A Better Way to Picture the Roots of Trees and Other Plants

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Speaking with gardener extraordinaire Robert Kourik reminded me of how little I—and most other gardeners, I think—know about roots, and how much he does know. That’s important, because anyone who has ever planted a garden realizes the essential role that the underground parts of our plants play in their overall success. Indeed, a plant cut back to the ground, perhaps by frost or by a grazing animal, will often survive this trauma, springing up anew from the roots. Destroy the roots, however, and in almost all cases your plant will die.

Yet most of us have only the sketchiest, flawed idea of what a plant’s root system looks like. The form varies from species to species. Some root systems reach deep, 100 feet or more into the soil, whereas others make shallow, spreading mats. In virtually all cases, however, it does not resemble the typical picture most of us carry in our heads, and which is commonly reproduced in gardening books, of a system that mirrors the aboveground growth of the plant or the tree.

As Robert Kourik points out in Roots Demystified and its companion volume Understanding Roots, a 60-foot-tall oak tree will not have a 60-foot-deep root system that extends only as far out as the tips of the branches. In fact, most of the average tree’s roots will grow within the top 18 inches of the soil, and if the soil is loose and sandy, they may extend outward 3 or more times as far as the branches. Even in dense clay soil, the spread of the roots is likely to be 1.5 times the spread of the tree’s canopy.

But the profile does vary with the type of plant. A humble dandelion, for example, may send its tap root more than four feet down into the soil. Horseradish is another deep-rooted plant. Knowing that it can be invasive, when I planted horseradish in my garden I encircled it with a bottomless plastic trash barrel sunk to its lip in the soil. This was not sufficient to contain the spread of the horseradish, which within a year or so had popped up outside the barrel enclosure. I shouldn’t have been surprised, as Robert reproduced in Understanding Roots a drawing of the excavated root system of a 10-year-old horseradish that had penetrated more than 14 feet down into the soil.

What to Do With This Understanding

Other than the wow factor, what is the relevance of this information to the average gardener? It should reset the rules for cultivation. For example, most gardeners will surround a newly planted tree with a circle of mulch a few feet across. This will do little to benefit the tree, especially after a few years, when the root hairs that sprout from the root tips have migrated outward. If you want to mulch a tree effectively, you must cover the earth in a doughnut pattern. That is, leave the soil surface untouched close to the trunk and lay down your mulch in a band starting near the drip line of the tree, the area under the tips of the branches. Then extend the mulch outward from there, because that is where most of the root hairs are growing.

Irrigation and fertilization should be distributed in the same doughnut-shaped pattern. The relatively shallow depths at which the bulk of the roots grow and their partnership with beneficial mycorrhizal fungi in the soil also make a strong argument for no-till gardening methods. Cultivating the soil will obviously harm such shallow-growing roots and fungi. Indeed, just stepping on the soil compresses it sufficiently to have negative consequences for both of these. This compaction detracts from nearby plants’ ability to secure from the earth the minerals and moisture they need. Confining your strolling to clearly defined paths is one of the kindest things you can do for your garden’s roots.

To learn more about the habits and needs of plant roots, I recommend reading Robert Kourik’s books. If you want to listen to Robert discuss the results of his research, you’ll find our interview on my podcast, Growing Greener.