landscape designer and author Robert Kourik to demystify roots and the amazing unseen things roots do.
Robert is a self-taught garden designer and a pioneer of edible landscaping who lives in a San Fransisco Bay Area. He began his career in natural landscape design and maintenance in 1974 with one of the first sustainability-oriented organic gardening businesses in the country. 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.” And coming out in October is his latest book, “Sustainable Food Gardens: Myths and Solutions.”
How Robert Kourik Got to Know Roots
Robert’s interest in roots traces back to 1978 and the University of California, Berkeley Agricultural Library. He recalls pulling a book off the shelves that had a drawing of roots on every page. It 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. He would excavate a trench 6-12 feet long around a plant then dig 5-7 feet deep and 2½ feet wide to reveal the plant’s root system.
Weaver shed light on just how large root systems can be. 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.
“Over and over again, I tell people what you see above ground has nothing to do with what’s going on below ground,” Robert says. “There are very few tree roots that are 18 feet deep.”
Roots Go Beyond the Drip Line
As gardeners, we often hear that the drip line is the best place to fertilize a plant because that’s where the roots end — but they can go much much, much farther out.
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.
Soil Type Effects Root Growth
In clay soil, the roots may be half-again wider than the foliage above ground, Robert says. In sandy soil, they typically grow three times wider but can be up to five, six, or even seven times wider than the foliage.
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. In fact, in clay, 90% or more of the roots will be found in the top 12 inches of soil, including as much as 10-30% in the top 4 inches.
The top 4 inches are the most critical because that’s where soil is its 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 (microorganisms, animals and plants that live in soil) to convert minerals into soluble nutrients that can then be taken up by roots.
The top 4 inches have twice as much biological activity as the next 4 inches down, Robert says.
Robert recommends digging a trench to see how deep or shallow the topsoil is and where the clay below begins. Tree roots don’t like to grow in clay, so when they do hit clay they tend to start growing sideways.
Absorbing Roots vs. Non-absorbing Roots
As a root system of a tree grows farther out, the oldest roots, closer to the trunk, grow bark. Those 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.
Deep Roots Doesn’t Mean Better Roots
Oak trees, pine trees, nut trees and persimmons are examples of trees that have taproots. However, fewer than 5% of trees have a taproot. So how do trees stay standing?
Robert explains that most trees have a root system that is wide in diameter like a tripod: That width stabilizes a tree more than a taproot can. He also points out that 50% of a tree’s roots are found in the top 18 inches of soil.
Even for the trees that have them, taproots 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.
Sinker Roots and Root Grafting for More Stability
Fruit trees gain extra stability with what are known as “sinker roots.” Along the length of the lateral roots, these sinker roots grow vertically.
Other trees, like redwoods, use root grafting for more stability. In redwood groves, the roots of one tree grow into the roots of another. This means a redwood standing by itself is much more prone to blowing over than a redwood in a grove, Robert says.
Root grafting will naturally occur between trees that are in the same genus, Robert explains. It happens when two roots are growing with a gap between them, and a root from another tree grows through that gap. As the roots grow bigger in diameter, they form a graft.
Though the roots joined together over a long period of time, researchers doubt that they transfer any nutrients from one tree to the other this way. Instead, it’s more likely that the mycorrhizal networks in soil move nutrients.
The Surprising Depths of Vegetable Roots
Robert recalls being blown away in 1978 when he learned in John Weaver’s book that there are vegetables with roots that grow 4, 5, or 6 feet deep. Still, the roots are concentrated in the first foot, even in vegetables that grow much deeper. That fact is why Robert recommends raised bed gardens be at least 1 foot deep, to allow for 60% of the potential feeding root system. Even better is 2 feet deep, for up to 90%. Those roots that go the deepest — as far as 8 feet deep for kohlrabi — are not the critical roots.
How Nitrogen-Fixing Bacteria Work with 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, inoculant is needed.
That 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 — called “shedding the nodules” — 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. But this should be done before the plants have gone to seed so the nodules are still packed with nitrogen.
One way to break up compacted soil is by planting a deep-rooted crop. Daikon radishes are one crop used for this practice, which is known as “biodrilling.”
