Watering Newly Planted Trees - Part 2

Newly planted trees only have roots in the root ball (See Watering Newly Planted Trees: Part 1) so for the trees to be able to establish rapidly, the root ball must be kept moist during establishment.

However, trees are lazy things and, if moisture is regularly applied in the same spot, for extended periods of time, the trees can grow well but will not “bother” to develop a root system that has spread far enough to either support the tree physically or be able to access enough water for sustained growth, once watering ceases. Therefore, when trees are to be watered for longer periods of time (e.g. >12 months) we need to vary the way in which water is applied during the later parts of the establishment period in a way that “encourages” roots to grow outwards.

Our Field Guide for Watering Newly Planted Trees provides indicative rates for the application of water for trees over a range of different planted sizes.  It also lists indicative required, and recommended, timeframes for watering to continue.

Simplistically, when establishment periods exceed, say, 12 months, and the roots have now grown beyond the root ball itself and out into the surrounding site soils/media, we need to increase the amount of water applied, per application, reduce the frequency of application, and increase the area over which the water is applied.  This will increase the volume of soil/media “wetted up” each watering and allow the root ball to dry out a bit, between waterings, encouraging the roots to grow outwards and increase the diameter of the root plate.  This increase in the tree’s “footprint” increases stability, while the associated increase in the volume of soil/media the root system can now access also increases the amount of soil moisture available to the tree.

The base watering rates shown in our Field Guide for Watering Newly Planted Trees have been calculated such that the water applied should wet the planted root ball from top to bottom (i.e. 500mm-600mm deep), and it appears to work quite well. We are not aware of any research that will tell us what the “ideal” increases in the amount of water applied, per application, or what the increase in the surface area it is applied over should be, as the tree begins to establish.  However, if you follow the suggested increases in application rates shown in our Field Guide for Watering Newly Planted Trees (i.e. 1.5 x the base rate shown when watering between 13- and 24-months post planting and 2 x the base rate when watering between 25- and-36 months post planting) you will be headed in the right direction. Try increasing the diameter of the area watered to 1.5 and 2 x planted root ball diameter, when the water regime reaches the 13th and 25th months, respectively.  These increases in the area over which water is applied should be feasible and will encourage lateral root development.

As with any suggested irrigation regime, monitor it on site, and adjust as your specific site conditions dictate.