How deep do roots go? Rooting depth is dependent on a lot of different conditions. After reading this post, you may never look at your garden plants the same way again. I know I don't!
Thrive or survive?
We’ve all seen examples of tenacious, wind-battered trees growing impossibly out of rocks, but we want life for our garden plants to be better than that, don't we? We don’t want our plants to simply survive, we want them to thrive! This is where rooting depth becomes so important. Plants will make do with whatever they have available to them. By providing enough loose, healthy soil, our plants will be more productive and less likely to get sick.
How deep plant roots go depends on several variables: species, soil structure, soil health, soil moisture levels, and probably a thousand other things. Roots will go where they need to to find water and nutrients. Imagine carefully digging up specimens of your garden plants to see what their root systems really look like. Ends up, it’s already been done. Back in 1927, a couple of researchers, Weaver and Bruner, dug up a bunch of vegetable plants to examine rooting depth and structure. We can use what they learned to make sure we put our plants where they will grow best.
And that lawn, its root system is pitiful compared to native plants.
In my case, I have to assume that plants installed directly in the ground are going to have a tougher time moving around in the soil, for at least another year or two. Because of this, I try to remember to install shallow-rooted plants in those places. I generally use my 12” deep raised beds for plants with moderately deep roots and my 24” deep bed for the plants with deeper roots, though not always. Because my raised beds are open to the native soil at the bottom, rooting will get progressively deeper as the preexisting soil improves.
Strawberries are classified as shallow-rooted, but my deep bed has newly built netted panels that keep birds away, so that’s where my strawberries live. [As you read this post, you will learn that strawberries are not nearly as shallow-rooted as many, myself included, once believed.]
Listed below are categories of minimum rooting depths, under ideal conditions, for many common home gardening plants. Remember, these numbers are bare minimums, assuming nutrient-rich, loose soil. Your results may vary.
Shallow rooted plants
These plants are your best choices for containers, towers, and compacted soil. Basil, chives, cilantro, endive, escarole, ginger, lettuce, oregano, parsley, radish, scallions, spinach, summer savory, tarragon, and thyme can all be grown in less than 12” of soil. Of course, more is generally better. In one study, biologists found that “doubling plant pot size makes plants grow over 40 percent larger.” And look what happens to a lettuce plant, given its freedom to grow! [By the way, the squares in all of these illustrations are one foot by one foot.]
Slightly deeper growing plants, arugula, bok choy, celery, fennel, garlic, Jerusalem artichokes, mint, onions, rosemary, shallots, strawberries, and Swiss chard need at least 12” of soil but perform better in 18” or more soil.
Moderately deep rooted plants
Your cabbage, carrots, chiles, okra, peas, beans, beets, blueberries, broccoli, collards, Brussels sprouts, cauliflower, cucumbers, eggplant, horseradish, kale, leeks, kohlrabi, mustard greens, Napa cabbage, peppers, potatoes, rutabagas, sweet potatoes, and turnips need at least 18" of soil to grow properly.
Artichokes, cantaloupe, cardoon, cereal grains, citrus, figs, lima beans, melons, parsnips, peaches, pumpkins, sage, squash, tomatoes, and watermelon need 24" of loose, nutrient-rich soil.
Deep rooted plants
Your asparagus, cherries, fava beans, hops, olives, pears, prunes, rhubarb, and spring wheat will ultimately go down 3 feet or more. Alfalfa, almonds, apricot, and corn may have roots that are 4 feet long, while walnut trees and winter wheat may reach 5 feet. The roots under your grape vines may be 20 feet long.
Remember what I said earlier about strawberries? Well, if you are like me, strawberry pots have never seemed to work out. Believe me, I’ve tried! Experts all say strawberry plants have a minimum rooting depth of 12”. What they don’t tell you is that mature plant roots might go down 3 feet! And what about that containerized horseradish? A 10-year old horseradish plant may have roots as deep as 14 feet!
Rooting depth depends on species, soil, and several other variables. Knowing more about rooting depth can help you select plants suited to your soil, container size, or planting beds.
You can grow a surprising amount of food in your own yard. Ask me how!