The radishes are allowed to die and rot in place to loosen up the soil. In cold climates, the radishes will die when they freeze, but in warmer areas, a gardener may have to cut the tops of the radishes to kill them.
Hydraulic redistribution, formerly known as hydraulic lift, is what it’s called when deep roots grab moisture from the deeper zone and bank it up near the surface during the night. During the day, when it’s hot and dry, the plant can take up that water and put it into its foliage, where it transpires.
Chaparral plants — drought-tolerant plants in shrublands — are known to use hydraulic redistribution. One of the most fascinating things about it is that the water brought closer to the surface is not just used by the chaparral plants themselves, but also by neighboring plants.
Why the Rhizosphere Is Critical
The beneficial critters in soil — bacteria, blue-green algae, fungi — prefer to live in close proximity to roots. This area is known as the rhizosphere, and it’s where roots can influence the microorganisms in soil.
The microorganisms give the roots nutrition by dissolving minerals into plant-available nutrients, and the roots, in turn, exude carbohydrates, proteins and fats to feed the critters. 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.
Phytoremediation removes or neutralizes contaminants in soil. Robert says another word for it is hyperaccumulation, as the idea is that some plants’ roots grab large amounts of certain minerals and chemicals in soil. Some plants are even effective at absorbing radioactivity through their roots.
So for example, in an area near a mining operation where there is zinc toxicity, plants that readily take up zinc will be grown. When the top of the plant is cut off and hauled away, zinc has been removed from the site too. It’s much cheaper than hauling the contaminated dirt away to a toxic dump.
Robert says there is a misconception among gardeners that they can detoxify their soil by growing plants. But in reality, the right plant must be chosen, it must be grown for years, and the top growth must be disposed of. The toxic material concentrates in the top growth, so if that top growth is composted, the toxicity will still be present.
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. Robert’s book uses fava beans as an example, and researchers continue to learn of others.
How Street Trees Find Water
How do street trees, surrounded by few permeable surfaces, stay watered? Roberts says that in addition to the water that does land on the limited permeable surface, there is water that moves horizontally through soil by capillary action.
After heavy rain, pavement acts as a mulch for the soaked soil beneath it. That soil acts as a reservoir that the street trees can tap into for water later.
Limited access to water is adequate 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 get by with a limited volume of soil, as it charted in his book.
How Trees Hang On to Rocks
Have you ever been hiking and noticed conifers hanging precariously out of craggy rock surfaces? I know I have marveled at this sight.
Robert says there are many more fissures in rocks than the naked eye sees. The microscopic root hairs can get into even the smallest cracks. The exposed roots will eventually grow bark, but the absorbing roots will keep going deeper into fissures.
Considerations When Mulching Trees
To suppress weeds and protect tree roots, Robert recommends 6-8 inches of arborist’s wood chips as mulch. If you don’t have enough wood chips to lay them down that deeply, he says a workaround is to use cardboard in conjunction with just a couple of inches of wood chips or compost. However, until the cardboard rots, it will slow the exchange of gases between the air and the ground, which will negatively affect root growth.
I hope you enjoyed my conversation with Robert Kourik. 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.
What surprising 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.
joegardener Online Gardening Academy™: Popular courses on gardening fundamentals; managing pests, diseases & weeds; seed starting and 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 Beginning Gardener Fundamentals: Essential principles to know to create a thriving garden.
joegardener Online Gardening Academy Growing Epic Tomatoes: Tomato expert Craig LeHoullier joins me in leading this course on how to grow healthier, productive tomato plants and how to overcome tomato-growing challenges.
“Sustainable Food Gardens: Myths and Solutions” by Robert Kourik
“Understanding Roots: Discover How to Make Your Garden Flourish” by Robert Kourik
“Understanding Roots: Discover How to Make Your Garden Flourish” — Autographed
“Roots Demystified: Change Your Gardening Habits to Help Roots Thrive” by Robert Kourik
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, Exmark, Greenhouse Megastore, High Mowing Organic Seeds, Territorial Seed Company, Wild Alaskan Seafood Box and TerraThrive. 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.