Written by  Judy Tisdall & Bas van den Ende

Manage soils & water to control tree growth & increase productivity (part 7)

Root restriction provides orchardists with a useful tool for tree growth manipulation.

Restricting root growth (continued from last month)
Root restriction provides orchardists with a useful tool for tree growth manipulation
Strategies to achieve root restriction include close planting of fruit trees, regulating the supply of irrigation water, applying growth regulators, using rootstocks which induce precocity (if available), pruning, and keeping the soil in optimal condition to implement the long-term effects of root restriction on controlling root growth, tree vigour and fruit production.
Close planting restricts roots
Fruit trees with confined root systems do not grow much and if properly cared for in regard to water and nutrients, can remain healthy for a very long time.
Fruit trees planted closely together experience root competition. Root distribution, as well as abundance of roots, is influenced by the root growth of other trees planted close by.
Roots will in preference always grow into areas of soil free of competition from other fruit trees.
It must be assumed that the roots exude or produce substances that are inhibitory to root growth of fruit trees of the same species or variety.
High-density planting suppresses tree growth, and while competition for sunlight may cause shoots to extend and result in taller fruit trees, girth of the trunk, expressed as trunk cross-sectional area (TCA), is decreased.
The weight of above ground parts of a tree, excluding the fruit, is proportional to TCA, and thus, root competition decreases vegetative growth above the ground.
When management becomes a challenge
Early bearing, high yields and high pack-outs per hectare—generated by high tree density—have the most effect on profitability of orchards, and consequently have been the aim of much tree management research.
In orchards, yield is the product of the number of fruit trees per hectare, the number of flowers on each tree, the number of flowers that set fruit, and the ultimate size and quality of the fruit.
Maximising orchard performance means maximising each of these factors early in the life of the orchard, each year.
In young orchards there is nothing wrong with strong growth, so long as it is properly directed towards building a full, productive canopy.
It is once you have filled the allotted space that excess vigour becomes a wasteful problem.
Young orchard performance depends on how fast the new fruit trees develop their canopy.
Limiting factors to tree growth are: water stress, weed competition, inadequate control of pests and diseases. Young fruit trees have small root systems, so cannot fully use water reserves during dry periods.
Fruit trees have the capacity to crop when they are young and small and management practices are developed to take advantage of that fact.
High-density plantings frequently become uneconomic because tree size, although decreased by root competition, cannot be controlled sufficiently to prevent competition for sunlight and internal shading; and fruit trees become increasingly difficult to manage.
Switch from wood prodction to fruit production
The secret of managing physiologically balanced fruit trees is to switch over from wood production into fruit production and achieve regular yields, high quality fruit and extend an orchard’s lifespan.
Management becomes a challenge when closely-planted fruit trees have filled their allotted space, because most fruit trees are planted at higher densities or allocated less area than the fruit tree normally occupies when fully grown. We are no longer in the canopy development phase.
Maintaining an efficient productive fruiting canopy
As we are no longer trying to increase canopy volume, but contain it within its allotted space, we need calm fruit trees that divert most of their photosynthates production into fruit, rather than unnecessary shoot growth that needs to be pruned out annually.
Working with generative wood is the main benefit of high-density planting. As we move towards more intensive plantings with simple tree forms, setting the trees up with the right bud numbers and fruit tree form, should become much easier.
The fundamental message is that orchard efficiency is determend by establishing a light-efficient canopy as quickly as possible.
As orchard plantings intensify, quality standards refine, and more stringent market requirements must be met, good tree vigour management becomes critical.
Regulating the supply of water restricts roots
In dry climates low-flow irrigation (drip or microjet) can be managed to restrict root volume simply by controlling the amount of the soil which is wet during irrigation.
A smaller root volume under drip irrigation can decrease tree size.
(continued next month)

See this article in Tree Fruit Feb 2018