Written by  Judy Tisdall and Bas van den Ende

Manage soils & water to control tree growth & increase productivity (part 9)

Management of root growth is an important method to control vegetative growth and production of fruit trees.
Regulation of water supply is a convenient way of achieving this management.

Deficit irrigation
The natural barrier in shallow soils, allow regulating the supply of soil water, and becomes a powerful management strategy and convenient means of manipulating tree growth for greater fruitfulness and less vegetative growth, in much the same manner as dwarfing rootstocks.
Irrigation is a powerful management strategy
Irrigation can be developed into a powerful management strategy to manipulate tree growth for greater fruitfulness and less vegetative growth.
The growth rate or vigour of the vegetative portion of the above-ground part of the tree is directly correlated with the growth rate of the roots.
On many heavy orchard soils, low-flow irrigation will not decrease tree vigour to an extent that allows adequate penetration of sunlight in high-density plantings.
A method has been developed, called regulated deficit irrigation or RDI, for stone fruit and pear trees with medium and long fruit development periods, to reduce tree vigour and overcome the shading problem.
With RDI, the level of irrigation is decreased during the period of fast shoot growth by cutting back on the quantity of water delivered but maintaining irrigation frequency.
At the start of fast fruit growth, optimum irrigation levels are resumed.
RDI has not only decreased shoot growth and the need for summer pruning, it has also increased fruit size and yield.
The phenomenon associated with RDI is compensatory growth. When fruit trees are temporarily deprived of adequate water, fruit growth slows down. On the assumption of ample irrigation, growth accelerates.
RDI is of significant value where:

  • Water is limiting
  • Excessive vigour is an issue—an early season irrigation deficit can strongly decrease tree vigour and does not affect fruit size at harvest.

How much deficit?
The most difficult question to answer is how much of a deficit should be imposed on trees.
To get positive benefits the deficit needs to be such that soil wetness is being held close to levels such that all readily available water is depleted.
Fruit trees will still be able to extract water from the soil at these levels, but will be under pressure to do so, and hence the positive and negative effects will accrue.
Over watering (above field capacity) can lead to leaching of nitrogen and causes anaerobic soil conditions (water logging), reduces the fruit trees’ normal water uptake, limits root activity or even root death.
RDI is to be avoided for young fruit trees until their canopies have been fully developed.
The high root density obtained with close planting and drip irrigation is designed to allow controlled management of root growth.
Because the tree roots are confined to a relatively small area of soil, it is important to create a good environment within this restricted area. Such an area often consists of surface soil only.
Growth regulators restrict roots (cont next month)

See this article in Tree Fruit April 2018