Can shade nets reduce water use?

Research shows that netted apple orchards need less water – but growers should read the fine print. By Anna Mouton.

Deciduous-fruit growers are adopting nets to protect their crops against everything from hailstorms to sunburn. Can nets also help trees use less water? And how much less? A project funded by Hortgro and the Water Research Commission provides some answers.

The study was conducted by a team of researchers, including Dr Theresa Volschenk of the ARC Soil and Water Science Programme and Dr Elmi Lötze of the Department of Horticultural Science at Stellenbosch University, among others from Stellenbosch University, the CSIR Hydrosciences Research Group, and the Western Cape Department of Agriculture. Edward Lulane was the PhD and Stephen Jordaan was the MAgric student on the project.

It was initially led by Dr Stephanie Midgley, currently Climate Change and Risk Scientist at the Western Cape Department of Agriculture. Dr Sebinasi Dzikiti of the Department of Horticultural Science at Stellenbosch University led the project in its final year. Fresh Quarterly spoke to Dzikiti about their results.

Two of everything

The researchers examined full-bearing apple orchards of two cultivars in two regions and under two types of netting — see Table 1 for details. A section of each orchard was netted, and the unnetted section was the control.

Table 1: Summary of the orchards and treatments.

The fixed nets in the Rosy Glow orchard had been in place since 2014. The draped nets in both Golden Delicious orchards were applied after fruit thinning in early summer and removed at harvest. All the orchards had micro-sprinkler irrigation.

Data were collected from September until complete leaf drop in July each season. The researchers measured shoot and fruit growth rates, tree sap flow, leaf photosynthetic and transpiratory gas exchange, leaf area index, and leaf and stem water potential on a representative sample of trees. Fruit yield, maturity, and quality were recorded after harvest.

The data also included cover-crop transpiration, orchard-floor evaporation, soil water content, and irrigation volume. Each orchard had an automatic weather station to monitor the microclimate.

These data were used to calculate water consumption in netted and unnetted orchards — transpiration for the trees and evapotranspiration for the trees and orchard floor together — and to figure out the reasons for any differences.

Fixed white netting

The same Rosy Glow orchard was monitored for two seasons. In both, the trees under the fixed nets transpired less than those in the open — see Figure 1.

Figure 1: Total tree transpiration and yields for a netted and unnetted section of a Rosy Glow orchard in two seasons.

“Tree transpiration rates differed because the microclimate was moderated by the nets,” says Dzikiti. “Available water was optimal in most cases. If the tree has enough water, but the microclimate is milder, then transpiration rates are going to be lower.”

Yields did not show a consistent pattern, but fruit under nets were larger in both seasons.

In both seasons, the pack house reported 5% sunburn and 4–7% hail damage in the unnetted fruit, whereas the netted fruit had about 1% each sunburn and hail damage. But it was not all good news for nets — red colour was significantly poorer, but this did not negatively affect the pack out.

The researchers compared physical and economic water productivity for netted and unnetted trees. Figure 2 shows water productivity calculated from tree transpiration rates without taking water lost from the orchard floor into account. Netted trees consistently yielded more fruit mass and income per unit of water than trees in the open, despite the decrease in red colour.

Figure 2: Physical and economic water productivity based on total transpiration for a netted and unnetted section of a Rosy Glow orchard in two seasons.

One challenge with analysing the data was that the netted and unnetted orchards received different irrigation. “The grower was giving more water under the nets because he observed that the tree growth was more vigorous,” says Dzikiti. “The irrigation application was matched to the growth.”

Dzikiti thinks it should have been the other way around. “Even though the netted trees are growing more vigorously, their water consumption is lower than in the open, so they should get less water.”

Draped black netting

The draped-net trials took place in Golden Delicious orchards in two different areas. Each orchard was monitored for one season.

Tree transpiration was similar in the netted and unnetted trees in one orchard and much lower in the netted trees in the other — see Figure 3. The researchers suspect they may have underestimated transpiration values for the Koue Bokkeveld orchard due to technical difficulties.

