Roots
By Bob Chapman
The roots of plants, shrubs and trees are seldom seen by gardeners. Occasionally the roots of shrubs and, more often, trees will show at the surface of the soil, but these are but a small part of the extensive underground parts of a plant. The role that roots play in the ongoing struggle for survival is little understood by most. Gardeners will benefit from knowing a little about the roots of plants, how they grow and what their functions are.
It starts as a seed.
When we plant a seed in the soil we witness one of the miracles of life. The seed absorbs water, germinates, its casing opens and it sends out a small root. The root grows downward due to the force of gravity and effects of light and temperature. The new seedling is fed by the reservoir of food within the seed. This supply of food keeps the young plant growing until the leaves are formed. Then all the nutrients a plant needs (about 17 different ones!) plus oxygen and water must be available at all times.
Roots serve as anchors.
Roots serve as an anchor, holding the plant in the ground. They also carry the water and mineral nutrients they have extracted from the soil to the leaves. These mineral nutrients are converted by the leaves into food (primarily sugars) and the food is transported back to the roots.
How roots grow.
The root is that part of a plant which grows downward into the soil. While its primary purpose is absorbing water and mineral nutrients, it may also serve as a food storage organ (for example, onions, gladiolus and iris), a reproductive organ or some other function. Roots take many physical forms. They vary in extension from a few inches to hundreds of feet on a single plant. The root differs from the portion of the plant above the soil primarily in structure. Roots do not normally bear leaves or buds and are not divided into node or internodes.
Root tips
The tip of each root contains a growing point that divides repeatedly. It produces a region of cells that elongate and push the root farther into the soil. The outer layer of cells sloughs off as the new cells are formed. These old outer cells can serve as a lubricant, easing the way of the root tip through the soil particles.
The root tip also produces downy root hairs in a special zone of the root. These root hairs absorb most of the water and dissolved mineral nutrients used by the plant. Root hairs are not much over 1/100 of a millimeter in diameter and a few millimeters in length. Each root hair is in outward prolongation (or extension) of a portion of an epidermal (outer) cell of the root. The outer, delicate walls of the root hair consist partly of pectic materials, which are gelatinous. These gelatinous materials enable the root hair to cling to soil particles and to absorb water and mineral nutrients in solution. It is common to find 200-300 root hairs per square millimeter of epidermis in the root hair zone!
Root hairs absorb mineral nutrients.
In a complicated chemical process called ion exchange, root hairs attract and absorb the nutrients into the cells. Once inside the cells the nutrients move into the vascular system and are drawn up into the leaves. Once there, they are converted to foods that the plant uses to grow.
Old age
As the root ages it turns yellow, then brown and the root hairs disintegrate. By the time the root turns brown, it no longer absorbs very much water. It has formed a protective outer layer resembling bark.
Old wives tales
There are many things said about roots, their functions and where you’ll find them. Here are some of the “old wives tales” and some facts.
1) Drip line. The drip line (the perimeter of the shrub or trees canopy) is as far as the roots spread. We used to believe that to properly take care of shrub or tree we needed only to water and feed that section of the soil beneath the drip line. We now realize that the roots extend far beyond the drip line. One tree had roots that extended 800 feet from the trunk! The root hairs were extracting moisture and nutrients a considerable distance away from the drip line. To keep the shrub or tree growing optimally, we need to fertilize and water the entire area wherein the roots are found.
2) Surface roots. Who hasn’t seen roots at the surface? These roots were always there, they just got larger and thus more visible. Some trees, like the Raywood ash, create such a problem for lawn mowers that it often is impossible to operate one over the area where these surface roots are found
3) Damaging foundations. The roots of some aggressive and fast-growing trees and some larger shrubs are thought by many to damage the foundations of buildings. These roots, when encountering the foundation, are deflected and tend to run parallel to it rather than going down and under and surfacing on the other side. The roots will enter any cracks found but will not by themselves cause any cracking.
4) Lifting walks and driveways. Tree roots can and do lift sections of sidewalks and driveways. Careful selection of street trees and shade trees is needed to eliminate the problem.
5) Chemical versus organic sources of nutrients. All nutrients absorbed by the roots must be in solution. It makes no difference to the root hair if the minerals are derived from manufactured fertilizers or from organic sources. This point is often overlooked when gardeners espouse the virtues of either form of fertilizer sources. Mineral nutrients and moisture must be present for the root hairs to do their work in providing the leaves with a source of nutrients for their use in manufacturing food for the plant to use.
6) Seeking water. Roots do not “seek” water. They use it if found. Roots do not have a “sixth sense” about finding a source of moisture when growing through the soil. They simply grow and, if a source of moisture is found, will use it.
7) Tap roots. Not all plants and trees have them. Some trees will discard the tap root after they grow to a certain point. Some plants and trees will send down “sinker roots” to help anchor the plant.
8) Roots get nutrients far beneath the surface. Eighty to ninety per cent of roots are found in the top twelve inches of the soil. The rest are normally found down to a depth of three to four feet. There are exceptions, of course, but roots must have air to survive and grow. Air becomes very scarce to non-existent at the deeper depths.
9) Root barriers. These barriers are made of plastic material with chemicals inside that deter roots when they come into contact with the barrier. Many aggressive tree roots, as well as those of other shrubs, will come in contact with the barrier, grow down and under the barrier to rise to the surface and continue their horizontal growth. It’s better to plant only those trees and shrubs known to lack aggressive roots than to try to contain the roots of those known to be aggressive.
10) Root feeders. By this time the reader should be aware that the spread of the roots of a typical plant, shrub or tree makes it impossible to “feed” them with the probe featured in the advertisements. The area covered when using the device is but a very, very small portion of the root system.