Roots are the portion of a plant that interface with the ground. They perform three primary functions, anchoring the plant, absorbing water/nutrients/minerals, and also act as storage of energy for future use. There are generally two types of plant roots, fibrous and tap-root.
Although mostly hidden from our view, plant roots are one of the more simple, yet fascinating parts of a plant. In this post I plant to go through a detailed explanation on plant roots and how they work – ie, what type of roots exist, what functions to the perform, how do they grow, and some other interesting facts.
What types of roots exist
There are two main types of root systems, fibrous (diffuse) and tap roots. And typically most plants have one or the other. Although there are some plants that contain both. 
Fibrous or diffuse roots are thin, somewhat stringy roots that spread around the plant, occupying a large volume of shallow soil. Fibrous roots absorb water as it infiltrates the soil, generally catching it in shallow depths. Common turf or lawn grass is an example of fibrous roots.
However, fibrous roots can go deeper depending on the species and water demands. For an example, one needs only to look to Weaver & Brunner (1927)  to see the quick progression of fibrous tomato roots over one month, growing from 3 to 7′ wide, and from 2 to four feet deep!
A tap root system is a thicker, sparsely branched root that grows straight down into deeper soil where it can access deep water tables and minerals. In sandy or shifting soils, tap roots do a great job anchoring the plant.
There are some plant species that will have both fibrous and taproots. And even some plants will change their root style as they grow. For example some trees begin producing taproots such as Red & White Oak, only to change and switch to a more shallow root depth as the tree matures.
What do plant roots do?
All plant roots have three primary functions:
- Anchor the plant
- Absorb water and minerals
- Store excess food
No matter what species of plant, the root is the part of it that will exchange water, minerals, and nutrients to support the upper portions of the plant. And storing food underground in the roots is a smart evolutionary method, as it is relatively hidden from most animals.
Anchoring the plant
Roots occupy large volumes of soil either a fibrous root system or tap-root. But, roots bond with the soil and grip, anchoring the plant from tipping over, washing or blowing away. Fibrous roots will spread out far and wide from the base of the plant, gripping hold of all the local shallow soil while helping to prevent erosion. Tap roots send on or more sparsely branched roots straight down to deeper ground, gripping the deep ground.
Absorb water and minerals
Roots support plants by absorbing water, nutrients, and minerals from the soil. As rainwater begins to percolate into the ground, fibrous roots will absorb it when needed. Where as tap roots access deeper water tables.
Store excess food for future needs
Both fibrous and tap roots are used to store food for future use. In perennials, this stored food & energy is what the plant draws on in Spring to reemerge until it can regrow leaves and restart photosynthesis.
Common root depths
Given proper soil texture and structure, even roots of annual crops can grow to incredible depths. Weaver & Bruner observed in their 1927 study that annual lettuce crops could reach depths of up to 7′ deep in a single growing season. This illustrates that even smaller crops and plants may have extensive, albeit hidden growth underground.
Surprisingly, many tree roots only occupy the top 3′ of soil. In fact if you hike in an older oak forest you may come across a tree that has tipped over. If you look at the root ball, you will clearly see that there is no taproot, only shallow woody roots. This is in contrast to most conifers (evergreens), which are firmly anchored with a tap root. Most fruit and nut trees will produce the majority of their roots in a circle, extending to the ‘drip zone’ of the canopy, which is the area where most of the rain is directed via the canopy.
How Roots Grow
Since the primary purpose of roots is to find and absorb water and minerals, that is the direction they will grow. Probing in between soil aggregates and pushing through in a never-ending quest to supply the plant with adequate moisture and nutrients.
The amount of roots that will be present at various depths and sizes will vary with the species of plant being studied, as well as the soil type, structure, and available water. Watering infrequently, but for longer durations will encourage roots to grow deeper. Grass roots (and most other plants) will grow deeper roots if they have a need to access water that is deeper in the earth. This should be done with lawns and most plants, as if they are watered frequently they may not have a reason to grow them deeper to access water tables.
This interesting root system is actually growing down into an abandoned mine. It is likely that the root continually was accessing water and minerals, and kept growing deeper.
Root Hairs and Branches
Fuzzy root hairs are what a root uses to absorb water, starting approximately 1/4″ (6 mm) from the tip of the root and going back towards the plant. These will occur radially around the root, giving it an almost ‘fur-like’ coat. These hairs are easily broken when a plant is dug up from the soil.
Roots will branch as they age and grow in an attempt to locate more water an nutrients in a somewhat exploratory manner. They originate from the parent root and grow at approximate right angles to it. And will continue to grow and branch in areas where they have success. So, once large enough, branches will begin to produce branches.
Are stolons, rhizomes, or suckers roots?
Although they resemble roots, and look like them, and even sprout roots….neither stolons, rhizomes, or suckers are actually roots.
- Stolons (which are also referred to as runners) are actually horizontal stems that grow above ground. Emerging from the plant crown, they grow horizontally out, eventually bending down to ground. When they touch ground they will sprout roots and stems, making a new plant. And the runner will continue onwards. See Buffalo Grass or Wild Strawberry for good examples of a plant that produces stolons. 
- Rhizomes are underground runners, aka underground horizontal stems that will also periodically sprout roots and stems. For examples of plants that produce runners, see Canadian Goldenrod, Black Locust Tree, and Red Bee Balm.
- Suckers are upright shoots that come from a horizontal roots. This is common in Blackberry and raspberries.
 – Capon, Brian. Botany for gardeners. Timber Press, 2010. pp.279
 – Weaver and Bruner, Root Development of Vegetable Crops. McGRAW-HILL BOOK COMPANY, Inc. 1927.
 – Ellis, Barbara W. Complete gardener’s dictionary. Hauppauge, N.Y. : Barron’s Educational Series, 2000, pp387.
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