Figure 3: Total tree transpiration and yields for netted and unnetted Golden Delicious orchards in two seasons.

Yield and fruit size were not significantly different between netted and unnetted trees in either draped-net trial. However, the pack-house data for the Koue Bokkeveld indicated a higher yield and smaller fruit from the netted trees. Pack-house data were not available for Villiersdorp.

The netted fruit had a slightly better green colour and less sunburn at both sites, and netted fruit from the Koue Bokkeveld also had less hail damage. Class 1 pack-outs were about the same for netted and unnetted fruit from the Koue Bokkeveld. Fruit maturity was advanced under nets at both sites.

Dzikiti thinks the shade factor may have been too high for Golden Delicious with high crop loads. “The nets were cutting about 30-40% of the light under the draped nets — I think that overdid it.”

For the draped-net trials, water productivity was calculated based on total tree transpiration and total evapotranspiration. Whereas tree transpiration only measures the water directly used by the tree, evapotranspiration also takes other losses into account, including transpiration by orchard-floor vegetation and evaporation from the soil.

Figures 4 and 5 show the two sets of water-productivity numbers. Note that the scale of the charts is the same. Productivity based on evapotranspiration is lower because it includes evaporative water losses.

Figure 4: Physical and economic water productivity based on total transpiration for netted and unnetted Golden Delicious orchards in two seasons.

Figure 5: Physical and economic water productivity based on total evapotranspiration for netted and unnetted Golden Delicious orchards in two seasons.

In both orchards, water productivity based on transpiration was higher for the netted trees. But physical water productivity based on evapotranspiration was very similar for the netted and unnetted trees in the Villiersdorp orchard in 2020–2021 because of higher evapotranspiration values for the draped-net treatment. This could be due to evaporative losses from the uncovered work row.

The fine print

“I think the take home is that both types of netting tend to reduce water consumption,” says Dzikiti. “Under fixed netting with white nets where you cut off about 20% of the light, yields can be maximised with less water.”

He cautions against over-irrigating under fixed nets. Besides exacerbating vigorous tree growth, it reduces water savings.

The results also showed that micro-sprinklers under fixed nets increase evapotranspiration because of the luxurious vegetation on the orchard floor.

“The cover crops and the weeds tend to grow much more vigorously under nets because the conditions are more favourable,” says Dzikiti. “Management of the orchard floor is very important to minimise evapotranspiration — if vegetation is kept in check, the water savings are much more significant.”

This trial only looked at orchards with micro-irrigation. Drip irrigation will likely reduce evapotranspiration even further because the wetted surface area is smaller, and water is not lost to orchard-floor vegetation.

Dzikiti believes that nets also benefit trees by reducing damage caused by too much light. Photosynthesis is saturated at about 900–1 000 µmol per m² per second of photosynthetically active radiation. But out in the open, the plant receives over 2 000 µmol per m² per second on a bright day in summer.

The problem, he explains, is that the excess light damages the photosystem of the plant and inhibits photosynthesis. When photosynthesis drops, the stomata close and transpiration decreases, increasing canopy surface temperature and carbon losses through high respiration. The excess light also contributes to sunburn.

“Production under full light is probably not best for our circumstances,” comments Dzikiti. “The results say that cutting off about 20% light using a shade net will give you more production with less water.”

In the case of draped nets, results were promising, but there is still work to be done. “We need to find the sweet spot in terms of balancing our light reduction in relation to how much we affect the yield,” says Dzikiti. “To what extent can you cut back on the light in the tree and achieve maybe even better yields than what we’re getting at the moment? That is still a research question out there.”

What are people saying about this research?

“Research on irrigation is a long-term investment for the industry. Everyone forgets that we had a drought, but when the next crisis comes, we need this type of information to make the right decisions.”

Willie Kotze. Technical adviser, Dutoit Agri.