Five questions around moisture supply for trees, directly after planting

René Voogt discusses pressing issues around moisture supply for trees

When trees are being planted, several issues can arise. Moisture supply is one of those. Trees can potentially suffer from a shortage or precisely the opposite: an abundance of water. This leads to many questions for arborists, especially the question what to do about these problems.

Author: Sjoerd Rispens

In November 2019, René Voogt from ConnectedGreen won the Golden Watering Can award at the Climate Expo in Utrecht. ConnectedGreen helps landscaping companies to save on water, project visits and failing plants/trees. In ConnectedGreen, ‘projects’ are defined (for example per landscaping project, street, or neighborhood) which are divided into indication trees, planters, or beds. The sensors are calibrated by entering the tree/plant name and soil type. Besides optimization of watering, ConnectedGreen provides insights which help to improve growing places.

With ConnectedGreen, Voogt is involved daily with questions about moisture supply for trees, directly after planting. Here below he answers five pressing questions about this topic.

Question 1: How can a planting place dry out while being watered frequently?
‘Particularly in more porous and sandy soil types, like tree sand or planting soil, the process of drying can go faster than the compensation from watering’, says Voogt. ‘In certain periods, this means that watering once a week is not enough, and that a downward trend becomes visible in the moisture levels. This can even lead to the moisture level dropping to near zero between two waterings. And the more dried out, the more difficult it is for a soil to absorb water, so this is an accelerating effect (hydrophobic soil).’ The solution according to Voogt: ‘More frequent watering, in smaller quantities. These more frequent waterings can be compensated in cooler, wetter periods, when less frequent watering is required. Another option is to add more organic and/or granular material in the planting place.’

Question 2: How is it possible that rain or watering is not noticed by a sensor?
‘The sensors measure the soil moisture percentage at root depth’, explains Voogt. ‘That is roughly between 15 and 60 centimeters deep, dependable of the plants. If the top layer dries out, we observe the phenomenon that (rain)water simply washes off the top layer and cannot penetrate to root depth. This effect is called “hydrophobic” soil. In that case, the soil literally repels water. The water flows away via “flow channels” and does not enter the ground. The problem here is that the speed of drying out can be much higher than the speed with which the soil can absorb water.’ The solution according to Voogt: ‘Open up the top layer (for example with a rake) before watering or create a tree planting system, for example with reverse drainage’.

Question 3: How is it possible that different sensors in the same project show different soil moisture values?
‘This question pops up frequently when sensors show different values in beds and tree planting places that are cultivated in the same way’, says Voogt. ‘Looking through all projects in our systems, we see it is more an exception when all beds or trees in a project show the same percentages. Here, several variables are in play:
-The interaction between the top layer, underground, cultivated soil and root ball. Especially the top layer and underground are very divers in our country. Also, the soil that is added to the project is not always exactly similar in every bed or with every tree.
-The shadow of buildings or other plants/trees plays a rol, besides orientation/vulnerability to the sun or wind.
-Sometimes, very local impermeable layers or horizontal water flows play a role.’

Question 4: How come that some growing places are wet in the top layers and dry at the bottom, while this is the opposite for other growing places?
‘Here too, environmental factors play a big role. Soil with a high amount of organic matter can hold moisture very well. Sometimes, there is more organic matter in the top layers and sometimes more in the lower layers. Besides that, the drainage capacity of the growing place is essential. If an impermeable layer or heavy clay is present, we often see the growing place ‘filling up’ like a bathtub. First the moisture levels rise in the lower levels, and then in the top levels too. We also see the opposite, when plants and trees are planted on a slope or very sandy underground and the water just flows away.’ The solution according to Voogt: ‘Impermeable layers or clay can be penetrated or dug up. In very porous soils it will help to add clay to the bottom of the planting hole.’

Question 5: Why is the soil moisture percentage a few days after a(big) watering sometimes lower than before?
This is a remarkably interesting phenomenon, that Voogt has encountered in a couple of occasions: ‘After watering, the moisture percentage rises really quickly, to then drop dramatically. It is widely known that excess water drains away if it is much more than the soil can hold/absorb. It seems that, in certain situations, the water that is already present, is ‘drawn down’ with the water that flushes out. We have seen this in a couple of places. There are several theoretical explanations, but additional research is needed to find out the real cause. The observed pattern seems to support the theory that it makes no sense to give (much) more water than the soil can hold (more than the field capacity). On many graphs in the summer period we observe these high peaks, followed by lower levels than before. The preliminary results with giving less water (30 to 50 percent of the original quantities) are indicating that the soil moisture percentages drop much less quickly, thus being better maintained.’

Afterburner
And as afterburner: why is it important to receive a signal if the growing place is too wet? Aren’t these sensors meant for watering in case of drought? Voogt: ‘After having hundreds of sensors for more than two years in the field, we know that it is evenly important to receive signals if it is wet, even in wintertime. There are several examples known of projects where many trees and plants have failed due to an abundance of water. This can have many causes which were not known in the design phase, like:
-The growing place or bed is lower than the rest of the area, so all water flows there
-There is an impermeable layer or heavy clay, which prevents excess water from draining
-The planting place is next to a (sloping) road, a square or path, which makes all water flow in one direction
-The soil contains too much (raw) organic matter, holding large quantities of water
Often the notification “too wet” comes as a surprise (we have seen more than one occasion of broken water pipes or valves). It is however extremely useful to know, because the cause can be investigated and taken away – for example with vertical or horizontal drains.’

Source (NL): Stad+Groen https://www.boomzorg.nl/article/32511/vijf-vragen-rondom-vochtvoorziening-bij-bomen-direct-na-het-planten