ROOTS. They’re the engine of the plant, but remain mostly unseen—unless something’s being added to the garden or dug up, when we might catch a quick peek. Gardeners make some assumptions about that unseen system, imagining that it’s a particular width and depth, and watering and applying mulches accordingly to a particular “zone” we think they inhabit. Though we’ve probably never seen living evidence, we refer to some plants as “shallow rooted” or “tap-rooted,” as if we had.
How accurate are we?
To say Robert Kourik is a student of roots would be an understatement, as they have fascinated him for decades. Robert is the author of 15 books–including classic ones on edible landscaping and drip irrigation.
His latest is “Understanding Roots,” which builds on an earlier volume with more looks at what’s going on down there, and more answers to what gardeners should be doing to help. Robert lives and landscapes in Northern California, in Wine Country, where roots and every other part of the plants have been getting quite the test from prolonged, extreme drought.
I asked Robert to take us below the surface on my public-radio show and podcast. (Listen in using the player below, or at this link, or read along—or both. It’s the August 31, 2015 show.) Did you know that a tree’s roots can extend to five times the canopy’s width, or that a carrot’s may go down 4 (or even 7) feet? Or where to water to keep mature trees growing adjacent to once-irrigated lawns alive? (Hint: The sweet spot is located nowhere even remotely near the trunk.)
Read along as you listen to the Aug. 31, 2015 edition of my public-radio show and podcast using the player below. You can subscribe to all future editions on iTunes or Stitcher (and browse my archive of podcasts here).
my q&a on roots, with robert kourik
Q. I mentioned in the introduction that you’ve been looking at roots for a long time. How did you get started?
A. About 1978, I was cruising around in the stack at the U.C.-Berkeley Ag library—they let you do that then; you could actually just look at books. I would pull out all these books from different sections, and lo and behold, I found this book from 1930, from Dr. John Weaver, and inside were these amazing root excavations with drawings—including a lot of vegetables. That’s how I got started.
Q. So you saw these underground cross-sections—and we should say that in your new book, “Understanding Roots,” there are a lot of these. They’re kind of like an ant-farm view; as if you’re looking through the glass at a cross-section underground.
A. Weaver dug a trench, and followed the root system down as far as it went. He got so detailed that he was using a brush to get at the finer roots. And then he drew it all. It was a phenomenal amount of work; how he did it, I have no idea. [Above, Weaver in trench from University of Nebraska-Lincoln archives.]
Q. Talk about dedication; a life’s work. So in late 1970s, you discovered those and it fascinated you. A lot of the other subjects that you’ve written about, such as drip irrigation, water-wise thinking about gardening, and even edible landscaping and other sustainable practices—the roots are where it all begins.
A. You got it. And there are a lot of problems with what people think about roots. The first I think is the most important: Roots are not a mere reflection of the foliage. A lot of people think they look the same underground; not true at all. In fact most tree roots grow very far sideways, mostly in the top 12-18 inches.
Most people think they grow as wide as the foliage, but in a heavy clay soil it can be as five times the width of the canopy. In a regular clay, maybe half again; in a loamy, sandy soil, maybe three times as wide as the foliage.
Q. So it’s not just that “this is a such-and-such tree, and it’s roots are going to be this way,” like it’s a template, the way its leaves will be recognizable as being of that species. The roots will adapt to what they encounter on their way down or sideways, and adjust.
A. Yes, they adjust. If you have a hardpan, say, down less than a foot, that will force the roots to go sideways, and that could be up to five times greater. That’s because they are trying to get enough volume of soil to absorb the nutrients and water they need to grow.
Q. And that speaks to what is the purpose of a root system, anyway. You just mentioned two things—to get water and nutrients. Do they also have a stabilization or structural role; a physical role?
A. Fruit trees in particular have what are called sinker roots. They are vertical roots that come down from different places along that wide horizontal root. That’s what I think of as a tripod. It helps hold the plant down and protect it from wind.
But not tap roots—a lot of people think every tree has a taproot. Only about 2 percent of all trees have taproots. The exceptions are primarily oaks, pines, and nut trees grown from seed. I say grown from seed, because if you harvest the plant as a bare-root plant, or transplant it from a container, you destroy the taproot and it doesn’t regrow. [Below, root system of a red oak that was root pruned, and has no tap root. Above, a red oak that grew naturally.]
A. A fruit tree like a persimmon actually has a taproot, but again when I get them from a nursery in bare-root season, the taproot is totally damaged.
Q. This other arrangement, the tripod, is interesting visually; I kind of get it, the idea of the sinker roots. It sounds like a good structure to have if I were trying to keep myself upright.
A. Just the width alone helps stabilize the tree. Some of the drawings in my book show tree on slopes—the first time I think most people have ever seen a root system on a slope. Oftentimes it’s the downhill side that’s farther away from the trunk than the uphill side. But just to be on the safe side, you should put your mulch, your compost, your water beyond the drip line, where there are more root hairs ready to absorb the nutrients.
