Are water savings in apple orchards under shade netting an additional benefit?

New research project launched to quantify apple water use under shade netting compared to the open.

Stephanie Midgley (1), Sebinasi Dzikiti (2), Theresa Volschenk (3), Elmi Lötze (1), Mark Gush (2)

1 Department of Horticultural Science, Stellenbosch University
2 Council for Scientific and Industrial Research, Natural Resources and Environment
3 Soil and Water Science Programme, ARC Infruitec-Nietvoorbij, Stellenbosch

Figure 1. ‘Rosy Glow’ Orchard of the Future at Paardekloof, Witzenberg Valley, under shade netting.
The water use of this block and the adjacent block in the open will be quantified and the water use savings calculated.

The installation of shade netting over apple orchards is gaining momentum in South Africa. The high costs can now be offset by the benefits to fruit quality and profitability in many instances. The surge in interest in this technology is driven by the need to reduce the damage caused by stressful climatic conditions that lead to sunburn, wind marks, poor green colour in green and yellow cultivars, and undesirable pink blush in ‘Granny Smith’. Given the water crisis and efforts to increase water use efficiency in apple production, the potential water savings under netting may provide an additional reason to promote and consider shade netting technologies. This article aims to inform apple growers of a new four year research project which will quantify the water savings under both fixed and draped netting types.

Why should netting reduce water use in apple orchards?

Through reductions in direct solar radiation, air temperature and wind speed, and sometimes increases in air relative humidity, the microclimate under shade netting is milder (Smit, 2007; Mupambi et al., 2018). This can lead to increased stomatal opening and photosynthesis, but the lower vapour pressure deficit (VPD) between the leaf and the air usually leads to lower water loss from the leaf. Consequently, whole tree and orchard water use is potentially reduced. Although most of the science suggests a high likelihood of water savings under netting, this is by no means a given. In a scenario of very vigorous vegetative growth, or certain changes in atmospheric conditions and soil temperature/moisture, it is possible that much smaller (or no) reductions in tree water use could result. A comprehensive review of the scientific literature (Midgley et al., 2018) has confirmed that very little field-based research has been conducted globally or in South Africa on changes in tree and orchard water use under netting. Specifically, sap flow measurements have not been reported, and neither have the effects on orchard floor evaporation or cover crop transpiration, which are important components of orchard evapotranspiration, especially in younger orchards. Furthermore, no studies have calculated physical (kg fruit per m3 water used) or economic water use productivity (rand income per m3 water used) of apple production with and without netting. Interviews with local technical experts and advisors in the apple industry confirmed that they have very little science-based information on which to base decisions and advisory to clients, and possible water-use benefits are not yet included in the sums. A study which has just been completed (2014 – 2018), jointly funded by the Water Research Commission of South Africa (WRC) and Hortgro, has determined the water use of high-yielding ‘Cripps Pink’ and ‘Golden Delicious’ orchards in the Koue Bokkeveld (KBV) and the Elgin-Grabouw-Vyeboom- Villiersdorp (EGVV) regions (Dzikiti et al., 2018). In-depth measurements of both components of water use (transpiration and evaporation) and the development of a water use model have paved the way for further studies and application of the model to practical technologies such as netting.

Figure 2. Apple tree instrumented to continuously measure sap flow, from which whole tree water use can be calculated for the growing season.

The new research project and its objective

The WRC and Hortgro have again pooled their resources to fund a research project to quantify water use savings under shade netting. The study will be conducted over four years (2018 – 2022) in the KBV and EGVV regions on both fixed and draped netting systems. The open/fixed netted site will be the ‘Rosy Glow’ Orchard of the Future at Paardekloof (KBV), specifically the section under 20% white netting. This orchard will be studied for the first two seasons of the project. The open/draped netted sites will be selected from suitable full-bearing ‘Golden Delicious’ orchards in both production regions. Each orchard will be studied for one production season, 2019/2020 for KBV and 2020/2021 for EGVV. The project’s overall objective is to compare water use of a high producing open and netted (fixed and draped) full bearing apple orchard under optimal management and unstressed water use conditions, in order to determine water savings per hectare and per ton.

Figure 3. Apple tree row under drape netting in the EGVV region.

Figure 4. The seasonal tree and orchard water use of ‘Rosy Glow’ will be quantified.

How will this research benefit the apple industry?

The deciduous fruit industry will be supplied with information and products which it can use to provide effective guidelines for more efficient management of scarce water resources, in this case specifically by using netting. Growers, technical consultants and netting/ irrigation system suppliers and installers will be able to use the information for decision making on the installation of netting, new orchard plantings with or without netting, and irrigation design and scheduling under netting for optimal production and profitability. The information will also enable Hortgro to develop evidence for its Orchard of the Future programme, which aims to increase profitability through water-efficient and climate-adapted approaches and technologies for the next generation of orchards. The deciduous fruit industry and its water resource practitioners will also draw on this information when engaging with the Catchment Management Agencies and the Department of Water and Sanitation on efforts made to reduce non-productive water use without placing the future of apple farming at risk. The results will be compiled in a Practical Illustrative Guide on water savings achieved through netting, and growers will also be provided with crop factors suitable for apple orchard irrigation scheduling under netting.

Project team

The study is being conducted by researchers from three institutions:

Prof Stephanie Midgley: Dept Horticultural Science, Stellenbosch
University (SU), Project Leader;

Dr Sebinasi Dzikiti: Council for Scientific and Industrial Research (CSIR);

Dr Theresa Volschenk: Agricultural Research Council (ARC) Infruitec-Nietvoorbij;

Dr Elmi Lötze: Dept Horticultural Science, SU;

Dr Mark Gush: CSIR

One PhD and one MSc in Horticultural Science will become part of the team. The research management team is Dr Gerhard Backeberg and Dr Sylvester Mpandeli of the WRC and Prof Wiehann Steyn of Hortgro.

Conclusion

Apple growers who have installed shade netting are encouraged to monitor the water requirements of these orchards and any water savings realised through adjusted needs-based irrigation scheduling. The research team would like to ground truth their scientific findings against such practical evidence. While water savings by themselves likely do not justify decisions to use this technology, they could provide a compelling additional argument for the multiple benefits achievable.

Acknowledgement

We acknowledge funding from the Water Research Commission of South Africa (Project no. WRC K5/2815//4) and Hortgro Pome.

References

Dzikiti, S., Volschenk, T., Midgley, S., Gush, M., Taylor, N., Lötze, E., Zirebwa, S., Ntshidi, Z., Mobe, N., Schmeisser, M. and Doko, Q., 2018. Quantifying water use and water productivity of high performing apple orchards of different canopy sizes. Project no. WRC K5/2398//4), Final Project Report to the Water Research Commission and Hortgro Science.

Midgley, S., Dzikiti, S., Volschenk, T., Lötze, E. And Gush, M. 2018 (submitted). Review of the available knowledge on the potential of fixed and draped netting technology for increasing water use productivity and water savings in apple orchards. Deliverable 2 for WRC Project K5/2815//4. Water Research Commission of South Africa and Hortgro Science, June 2018.

Mupambi, G., Anthony, B.M., Layne, D.R., Musacchi, S., Serra, S., Schmidt, T. And Kalcsits, L.A., 2018. The influence of protective netting on tree physiology and fruit quality of apple: A review. Scientia Horticulturae 236: 60–72.

Smit, A., 2007. Apple tree and fruit responses to shade netting. MSc Thesis. University of Stellenbosch, South Africa.