To understand why conventional planting, mulching, and nursery practices can jeopardize a plant’s ability to thrive as discussed in part 1, we need to first outline how trees and shrubs work.
There are three basic parts to a plants above ground vascular system: the cambium, xylem and phloem. The cambium is an internal layer of actively dividing cells between xylem and phloem tissues, and is responsible for the growth of stems and roots. Cambium cells divide to produce xylem and phloem cells.
The xylem is plant vascular tissue that transports water and dissolved minerals up from the roots to the rest of the plant. As the plant ages primary or inner xylem cells will die and lose conducting function. Dead xylem tissues become the heartwood and will provide strong structural support for the plant. The function of water and mineral uptake is the responsibility of the secondary or outer xylem otherwise called sapwood.
Phloem is the plant’s vascular tissue that carries foods (sugars) or photosynthate made in the leaves to all other parts of the plant. Secondary phloem cells are toward the outer bark. The inner phloem consists of columns of cells and provide the channels in which food substances travel. As water and minerals are drawn up and food is produced the unused chemical compounds are pushed back out of the roots.
Lastly and certainly no less important are the plant roots. Plant roots contribute biologically active chemicals into the soil environment known as root exudates. These sprawling root systems produce prolific amounts of root exudates. Root exudates contribute many types of organic compounds, simple and complex sugars, growth regulators, amino acids, organic acids, flavonoids, enzymes, and vitamins to the root zone or rhizosphere. This is where the amazing and wonderful world of soil biology takes off. It is this interaction between plant and soil, the communication between plant organisms and soil organisms that many take for granted.
Plant roots are genetically made to stay underground and remain damp and in contact with moisture at some level. The buttress roots, the root collar or flare on upward is made to remain dry and above grade and should have no contact with moisture. So what happens when the root flare and trunk area is covered? By covering the root flare/collar you open up the plant to all types of root rot issues and insect attack. The woody trunk of a tree or shrub not only transports nutrients throughout the plant – it is porous, and gasses like oxygen and carbon dioxide must be able to pass through for the tree or shrub to remain healthy and for the vascular system function normally.
In the third and final part of this article, we will discuss the steps we need to take to remediate these issues prior and during the installation of trees and shrubs in the landscape. For more information on this topic and tree health improvement services we offer, click here.