This article has been modified for the southern hemisphere.
When we think of lychee trees what we think of is the spectacular evergreen foliage and the clusters of delicious red fruits. What often is not considered is occurring below ground in the root system of the tree. A healthy root system is essential for growing a healthy lychee tree. Understanding how the lychee root system works can help you grow more vigorous and productive trees.
Two Types of Root Systems Lychee seedlings and grafted trees that have reached maturity have well developed vertical tap roots that can extend a few meters down into the ground towards the water table. These deep dwelling tap roots are accompanied by some lateral branching surface dwelling roots. Air layered trees have no main tap root and instead have entirely shallow lateral branching root systems often called skeleton roots. These skeleton roots have their entire network in the top layer of soil to a depth of about 20cm. The lateral absorptive roots of a lychee tree extend out to about the drip line of the canopy. The diameter of the canopy is a good indicator of the extent of the root system. A clear above ground indicator of a healthy root system is the ability of a tree to resist lateral pressure, such as wind. This is especially true in small trees. Trunks that easily flop from side to side or feel loose, when they are pulled on, are either not healthy or lack an adequate network of roots to support the tree. If some of the root system is exposed, mature healthy roots should be flexible and should display a light green when gently scraped. The smaller branching root tips should be whitish and flexible and should not easily fall apart when tugged. The root system of a lychee tree typically experiences 3 climaxes of growth each year. Not surprisingly, these root system growth events seem to occur when the surrounding soil is warm. They occur from mid-November-December, January-February after harvesting, mid-April when autumn shoots get matured.
How the Lychee Root System Works A lychee tree has two basic functional systems: the canopy and the root system. It is important to understand the interactions between these two systems and the role of these systems in the health of lychee trees. The canopy, consisting of branches, stems and leaves is where photosynthesis occurs. The products of photosynthesis - sugars and amino acids - are conveyed through the bark to the roots, growing tips of stems and leaves, flowers and fruits. The root system is where water, minerals and nutrients are absorbed from the soil and conducted to all other parts of the plant. Gas exchange also occurs in the root system. The area directly adjacent to the absorbing roots is called the rhizosphere. In a natural, undisturbed forest environment this area is composed of weathered minerals and deteriorating organic matter. Living within this mix are populations of earthworms, insects, diverse invertebrates, protozoa, bacteria and fungi. These organisms participate in the breakdown of organic material into essential inorganic elements that may be used by forest plants. Bacterial microbes break down and solubilize minerals such as phosphorus for plant availability, recycle nutrients and help to protect the plant from soil pathogens and nematodes. Beneficial Mycorrhizal fungi colonize the fine absorbing roots of trees to derive most, if not all, of their food supply (carbohydrates, vitamins and amino acids) from the roots of the plant. In return the mycorrhizal fungi aid in the growth and development of their host tree by significantly increasing the absorptive surface area of the roots, as well as assisting in the decomposition of organic matter. These Mycorrhizal fungi enhance the tolerance of plants to drought, seasonal temperature fluctuations and pH balance as well as minimizing the negative effects of transplant shock. Mycorrhizal fungi also help to protect the plant from harmful diseases and pathogens and reduce the dependency of plants on supplemental fertilizers. In artificial soils, such as what we purchase and use for growing plants in containers or in man-made or urban developed landscapes, these natural soil components are missing and should be replaced for healthy plant growth.
Root shading and cooling: During late Spring, Summer and Autumn in the sub-tropics the shallow root system of a lychee tree is exposed to the daily cyclical extremes of heating and cooling. This stress is especially pronounced during the mid summer post harvest months. Although lychee trees need partial to full sun, the roots prefer to be shaded for optimum health and growth. The hemispherical habit of growth and shape of the lychee tree naturally keep the root system shaded under the canopy. Mulching provides an excellent way of maintaining a healthy growing environment around the absorbing roots of a lychee tree. Mulch retains moisture, promotes and created a home & food for beneficial soil bacteria and mycorrhizal fungi, earthworms, insects, protozoa and effectively shades the root system from direct exposure to the sun. This helps to regenerate the soil around your lychee tree to approximate forest soil characteristics which are missing in artificial landscapes.