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MI State UniversRoots: The Hidden Half of Growth
MSU has an excellent site for fruit growers, below is an example of 
an article on root growth that can be found there:

Mark Longstroth 
Southwest Michigan District Extension Horticultural & Marketing Agent
 
Winter has come and the trees, vines and bushes that bear our 
Michigan fruit crops have lost their leaves and gone dormant for the 
winter. Fruit growers would be wrong to think that not much is 
happening in their orchards and vineyards. The above ground portions 
of our plants are dormant and the buds by some undetermined mechanism 
are keeping track of chilling hours. We may think that the whole 
plant is dormant but this is hardly the case. In the ground, the 
roots are active and growing. Unlike the above ground shoots, which 
are dormant and will not grow until completing their chilling 
requirement, the only thing holding back root growth is soil 
temperature. The roots are capable of growth at any time of the year. 
All the roots need are conditions conducive to growth and the lack of 
competition from other plant organs such as the shoots and fruit. 
All of us know that our plants have roots. But we just don't think 
about them. It's hard to image things we can not see, touch or feel. 
Most of us, when we think of the roots imagine a subterranean mirror 
image of the trunk. We often wonder how far from the tree do the 
roots go? How deep are they? We busily take care of the plant above 
ground but we seldom think about the roots. With this essay, I hope 
to give you a little insight into what is happening under your feet 
all year long.
We know the roots provide support for the tree. We are reminded of 
this when trees fall over. We also know that the root system absorbs 
and supplies water and nutrients to the tree. We are reminded that 
the tree needs roots when new plants leaf out and then just sit there 
not growing for the rest of the growing season. We often wonder what 
could we do to increase growth. Then the next year, the tree takes 
off, growing like a rocket. This situation often occurs when trees 
are planted late or the soil conditions are not favorable for root 
growth. The young plant is unable to produce enough of a root system 
to supply the needs of the plant. When the demands of the leaves 
overtake the ability of the roots to supply water, growth stops.
In woody plants, there are two kinds of roots; woody roots and white 
roots. These two kinds are very different. The woody roots are in 
essence the underground trunk of the tree. These are the roots that 
we all think of when we think to the root system of a perennial 
plant. There are also new white roots that are the result of active 
growth. These young roots are an important site of mineral 
absorption. White roots do not hang around long. The live span of a 
young root is measured in days and weeks. As the white portion of the 
root matures, it turns brown as the outer layers turn brown and die. 
Nine times out of ten the remaining small brown root also dies. 
Occasionally a brown root lives. The root thickens and becomes part 
of the woody root system. Root survival is probably due to whether 
there is still good growth in the root tips supported by that root. 
Root growth is similar to shoot growth but there are major 
differences. At the tip of an actively growing root is the root cap. 
The root cap is a loose assemblage of cells and a slimly substance 
called mucilage that protects and lubricates the root tip as it 
forces its way through the soil. Behind the root cap is the zone of 
active cell division were new cells are created. Immediately behind 
is the zone of cell elongation. This is where the new cells begin 
growth. Plant cells grow by pumping themselves full of water. They do 
this by actively moving salt ions, usually potassium, into the cell. 
They also move sugars into the cell. The name of the game is to be 
saltier and sweeter that the water around you. This causes water to 
move into the cell. The cells do not actively take up water. The 
water flows into the cells because there are more salts and sugars 
inside the cells than outside. This is the way all plant cells grow 
and the way roots absorb water from the soil. As the cells absorb 
water they increase in length and force the root tip deeper into the 
soil. 
Behind the zone of elongation is the zone of maturation where the 
different types of cells develop. The xylem, long cells that conduct 
water out of the roots and up the tree, form in the middle of the 
root. On the outside of the root is the cortex that absorbs nutrients 
from the soil. In-between the xylem and the cortex are two very 
important layers of cells. The outer layer is the endodermis. This 
layer of cells forms a thick waxy layer between all its cells called 
the casparian strip. This thick barrier forces water and nutrients 
moving into the xylem to go through the endodermis cells. This 
barrier prevents leakage out of the root. Inside the endoderimis is 
the pericycle. This layer still has the capacity for new growth by 
renewed cell division. This is where new roots originate and where 