Q. Let’s go back for a second to the range of root systems, and how appearances above-ground can be deceiving. What are some of the extremes in other plants. For instance, with comfrey, people think, “Oh, it has such a big root system, I can’t get rid of it.” But is that always the case: that something tenacious has a big root system, or is it just a successful root system where little pieces that break off can grow?
A. I think it’s more likely successful. There is a real problem in the garden trade where everyone says, “Oh, the roots of comfrey go 10 feet deep, and that’s why they can grab nutrients that no other plant can grab.”
Over and over again, when you study where the nutrients come from, it’s still in the top 1 to 2 feet—where 50 percent or more of the moisture and nutrients are in the top 12 to 18 inches, regardless of how deep the root system is.
The deep root system can provide backup nutrients and backup water during a drought, as an example.
A. When you pull up a carrot, you’re leaving a massive amount of root system back in the soil to compost, as it were. Dr. Weaver dug carrot roots down to 7 feet, in a very loamy soil. I have in the book drawings from Germany and they only have carrots going down 4 feet, but nonetheless it’s deeper than a lot of people figure.
Q. Seven feet of carrot roots! [Above, from Weaver’s work on vegetables, a mature carrot root system.]
A. And they have a taproot, unlike things like cauliflower or broccoli have a big mass of roots but no discernible taproot.
Q. Dr. Weaver was from Nebraska, and some of the first illustrations of his I had ever seen were of these cross-sections—again, like the ant-farm view—of prairie plants’ roots. In different habitats, like a prairie versus a woodland, do roots develop differently? Are prairie plants tending to be rooted in a particular manner compared to woodland plants—are there any of those kinds of assumptions?
A. When Weaver looked at prairies plants in Nebraska, a lot of them did have their roots below the top foot or so, because they were trying to struggle through dry periods. In California, we have a tree called Douglas fir, and it’s native to the area where I live. I have drawings in the book where the root system of a 46-foot-tall tree was 67 feet wide. It didn’t have a taproot; it had tremendous width. That seemed to be the case wherever there was a reasonable amount of rain.
Sometimes, in droughts, the lower sections of the root systems are the ones that are going to grab the moisture, but over and over again in the drawings I find, there is primarily a lateral root system.
Q. Again, looking at his images and others that you have unearthed (haha; I had to get a bad joke in, Robert)…
A. …It was a tree-mendous joke.
Q. I can see we’re going downhill fast now. [Laughter.]
Looking at a lot of those drawings elicits a lot of “aha” and “wow” reactions, but then practically speaking, what did they tell us to really take notice of and do differently? That’s what a lot of the new book has in it. For instance: Where should I be mulching what kinds of plants? Where should I be applying water?
A. Most people put their straw mulch, let’s say, around the trunk of a tree—let’s say out 2 or 3 feet. In reality, you should start at the drip line, if you want to save mulch, and go farther out 2, 4, 6 feet on a mature tree.
I don’t know if you have volcano mulching in your area.
Q. [Laughter.] My goodness, yes, especially at shipping centers and gas-station median strips. Shocking.
A. We don’t have it here yet. It’s a mountain of mulch, but only 2 or 4 feet wide. It’s not doing any good for the root system; I think it’s just some crazy way to protect the tree from a lawn mower. You certainly don’t need a mound like that, which can kill a tree off.
Q. You’re saying that the more important portion of a tree to mulch is farther out?
A. The root hairs are at the ends of the branching root system. There are a lot more root hairs exploring the volume of soil out in the area half-again to three times wider than the foliage.
You may not be able to come up with enough mulch to do that much width, but you should try to concentrate at least at the drip line, and as far out as you can go. When I do a drip-irrigation system to protect a tree from the drought, we have a circle of the drip system that goes out 60 feet away from the drip line. Those old trees were getting water from a lawn; the lawn is gone. But the water has to stay there to keep the tree alive.
Q. This is a tree that’s maybe how big?
A. In the order of 60 feet wide and 40 or 50 feet tall.
Q. And you’re watering it 60 feet out…
A. Where the lawn used to be. One of the problems with the drought is that people are taking away their lawns but not remembering to water the trees that are there.
Q. I see. Those trees acclimated to being neighbors to a lawn that had irrigation. They made a root system to access that water. Now that water is no longer being applied, but the tree roots are still out there. So we have to strategically figure out—based on the soil type, the stressors, the changes, the environment we’re in—approximately where the roots are.
Q. I have some really big, old apples at my place—like maybe 125 years old. (And no, I didn’t plant them myself, Robert.) [Laughter.] I’ve noticed with them and other older trees that roots come up to the surface, too, unlike with my herbaceous plants.
A. Roots come up to the surface, yes. I think that’s because we took the mulch away; that zone that the roots would like to be in, a zone that’s very aerobic. Basically all roots are trying to fight it out in the aerobic zone, both vegetables and trees.
