Understanding Plant Roots - March 9, 2016
Jeff Schalau, Agent, Agriculture & Natural Resources
University of Arizona Cooperative Extension, Yavapai County
The word "root" has meanings and uses in the English language. "Root" not only refers to an underground portion of a plant, but also has meanings in the disciplines of music, mathematics, anthropology, and even dentistry. In all cases, "root" refers to a basis or origin. In plants, roots anchor the plant, absorb and conduct water and nutrients from the soil to other plant parts, and, in many cases, store energy for later use by the plant.
Germinating seeds have a primary root that grows downward into the soil. On young roots, new cells are formed at the root tip. On the very end of the new root is a thimble-shaped cluster of cells called a root cap. The root cap is actually a sacrificial structure that helps the root penetrate the soil matrix. As the root grows downward, the root cap cells are sloughed off creating a slimy surface that lubricates the root as it goes deeper. Just behind the root cap is a growing point called the apical meristem, which produces new root cells and a new root cap to replace the sloughed off cells. The root cells then elongate and push deeper into the soil.
After the root tip has passed through a region of soil, the new root begins to mature producing root hairs. Root hairs are relatively short-lived and form after the root cells have elongated. Root hairs are very important to gardeners because they are the primary surfaces that absorb water and mineral nutrients from the soil. These are often called "feeder roots". When transplanting, it is important to retain as many feeder roots as possible to ensure proper uptake of water and nutrients and allow quick recover from this disturbance.
As plants mature, roots branch and expand their zone of soil contact forming a root system. In general, plants have one of two different types of root systems. Conifers and their relatives (gymnosperms) and broadleaved plants (dicotyledons) both have a taproot that grows downward usually branching along the way. Grasses and their relatives (monocotyledons) produce fibrous root systems that radiate out into the soil from the base of the plant. Taproots are often deeper, fleshy or woody, long-lived, and have more branching. Fibrous roots tend to be smaller, shorter-lived, and branch less.
When thinking of a taproot, a carrot may come to mind. While a carrot is an example of a taproot, don't think that mature woody plants and trees have a large carrot-like structure below ground. They usually don't. Instead they have a highly branched, complex network of woody conducting tissue to anchor the plant and conduct water and nutrients. Feeder roots are found near the soil surface (3 to 5 inches deep).
Grass root systems are often under appreciated. Grasses with their fibrous root systems are excellent at holding soil in place to reduce erosion. Perennial grasses that are planted in lawns and grow naturally on rangelands also contribute organic matter to the soil. Each year, old roots die and new roots are formed. The dead roots decay, are attacked by beneficial soil organisms (bacteria, fungi, protozoa, worms, and insects) releasing nutrients and producing organic matter. This turnover of roots is what contributes to the dark, rich soils found in some grass-dominated ecosystems.
On the practical side, learn to visualize the root system that is present on each plant species you grow in your garden and landscape. If a given plant was installed, the feeder roots are likely near the dripline of the plant and where irrigation has been applied. Native grown plants often have much more extensive root systems and have found water and nutrients on their own. Perennial grasses are excellent at mitigating erosion with their fibrous root systems. If you installed drip irrigation to establish trees and shrubs, consider where the feeder roots should be in 5 or 10 years and expand the system accordingly. These are just a few ways that your garden will benefit from your better understanding of root systems. See below for additional information on plant roots.
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Taproot and fibrous root systems in comparison (from: http://bugs.bio.usyd.edu.au/
Roots in Depth
Iowa State University Forestry Extension
Plant Structures: Roots
Colorado State University Extension
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Last Updated: February 29, 2016
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