We gardeners expend lots of energy to maximize the growth and yield of our vegetables. But just what is going on underneath the soil where we can’t see? I’ve come across an exhaustive study from nearly 100 years ago that provides answers to “how deep do vegetable roots go down?”
Given the opportunity, most vegetable roots will penetrate between 2′-6′ below the soil (60 cm – 2m), by 2′-7′ wide (60cm -3m). Often the maximum rooting depth matches or exceeds the vegetable plant height at maturity. This is based on a significant study from the mid-1920’s by professors from the University of Nebraska.
Roots, Root Depth, and Relationship to water/nutrient absorption
Each species of vegetable is different in it’s lifecycle and nutrient uptake. And plants are generally opportunists in that they will take up nutrients in the easiest manner. Studies have found that application of Nitrogen fertilizers will reduce root depth, possibly because the plant does not need to ‘work as hard’ to obtain the food it requires. 
Working Level of Roots
The working level is the depth where many roots penetrate where significant water and nutrient absorption occurs. Although a large amount of water absorption and nutrient uptake typically occurs in the first 12″ of soil, plants with deeper root systems will extract nutrients at deeper levels given the opportunity. 
Working Root Depth, Maximum & Minimum Root Depth, of mature vegetable plants
The table below lists the Working Root Depth and the Maximum/Minimum Root Depth. The Working Root Depth should be considered a threshold from which a large portion of roots has penetrated. Below which there is not a large amount of root mass, and not much nutrients or water is absorbed.
The Maximum Root Depth is the maximum root depth measured by Weaver & Bruner in their 1927 Book, Root Development of Vegetable Crops. 
While the minimum Root Depth is the minimum effective soil depth necessary for crop production per the University of Nebraska Extension Office. 
The minimum root depth can be useful if one wants to determine raised bed garden height for a patio garden.
|Vegetable Plant||Working Root Depth||Maximum Root Depth||Minimum Root Depth|
|Horseradish (@10 yr)||108″||168″||18″|
|Radish, Long Scarlet||36″||60″||6″|
|Radish, White Tipped||12″||60″||6″|
|Rhubarb (@4 year)||84″||120″||36″|
How the studies were done
Trenches were dug of substantial width and depth to carefully excavate root systems of the vegetable crops. Several specimens would be excavated at several stages of growth to provide an adequate sample to provide information as to the extent of the rooting system.
Trenches were initially dug to 5′ depth (1.5 m). As plants grew and excavations progressed, the trench depths would be increased. Final trench depths of 6′-11′ (2-4 m). 
Sketches were made of the root systems during the excavations. This was carefully done, and measurements were compiled at the same time. The method allows us to get a comprehensive picture of vegetable roots systems at their various stages of growth.
Soil Conditions at the study location
Weaver’s study was conducted at Lincoln Nebraska and at Oklahoma in the 1926. It was in a silt loam soil that had clover grown for two years prior, and a crop of corn. Weaver regarded the soil as having excellent tilth and significant amounts of humus. So, from his description it sounds like ideal soil for any garden.
Soil conditions in Nebraska
The study reported that the top 12-14 inches had a very dark silt-loam. Up to about 3′ it was more clay-like, and would crack once dried from air exposure in the trench. At 3′ depth, the soil became similar to loess and was easy to work and yellow in color, and this type of soil was observed to a depth of 12′. It should be noted that earthworms and their tracks were found up to 8′ deep.
One other observation that I found interesting was that the area had been subjected to drought for a few years prior. Cracks of up to 1″ wide (2.5 cm) had formed on the surface and extended 4-5′ deep (1.3-1.6 m). These cracks had become filed with black surface soil through wind or surface movement. This is another natural method of transferring organic matter deep into the soil that I would have never imagined.
Soil Condition in Oklahoma
The Oklahoma study location consisted of a fine sandy loam with lots of tilth and humus. It had been manured for many years and utilized for growing vegetables. Significant amounts of clay were present though, but not enough for the soil to exhibit cracking.
The lack of cracking made root penetration difficult when the soil was dry in the top 2′ (60 cm). As the depth progressed, soil type varied from red sandy clay, to heavy iron soil. At 7 feet the soil was very compact.
Do the soil conditions matter?
The rooting depth presented in this article is based on the soil conditions and growing seasons that were studied. In the landmark book, Root Development of Vegetable Crops, anecdotal studies of other sites show that soil conditions can have significant impact on rooting development of plants.
From the example of Linseed in India, where certain varieties that have adapted to moist conditions fair well in moist conditions and poorly in mesic soils. And the opposite with the same species, the authors clearly demonstrate the importance on local evolution to the growth and health of a plant.
Furthermore, the comparative studies done in New York illustrated that taproots of Cauliflower and other Brassica species fared poorly in compact clay. The New York site having compacted clay soil, the roots developed less than half the total length of their Nebraska/Oklahoma counterparts. 
Are deep roots necessary to produce vegetables?
Note that most vegetable plants do not need to have roots penetrate to deep soil levels to produce fruits or develop root vegetables. Given enough fertility and desirable conditions, one can grow almost any plant in even a small container and obtain a harvest.
For instance, think of those who grow tomato plants in 5 gallon (20 L ) buckets on the balconies of apartments. The rooting depth of a tomato plant in that condition is far less than the 60″ working depth that has been observed. None the less one may enjoy tomatoes, although at a lesser yield.
Images of Root Depth of Mature Vegetable Plants
The following are a few images from Weaver’s work on the Root Depth of Vegetables .
 – Crist & Weaver, Absorption of Nutrients from Subsoil in Relation to Crop Yield, International Journal of Plant Sciences. Volume 77, Number 2Apr., 1924. https://doi.org/10.1086/333295
 – Weaver, Jean, Crist, Development and Activities of Roots of Crop Plants: A Study in Crop Ecology. Carnegie Institution of Washington, Publication No. 316. May 11, 1922.
 – Weaver and Bruner,
- Root Development of Vegetable Crops
. McGRAW-HILL BOOK COMPANY, Inc. 1927. https://soilandhealth.org/
 – Water Wise Root Development of Vegetable Crops, University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources.
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