Trees play a pivotal role in the urban landscape as a symbol and key element of the green infrastructure. Their presence is essential for providing key functions such as cooling and regulating the water cycle, thus contributing towards the sustainability and resilience of the cities.

Nevertheless, these roles rely on one crucial factor: the robust and profound anchorage of the roots. So how can an integrated irrigation system contribute towards an optimum development of the roots? What are the unique features of irrigating a tree and root ball? This article offers some feedback on the experience gained after studying thousands of trees within the context of the tensiometric analyses and tests conducted between 2016 and 2023, mainly in France, Switzerland and Belgium.

Two types of irrigations to be identified

When considering the irrigation of the tree throughout its whole life, we will see that it is important to understand the intended purpose of the two types of irrigation:

• Irrigation for plant water security: this allows the tree to survive the first few years while its rooting system develops. Aim: to support the development of the roots as deep in the soil as possible, and as far as possible from the trunk. In fact, plant security irrigation generally corresponds to the length of the warranty provided for the tree.
• Irrigating for maintenance or servicing purposes: this applies when the tree’s warranty period expires. Its roots are thus already established at depth. The aim of this irrigation process is to compensate for the limited access to a water resource, either caused by unfavourable weather conditions or because there is a limited soil depth (e.g. plants on a roof terrace).

Distinguishing between these two types of irrigation is the key to achieving greater self-sufficiency for the plant, in terms of its water requirements, while, as far as possible, limiting the use of irrigation water once the plant is established.

Four golden rules for encouraging root anchorage

Perception: one irrigation zone reserved specifically for the tree, if not two. It is essential to create one irrigation zone and one schedule specifically for the tree. In fact, the irrigation of the trees is typified by:

• Applying irrigation amounts that are higher than those of the perennials or turfgrass (see below)
• Irrigation frequencies that are lower than those of other crops (see below)

In practice, it could be extremely useful to set up 2 irrigation zones exclusively for the trees. This would allow the grower, at the same time:

• To respond to the tree’s essential needs during the first few months by applying, quite frequently, localised amounts close to the trunk, so as to remoisten the plant’s root ball while waiting for the rooting system to develop.
• Following the start of root development, adapt the amounts applied by increasing the dosages, thus allowing for the whole volume of soil penetrated by the roots to be wettened, around the plant’s root ball.

The irrigation amounts applied are considerably higher than those applied to perennials or turf grass. For example, 60 litres of water are required to wetten the root ball of a tree with a 20/25 girth, during the first few weeks of growth in its first year. It, therefore, must be an amount applied specifically for the tree, concentrated around the foot of the trunk. Traditionally, the amounts of water applied could then vary between 100 and 250 litres, depending on how the rooting system develops.

Irrigation frequencies considerably lower than those of other crops. The key point here is to avoid irrigating in advance: apart from being of no use, they significantly reduce the development of the rooting system. By applying water too frequently, this development is inhibited because the resource becomes too readily available at ground level. Generally speaking, in Europe, activating the irrigation system every one or two weeks is quite sufficient, even in a Mediterranean context, with few exceptions.

In the event of wilting, the priority is to check the status of the soil. Over the last few years, we have seen a substantial rise in cases where heat stress is observed, frequently confused with plant water stress, as well as a significant increase in mortality among young plants.

The phenomenon of heat stress is observed in the plants when the air temperature increases unexpectedly (e.g. +20° in 48 h). The wilting observed can strongly resemble that caused by plant water stress. The global climate change tends to increase the frequency of this phenomena.

Applying a large amount of water during heat stress makes the situation significantly worse: the prolonged saturation of the soil with water can lead to root asphyxia and plant mortality.

When wilting is observed on a tree, then it is essential to conduct an analysis of the soil, using a hand auger to at least a depth of 30 centimetres, in order to check the soil moisture status. This is the quickest and most reliable way of differentiating between plant water stress (where there is a need to irrigate) and heat stress (where irrigation can be harmful or even fatal).

Role of the sensors and decision-making

Since the late nineties, a few applied research projects have allowed for a better understanding of the root development of plants in an urban environment and for an adaptation of the use of sensors for irrigation scheduling purposes.

More specifically for trees, the use of tensiometers and the appropriate processing of their data now makes it possible to locate the root drying front. This information is a determining factor for optimised irrigation: the water is applied, as precisely as possible, to the exact place where the roots are developing and where the soil is drying up, thus enabling the roots subsequently to reach the deepest zones, which are better supplied with water.

One important point: it is not only by combining the data received from various sensors that the localisation of the rooting system is possible: it is also a matter of analysing the difference in drying-up between a set of sensors (generally 3 to 6 probes) in order to be able to assess the presence or absence of active roots and the intensity of their activity.

Perspectives: irrigating the trees to enhance their cooling capability

Over these last few years, a study conducted by a consortium of researchers in the Lyons-Métropole region investigated the impact of irrigating trees, shrubs and perennials, with respect to their cooling capacity during a heatwave.

Thus, it could be demonstrated that maintaining an optimum plant water status enhances the plant’s cooling capacity, by delaying the closing of the stomata and allowing the evaporation of an outflow of water, in spite of the hot conditions.

These initial results, which are very encouraging, allow the user to plan for irrigation “scheduling methods” that apply specifically to the cooling of the plants during heatwaves.

Of course, there are still a few aspects to be studied before we can consider the widespread distribution of these tools: adaptation of the plant palette, agronomic quality of the soil, source of the irrigation water.  Nevertheless, the technique and technological groundwork required for the monitoring and scheduling of these sites has now been completed.        

Author : Coralie Tavassoli et Michael Fayaud, Urbasense