July 2023 - 16

IRRIGATION & WATER MANAGEMENT
Stressed
OUT
Sensor tech advances
help trees tell growers
when they are thirsty
BY CHRIS KOGER
MANAGING EDITOR
T
he traditional method of using technology to alert
growers of the need to irrigate involves soil-based
sensors, which do well at relaying moisture data
at a specific depth and location.
Dendrometers, which measure the expansion and
contraction of fruit tree trunks and fruit growth when
placed directly on apples or pears, were typically used
in early season fruit growth forecasting to shed light
on production. In recent years, however, dendrometers
in Washington apple orchards have also been used to
indicate tree stress and the need for irrigation, said Lee
Kalcsits, assistant professor of tree fruit physiology at
Washington State University (WSU).
In January, Kalcsits and Victor Blanco, a postdoctoral
researcher, ended a two-year study into different sensor
technology to detect stress in Washington and apple and
pear trees as a way to guide irrigation decisions.
" I think it's difficult for growers now because there is
a lot of technology available, and it's really important
that when growers adopt a technology that it truly leads
to either a change in practice or a clear decision-making
Dendrometers measure fruit growth, and can
indicate a fruit is stressed and needs water when it
shrinks slightly. Photos courtesy of Washington State
University Department of Horticulture.
pathway, " Kalcsits said. " And if it doesn't, then you
really question what the value of these technologies are,
because they are not cheap. Whatever technologies are
adopted need to be directly for decision-making. "
The project involved using the sensors in a Grandview,
Washington, Honeycrisp orchard that was dubbed
the Smart Orchard project. This is a Washington Tree
Fruit Research Commission funded effort with WSU
researchers and technology companies testing various
tech on the trees.
There are several ways to assess a tree's water
needs, Kalcsits said. Tracking soil moisture and water
availability can help growers estimate demand. Ideally,
directly measuring what a tree is experiencing inside
its trunk allows it to communicate stress and the need
for irrigation.
" We developed thresholds and models of how a tree
responds to soil stress or the environmental stress, and
the sensors would directly measure variables in the tree
that indicated whether it was stressed or not, " he said.
Of particular interest in the study was the use of
microchip tensiometers - microtensiometers - that
are " plugged " into tree trunks and use microchips to
detect stress triggered by low water content.
The advantage of the trunk microtensiometer's
pressure chip is that it directly monitors plant stressrelated
tension, whereas dendrometers measure it
indirectly. In high-stress situations, the relationship
between the dendrometers and the information they
receive from the trunk can break down, Kalcsits said.
16 | JULY 2023
July 2023

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