The use of tensiometers, directly measuring the suction or soil moisture tension,

has proved to be the most appropriate method for analysing the plant’s water requirements with precision from the time of planting. And a well-irrigated tree is well-rooted tree; and a well-rooted tree will look for and extract water from the soil and resist the vagaries of the climate.

From the first day of the first year of planting, a tree’s only source of water comes exclusively from the root ball. It is, therefore, dependent on rainfall and irrigation, which are important factors in ensuring that the roots emerge from the tree pit. Thus it is of interest to monitor the soil water status in order to schedule the irrigations, thus ensuring the recovery of the tree and its medium term resilience, while rationalising the human and financial resources deployed.

When planting, the water transfers and root colonization are often reduced because of the excessive compaction around the edge of the pit. Indirectly, between the budburst phase, marking the end of winter, and root emergence, a recently planted tree is particularly dependent on water being applied to the root ball. Only specific irrigation systems, such as Hunter’s RZWS or RWS of Rain Bird, or quite simply a pipe leading into a pan or earthenware trough, allowing for more or less significant quantities of water to be applied to the whole root ball, depending on the species concerned, its strength and the climate.

Constructing the watering pan / trough

Firstly, the life span of a pan or trough is 6 months to 2 years, Some troughs, kerosene-based, are very efficient. They deteriorate less quickly and less work is required by the green space agents. Specifically, the diameter of the bowl should correspond to that of the root ball. Using a shovel, the agents should also build up the edges of the pan to about 15 cm in height, no more. The thickness of soil over the root ball should be no more than 3 cm, or there will be a risk of water being held on the surface, to the detriment of the roots of the underlying root ball. In the majority of cases, there is no point in installing drains for the irrigation water because the amounts of water applied will wetten the root ball at too great a depth, and not evenly throughout the whole root complex. Finally, the amounts of water applied should be divided up so as to limit losses (about 5 to 7 irrigations per year.

For the young trees to flourish, they must receive sufficient irrigation and oxygen exchange at every level of the root zone. This encourages the roots to grow deep into the soil, where they will remain safe so the plant can become strong and long-lasting.

The irrigation systems consisting of an internal baffle system, directs water to the whole root zone.

Determining the soil water content

Nowadays, a well thought out irrigation technique is not only based on climate data, but also the availability of soil water for the roots. Directly measuring the soil water tension has proved to be the most appropriate. This principle is known as tensiometry. In the field, three tensiometers, which measure the suction or tension forces required by the plant to extract water from the soil, are placed at three depths that correspond respectively to the whole rooting zone.

“We are no longer in an era where we water blindly. We water because there are good reasons, and only good reasons! Continuing to water a tree that is more than 4 years old so that it does not wither in the heat means that the watering has not been effective or carried out during the critical 2-3 year period after planting. In short, it is not rooted enough to fetch water deep down! However, the landscape industry and cities no longer have the financial and human resources. Interventions must be rationalised. Hence the interest in precisely analysing water needs. This should no longer be an economic issue, but a necessity! We no longer question the use of quadripod staking now that the tree is growing well, so why should we question the analyses when they allow us to make significant savings afterwards (guaranteed recovery, scheduled irrigation, rationalization of irrigations…)? All the more so when the cost of analysis is between 20 and 50 €/tree/year, not to mention the fact that the water resource is also preserved”, explains Michaël Fayaud, co-founder and manager of Urbasense.

Reasoned irrigation management is not only based on the weather data, but also on the availability of water for the roots. Directly measuring the soil water tension has proved to be the most appropriate.

Irrigation: up to 24 months after planting

The objectives are clear. At the end of the first year, there must be significant root development outside the root ball, whereas, at the end of the second year, the personnel from the green space department simply have to check on the colonization of the roots, although that they should be active within a zone of 0.8 to 1 m from the trunk.

“Once the second year has passed, irrigation is no longer necessary even during harsh droughts”, said Laurent Mignonneau, head of procedures at Hunter. So why water the tree? The tree is supposed to be self-sustaining. “Still too often the managers do not know whether the tree has taken well, so they irrigate because they don’t have the appropriate measuring tools”, explained Michaël Fayaud. And even when the ‘adult’ trees have roots that extend beyond the planting pit, it would take a disproportionate amount of water (several hundred litres per tree) to satisfy the whole root volume. So would it not make more sense to improve the water uptake of the roots, which depends on the irrigation conditions during the first few years of planting in order to replenish the soil water reserve in a sufficient and sustainable manner in the years to come? All the evidence points to the fact that a well-rooted tree is a tree that is capable of extracting water from a greater depth of soil!

The problem of climate change

If the rains don’t clog the networks and strip away the soil, then failing to ensure that the water infiltrates correctly to a proper depth to supply the roots, will result in the prolonged periods of drought and heat waves damaging the vegetation, including the trees. To that we can add the mild temperatures at the end of February, provoking premature budburst, not to mention the heavy autumn rains. So much so that today the professionals must face a new challenge: what are they to do at the end of the season when the soil rapidly dries out and the groundwater is not replenished during the winter? “If the managers think they can do without irrigation in the winter, then they are very much mistaken”, explained Michaël Fayaud. “When the temperature are mild, it is highly likely that the revival of biological activity will trigger the beginning of ETP. In February 2017, due to a lack of water, conifers in the Île-de-France, therefore, had to be irrigated to ensure that they recovered in the spring and thus avoid losses.” Another example can be found in Montpellier. Usually, in autumn, the sap goes down and the tree does not draw water up from the roots. However, as they lack sufficient water reserves, deciduous trees are irrigated in winter in the South of France, just before budburst. “And then, we have to say something else: the losses suffered by trees due to the lack of water, in summer as well as in winter, has a greater cost than the actual measuring devices”, he adds. Therefore, there is no longer any doubt: we must analyze the requirements with great precision and irrigate repeatedly, in order to keep the trees looking at their best, strong and resilient!

Recommendations

According to the recommendations given in Section N° 35 of the General Technical Specifications (CCTG), the watering pan or trough should be of a toroidal shape and not spherical. After making the trough, the contractor in charge of the planation project must apply the first irrigation.

Notwithstanding stipulations to the contrary in these Specifications, the amounts of water required for this device (additional amounts of soil should be added to compensate for settling of the soil) are as follows:

• 10 L per young plant;
• 15 L per shrub;
• 40 L per tree with bare roots up to a force of 14/16;
• 100 L per tree with root ball of more than 14/16.