The Hidden Half of the Garden
Research indicates a misconception about root systems being subterranean mirror images of the plants’ above-ground structures.
By Thomas Christopher
One of the most inquisitive gardeners and writers I know is Robert Kourik of Occidental, California. When he gets ahold of a topic, he’s like a terrier with a bone. Whether it’s ways to drought-proof a garden, the design of graywater and drip irrigation systems, or the best recipe for a lavender-infused martini, he worries away at the research in the library and in the field until he gets to the root of the matter. Quite literally, too, in the case in point.
Like most of us, Robert had passed without comment the illustrations in horticultural textbooks that represented root systems as essentially subterranean mirror images of the plants’ above-ground structures. That is, if an oak tree rose to a height of 60 feet, with limbs reaching out to a similar extent, the image of the roots showed them sinking and spreading to 60 feet. Except that Robert has a tendency to question the accepted wisdom, and one day this led him to wonder whether the profile of a plant’s roots was really so simple. This, in turn, led to a compulsive exploration of road-side ditches and excavations to see how roots really behaved, and many hours of research in the University of California Berkeley Library, searching for actual data on exhumed root systems. What he found, and summarized in two books, Roots DeMystified (Metamorphic Press, 2007), and Understanding Roots (Metamorphic Press, 2015), is that the conventional imagining of roots bore little relationship to the truth.
One of Robert’s greatest finds were the detailed maps that professor John Weaver of the University of Nebraska made by excavating plant root systems in the 1920’s and ‘30’s. Later, Robert supplemented this with a series of German textbooks with similar illustrations of excavated tree, shrub, weed, and vegetable roots, as well as books and papers by other authors abroad and at home.
What he learned from all of this included the fact that most trees do not form tap roots, and that a shade tree growing on a clay soil is likely to grow 90-95 percent of its roots in the top twelve inches of soil. What’s more, on a very heavy clay soil, the roots are likely to stretch five or more times as far from the trunk as the tip of the branches. Even on a better drained and aerated loam soil, that shade tree is likely to keep the vast majority of its roots within 36 inches of the surface and extend them half again as far as the branch tips.
These facts have obvious implications about watering and fertilizing such trees. For example, the rule-of-thumb that fertilizer should be applied around a tree’s dripline (the area under the tips of the branches) is obviously in error.
The pictures Robert presents about vegetable roots can also change how you care for them. I had believed, for instance, the advice I was given that asparagus plants are shallow-rooted and so should never be weeded with a hoe. Yet Robert’s German source reveals that those excavators found an asparagus’ roots to penetrate down as much as four feet into the soil.
Commonly, the visible structure of the plant gives little indication of how the roots grow. Thus, a humble beet may sink its roots four feet down into the soil. A mature horseradish’s roots may reach downward more than a dozen feet, which explains why my attempt to confine horseradish by surrounding it with a bottomless garbage barrel sunk level with the soil surface failed.
One of the most interesting chapters in Understanding Roots is the one about mycorrhizal fungi, fungi that inhabit the soil in association with plant roots. These fungi tap into the roots to feed on the plant’s sap, but in return, they greatly extend the roots reach, for one cubic inch of soil may contain 8 miles of mycorrhizal filaments. This extended reach not only helps the roots absorb nutrients and water, it can actually connect one plant with another, creating a sort of super-organism. The presence of these fungi throughout the soil also creates a persuasive argument for no-till gardening as any digging causes widespread disruption of the mycorrhizal fungi’s network.
A look at either of these books will transform your gardening. Robert Kourik has also written a number of other books, most recently Lazy-Ass Gardening, whose motto is “Maximize your soil; Minimize your toil.” That sounds good to me.
Thomas Christopher is the co-author of “Garden Revolution” (Timber Press, 2016) and is a volunteer at Berkshire Botanical Garden. berkshirebotanical.org
Be-a-Better-Gardener is a community service of Berkshire Botanical Garden, one of the nation’s oldest botanical gardens in Stockbridge, MA. Its mission to provide knowledge of gardening and the environment through 25 display gardens and a diverse range of classes informs and inspires thousands of students and visitors on horticultural topics every year. Thomas Christopher is the co-author of Garden Revolution (Timber press, 2016) and is a volunteer at Berkshire Botanical Garden. berkshirebotanical.org.