Adapting Canopy Sensing Systems into Juice Grape Production

Contact: James A. Taylor

Canopy sensing to define vine vigor/size

The LERGP team at CLEREL are researching and developing methods to incorporate information from high-resolution canopy sensors into Concord (and Niagara) production systems. Two sensing systems are being trialed; the N-Tech GreenSeeker and Holland Scientific CropCircle. These canopy sensors operate by measuring the reflectance of visible (Blue, Green and/or Red) and Near Infra-Red (NIR) light from the leaves. The amount of reflectance is dependent on i) the number of leaves and ii) the health (color and cell structure) of these leaves.

By measuring the canopy response and looking at the variation within a field or vineyard, it is possible to identify areas of low, medium and high vigor vines. Understanding this variation within a field, will provide information that allows growers to quantify (properly measure) under producing areas and the cost of lost crop production. At the moment, the work LERGP is doing is focused on how to use these canopy sensors to map vigor (and pruning weight) in single top-wire cordon sprawl systems (the most common trellis system in the region). The canopy sensors could also be applied to other trellis systems, e.g. GDC, Umbrella Kniffin, VSP etc., and LERGP does intend to work with these systems in the coming seasons. However, the information on this webpage is primarily aimed at canopy sensing in single top-wire cordon sprawl trellis systems.

Sensor operation

The canopy sensors are operated by imaging (pointing at) the part of the vine where differences in vine growth are expected. Early in the season the sensor can be directed at growth around the top wire cordon. At this stage of growth the sensor will differentiate between weak and strong early season growth.

As the season develops, all vines develop a thick canopy around the top wire. This leads to a saturation of the sensor signal. Since the canopy is complete along the top wire there is little or no difference in the sensor response between low and high vigor vines.

At this stage the sensor can be moved from imaging the top wire to imaging the side curtain of the vines. Typically the sensor should be mounted about 18 inches above the ground (around the bottom wire if present). The rationale here is that vigorous vines will have a well developed curtain with long canes that will present a large amount of leaf to the sensor low down on the side curtain. Weaker vines, with fewer and/or shorter canes will have less leaf material in the same area.  The difference in side curtain development allows the sensors to map differences in vine vigor. 
What does the NDVI mean?

The NDVI value is a ratio between the reflectance of Red and NIR light from the canopy. Healthy green plants absorb Red light for photosynthesis (i.e. they have a low Red light reflectance), but strongly reflect NIR light. Good leaf cell structure (again indicating healthy plants) leads to higher NIR light reflectance. Therefore vigorous plants should have a low Red and high NIR light reflectance while weak plants will have a relatively higher Red and lower NIR light reflectance. The ratio between Red and NIR therefore forms a good basis for identifying low and high vigor plants based on their canopy response.

Interpreting NDVI maps

NDVI is a normalized or relative response. The sensor output will be very dependent on how the canopy (vine) is presented to the sensor. As stated above, if the top wire is imaged late in the season, then the sensor will always see a filled canopy and will interpret all vines to be of high vigor. The schematic presented below (Figure 5) shows how the sensor response may differ with different canopy architectures. On the top line it is easy to see how imaging the side canopy (the red zone) shows an increase in sensor response (from low to high NDVI) as vine size increases from 1.5 to 3.5 lbs/vine. The example here is a hand-pruned vineyard on a constant 6 foot high trellis. The second line shows some alternate canopy architectures. Machine pruning tends to generate a greater canopy density along the top wire than hand-pruning. Consequently, the side curtain may be less developed than in hand pruned vineyards for a similar sized vine. Both vines (b) and (d) are 2.5 lbs. vines but the NDVI response from imaging low down on the canopy side curtain may be relatively lower in the machine pruned vineyard. This is only due to the difference in canopy architecture between hand and machine pruning, not a difference in vine size or productivity. The same can be seen for vines (c) and (e) that are both 3.5 lbs. vines but again differ in pruning management and NDVI response. A change in the trellis system will also affect the sensor response. Vines (b) and (f) are both 2.5 lbs vines, however the trellis height for vine (f) is shorter so that the relative position of the canopy sensor is closer to the top wire. As a result, vine (f) presents more foliage and an apparently higher NDVI for the same size vine with the same pruning management. Note also the difference in NDVI response between the taller trellis machine pruned vine (d) and the shorter trellis hand pruned vine (f). Both vines (d) and (f) have the same pruning weight.
Management will affect the relative sensor response.
The patterns in the NDVI maps should only be interpreted within a single block or between blocks that have the same management!!
Comparing the NDVI response between blocks with a different variety, different pruning regime or different management is very difficult and can lead to misinterpretation. It should not be done. The main interest is to look for patterns within a block or within uniform management areas that indicate where vine vigor (size) is relatively small or large. If desired, the canopy sensor data can then be calibrated to an actual vine size by taking pruning weight measurements within the block. Otherwise, the patterns in the NDVI can be used to target sampling (soil, petiole etc) to identify production limiting factors in the low vigor areas and (hopefully) remedy these factors.

The canopy sensor data tells you where to look to identify problems in the vineyard. It does not tell you why vine vigor is low. Some agronomy and management expertise is needed to interpret the patterns and develop a management strategy for the observed variation in vine vigor.