But also I have a theory that plants that grew up in riparian habitats tend to have roots closer to the surface. So in our area, when people plant sycamores—London plane trees—in the lawn, or alders, we’re constantly seeing the roots appear at the surface.
Q. We have to figure out, and probably overestimate, where to place the mulch around trees, and apply water similarly. With herbaceous ornamental plants, how do you think about it?
A. I don’t have any root maps of ornamental herbaceous plants, but so far it appears to be so common in other types of plants, I’m guessing it’s a similar situation.
Q. So better to overdo it a little than underdo it.
A. The main thing is that roots really want to be in the aerobic zone, where they can get enough air to get down in the pore space, and spread into the soil. The roots want to be up there to get it first. They’re competing in the aerobic zone. I think that’s why herbaceous plants will tend to have the root system near the surface as well.
Q. In one section of the book I had a good laugh when you wrote about methods of transplanting and referred to it as the H-O-L-E truth. What’s your general philosophy about the old thought of making a $20 hole for a $5 plant, your transplanting wisdom?
A. My main approach: I plant on a mound, for a bunch of reasons. Particularly for the Mediterranean plants, and the natives here, because they don’t want to get moisture up against their trunks or you get Phytophthora, or crown rot.
With any planting, it can’t hurt to plant on a mound. The idea is to force the roots away from the trunk—in other words, if the mound starts to try out a little bit, the roots are going to explore for more moisture farther away from the trunk. It’s a way to push the roots farther into native soil.
I do make the mound out of native soil, and I use no amendments whatsoever. There’s lots of research as long ago as 1978, and current research as well, that shows that in most cases adding amendments to the planting mound or hole keeps the roots kind of localized, and they’re less like to grow into the native soil.
Q. I don’t think it can be stressed enough what you just said about the mound, or at any rate: not planting low, especially with woody plants. It’s like they suffocate if you do. And especially a large woody plant is going to sink, too, after planting.
A. You have to mound it up a little just for that. I pretty much plan on about 50 percent settling, in a loamy soil.
Q. Are you a no-till person? Where are you on that?
A. For other than vegetables I’m a no-till person. Again, when you realize that so many roots are in the top 6 or even 3 inches, any type of tillage severs those root hairs. It’s much better to work from the top down to improve the soil with mulch or compost or moisture than to cultivate.
It’s much harder with vegetables, but it certainly can be done. You have to do two things: 1, build up your soil to a nice tilth first, before you try no-till, and then usually plant farther apart.
People read Ruth Stout’s book…
Q. Oh; my favorite.
A. She’s the grand lady of mulch. If you read her books carefully, she says she rototilled and manured her asparagus bed for 14 years before she started to do no-till. You’ve got to get your plants’ soils up to a little bit of snuff to get the plants going with a good root system. I don’t think you should start out with any garden with no-till for the vegetables.
There is too much of a church-like attitude in gardening, where you say, “I’m no-till across the board,” and then they try vegetables and they start dying. If people want to double-dig, double-dig; or single-dig.
I wouldn’t recommend the rototiller, though, under any circumstances.
Q. It just churns the soil and destroys the tilth?
A. It kills off the pore space, so to speak.
prefer the podcast version of the show?
MY WEEKLY public-radio show, rated a “top-5 garden podcast” by “The Guardian” newspaper in the UK, began its seventh year in March 2016. In 2016, the show won three silver medals for excellence from the Garden Writers Association. It’s produced at Robin Hood Radio, the smallest NPR station in the nation. Listen locally in the Hudson Valley (NY)-Berkshires (MA)-Litchfield Hills (CT) Mondays at 8:30 AM Eastern, rerun at 8:30 Saturdays. Or play the Aug. 31, 2015 show using the player near the top of this transcript. You can subscribe to all future editions on iTunes or Stitcher (and browse my archive of podcasts here).
[Top-of-page photo: A 30-year-old apricot tree from the Hungarian researcher Janos Tamasi, with roots going down 7 feet in sandy soil but extending 39 feet beyond the dripline. Tamasi estimated that 82.5 percent of the roots were in the top 8 to 24 inches.]
enter to win ‘understanding roots’
I’LL BUY TWO lucky readers a copy of “Understanding Roots,” Robert Kourik’s newest book. All you have to do to enter is comment all the way at the bottom of this page, in the box below the last reader comment, answering this question:
Did something in this interview about roots surprise you, or is there some root “aha” or “wow” you’ve had on your own that you want to share?
I didn’t know how few trees had taproots, or that (of course!) taproots are destroyed by bare-rooting or transplanting.
Feeling shy, or no answer? Just say something like, “Count me in” and I will, but an answer’s even better. I’ll select winners at random after entries close at midnight Sunday, September 6, 2015. Good luck to all.
(Disclosure: Purchases from Amazon affililiate links yield a small commission.)