thickening will occur if the root survives and becomes woody. As the 
root ages the cortex dies and the endodermis is exposed to the soil. 
This is when the white root turns brown and soon after that the root 
usually dies. If the root lives, new roots from the pericycle may 
burst out through the old covering or the root may thicken and become 
woody.
There is a pattern and timing to root growth. The roots grow when the 
shoots are not growing. Competition with other parts of the plant$is 
the main determining factor to root growth. In fall, after harvest, 
sugars made in the leaves was transferred to the shoots, trunk and 
roots for storage and use during the next growing season. A large 
portion of these storage compounds are stored in the roots. The roots 
use this abundance of energy for growth and root growth is active in 
the late summer and fall. Root growth begins to taper off as the soil 
cools in the fall, but the roots are always capable of growth. As the 
soil becomes cold in the winter, root growth ceases. This is not 
because the roots are dormant as the buds are in the top of the tree, 
but because the soil is too cool to support growth. The crown of the 
tree, where the root system joins the trunk of the tree is the 
transition zone between the dormant top and the active roots. This 
transition zone, which often contains a graft union, is very 
susceptible to winter cold injury. Conditions that favor late root 
growth can predispose your plants to cold injury in the winter, 
particularly if the ground is not covered by snow. 
Root growth increases in the Spring. This is not because the roots 
have become active but because the soil is now warm enough to support 
faster growth. Root growth increases as the soil warms. Rapid root 
growth continues through bud break and only 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 the trees' 
reserves are used in above ground growth. When shoot growth ceases in 
early summer, root growth begins again. The rate of root growth 
increases through the summer reaching a peak in early fall and then 
declines as the soil cools. So, two of the main restraints on root 
growth are soil temperature and competition with shoots and fruit.
The other factor determining root growth is soil moisture. Roots grow 
were the conditions for growth are favorable. The old adage that 
roots grow to water is WRONG. Roots grow where the water is. Roots in 
dry soil show very little growth. In the 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. 
Lets go back to the analogy of the upside down tree to represent the 
root system. We can imagine that instead of one trunk that there are 
several and the trunk and branches of this root system are composed 
of woody roots. Now imagine that the leaves on this tree are not 
green and flat like an apple or a grape leaf but long and thin like 
the needles of a pine and white not green. Keep in mind that most of 
these leaves (white roots) will fall off, many a week or two after 
they form, and few become permanent woody stems. Unlike the leaves on 
a pine tree, white roots can change into a woody roots (stems) and 
grow more leaves (roots). 
I find that thinking 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 mix it with water to 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 were 
the ground is warm, moist and has the most nutrients. Over the years 
a frame work builds, with many branches close to the soil surface and 
several large trunks. Most of these large trunks lie close to the 
surface. Few roots are found deep in the soil. Forty to sixty percent 
of the rootsystem of perrenial plants is in the uppermost layer of 
the soil. On the large woody roots there are branches were the roots 
have found water and nutrients over the years. 
If you now take this root system and place it in the soil with a 
fruit tree above we can visualize the annual cycle of root and shoot 
growth. In the early Spring, many new roots grow close to the soil 
surface and remove water from the surface soil layers. As the leaves 
begin to appear on the tree, roots appear lower and lower in the 
soil. While the leaves above ground last until the end of the growing 
season, there are ten or twenty overlapping generations of new roots 
in the soil. You can almost imagine a time lapse movie of tree 
growth. Above the ground we see leaves form and shoots grow begin. 
Below ground we see a zone of white moving downward in the soil, 
broadening and narrowing as it moves downward. As active shoot growth 
begins, root growth slows and the white zone fades. All through the 
soil we see brief flashes of white as roots grow and die. As shoot 
growth ceases, we see increased activity in the soil as new roots 
form close to the surface and older, deeper roots die.
Almost half the sugars manufactured in the leaves are used and lost 
through root growth, which we normally do not think about because we 
do not see it. As I sit here and write this on a cold winter day, I 
can look outside where the trees are standing with no leaves on them. 
I know that in the soil roots are growing, getting ready to supply 
that explosive burst of green, leafy new growth that comes every 
spring. 




 

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