Chapter 14 How Roots Work

The challenges of root pruning and management are not unique to bonsai growers. Fruit tree and nursery growers deal with them routinely. By looking at standard nursery practices we can better understand what is going on below the soil.

This guide is a mash-up of four main sources (listed below) mixed with side notes I picked up from various other references, and my own observations.

14.1 Roots Don’t Take Winter Off

Most bonsai guidebooks say “report when fine white root hairs first appear.” This is not just a random rule of thumb; it is referring to a specific stage in the life of a tree’s root system.

According to Southwest Michigan Extension Educator Mark Longstroth, there is a predictable pattern and timing to root growth. Roots compete with other parts of a plant for resources, so roots grow most rapidly when top shoots are not growing. Above ground structures often have a minimum dormancy requirement; without it they start to fail. Roots remain active long after the above-ground portion of a tree has gone dormant, and grow at any time of the year so long as the conditions are favorable and soil temperature is high enough.

In fall, sugars made in leaves are transferred to the shoots, trunk and roots for storage and use during the next growing season. Part of what is stored in the roots is spent on root growth in the late summer through fall. Root growth begins to taper off as the soil cools in the fall, and as the soil becomes frozen in the winter, root growth ceases. We have a shallow frost zone here so roots of trees in the ground are capable of growing nearly all winter long. Roots on trees in pots are less active because they are not insulated by the soil, but they still grow.

Root growth restarts in the spring as the soil becomes warm enough to support faster growth. Rapid root growth continues through bud break then begins to slow as the shoots begin their growth. During the period of rapid shoot growth in the early spring new root growth virtually stops and all of the trees’ reserves are used in above ground growth (explaining why it is unwise to root prune actively growing trees). When shoot growth ceases in early summer, root growth begins again. The rate of root growth peaks again in early fall and then declines as the soil cools.

14.2 How Do Roots Grow?

Root cells grow by actively transporting in potassium ions (part of why plants need potassium) and sugars. The name of the game is to be saltier and sweeter inside the root than out in the soil, which pulls water into the root cells. As the cells absorb water they lengthen (like water balloons being filled with water) and force the root tip deeper into the soil.

Woody plants have two main kinds of roots: smaller, non-woody white roots and larger, darker woody roots. The tips of white roots are the only part of the root mass that actually lengthens and penetrates into the soil. Each tip has a thimble-shaped cap of cells that make a slimly film to protect and lubricates the root tip as it forces its way through the soil. Behind the root cap is the zone of active cell division where new cells form. Immediately behind that is the zone of cell elongation. This is where the new cells enlarge to their final size. This also is where most water and nutrients are absorbed.

A stained root tip. The looser, more open cells are the root cap. Link to image source.

Schematic of a root tip. The entire structure is about the size of a human hair. Link to image source.

Root hair cells projecting out from a white root. Link to image source.

White roots are fairly short-lived. As they lengthen the outer layers of older parts produce a thick layer of lignin. Nine times out of ten lignified brown roots die completely and decompose. Occasionally though a brown root starts to grow in diameter. These become the larger woody root system. They contain vascular tissue that transports nutrients and water absorbed by the younger and smaller white roots. Woody roots essentially are the underground trunk of a tree.

Lignifying roots are more likely to persist when they have strongly growing white root tips that are bringing in water and nutrients. If a root has access to nutrients and water, then it is beneficial to the tree to spend resources expanding that root. Conversely when there are no nutrients or water available, that particular root is not needed and is allowed to die back.

14.3 What Determines Where Roots Grow?

The old adage that roots grow towards water is wrong; roots grow where the water IS. Roots in dry soil show very little growth. Roots grow where the conditions for growth are favorable NOW.

In spring when the soil is moist and the surface is warm, roots near the surface show the most growth. As the soil dries out, roots near the surface die and new roots grow deeper in the soil. As the season progresses, the depth of active root growth moves progressively deeper and deeper in the soil. With each rain or irrigation new roots appear near the surface. These new roots absorb water and nutrients and then die back.

Longstroth describes root growth this way:

“…[T]hinking of white roots as leaves helps me to put the process of root growth in a visual framework that I can understand. The white roots really are like leaves. They are temporary structures that absorb water and nutrients. Leaves are temporary structures that absorb carbon dioxide and make sugar providing the energy for plant growth and fruiting. When their job is done, both roots and leaves are discarded by the plant.

“As the growth of our imaginary (upside down) root tree begins in the Spring, most of the roots sprout out close to the ground. As the season progresses, more roots sprout deeper and deeper in the soil. Instead of hanging on to the trunk all season, most of the roots fall off after two weeks. Rapidly the white rooted region of the tree moves downward in the soil, and every time it rains there is a new flush of growth near the surface of the soil from the older root system. Towards the end of the year, fewer new roots grow deep in the soil. Most of the new growth occurs close to the soil surface where the ground is warm, moist and has the most nutrients.”

14.4 How Do Roots Respond to Barriers?

Behind the zone of elongation is the zone of maturation where different types of cells develop. Some form long hollow cells that conduct water out of the roots and up the tree, and carry nutrients back for storage. Other cells divide and form branch roots that push out laterally. As long as root tips are moving down into the soil they inhibit branch roots from forming. When the root tip encounters a solid barrier it bends to move around it. If the root tip does not pass the barrier, the tip dies and new branches emerge from the zone of maturation.

We use this physiological behavior to our advantage when we root prune trees. Clipping off the outer layer of the roots stimulates new branch roots to form in the zone of maturation, which expands the total number of small roots available to absorb water and nutrients. Repeated root pruning over time pushes the woody root mass back to the base of the tree and creates a root mass that is mostly fine fibrous white roots.

14.5 How Do Roots Grow From Stems?

Most roots are branches or offshoots of the original root that emerges when a seed germinates. Adventitious roots develop directly from shoot tissues, not from another (parent) root. Most adventitious roots begin from cell division in the stem cambium. Growers propagate many trees by taking cuttings then treating the cuttings with plant hormones so they form adventitious roots.

Species differ in how easily they form adventitious roots. Trident maple is a species that develops adventitious roots easily. Other tree species do not root as readily. Junipers root slowly from cuttings, while Japanese maples (dwarf cultivars especially) rarely grow roots from stem cuttings.