How Plant Roots Work

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. [1]

Fibrous Roots

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) [2] 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!

Tap Root

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.

*If interesting in seeing a summary of common vegetable root depths, click here.

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.