Net essentials

What you need to know to get started. By Anna Mouton.

Pome and stone fruit production under nets is surging — but are nets appropriate for all orchards? Fresh Quarterly spoke to six people across the industry about their practical experience of using nets.

Do you need nets?

“You need to determine the commodity and the purpose. You can put nets up for hail and sleet, or for sunburn, or for wind damage,” says Charl Stander, agricultural consultant at Franschhoek Marketing. “The pioneering work on nets was done in the northern parts of South Africa, primarily for hail in peaches and nectarines.”

Most people agree that nets are essential in areas where hail and sleet occur. “If you don’t erect hail nets, you may as well give up,” asserts Daan Brink, technical adviser at Fruitmax Agri.

Hail nets tend to be fixed nets — these are nets that are supported by permanent structures. The nets are usually pitched over the rows to encourage the hail to fall to the ground instead of accumulating on the nets.

Brink contrasts nets for hail with nets for sunburn. “You want hail nets to transmit as much light as possible because they’re usually covering high-value crops for most of the season, and you don’t want to influence the red colour. For sunburn on green varieties you want a dark net so you can prevent as much sunburn as possible.”

Sunburn can reduce the yield of export-quality fruit to as little as 40% in susceptible cultivars such as Granny Smith. “Put a net up and you can shift your pack-out to 85%,” says Brink, “and your fruit quality improves dramatically. The fruit is greener.”

Nets for sunburn can be either fixed or draped. Draped nets are — as the name suggests — draped over trees instead of being supported by posts and wires.

Sunburn can also affect plums but the primary driver for using nets in plums is often wind damage. “I’ve erected about fifteen hectares of nets to prevent wind damage,” remarks horticultural adviser Petru du Plessis of his own orchards in Stellenbosch. Nets have increased his yield of export-quality fruit by 10%–12%.

What are the challenges?

Cost is an obvious drawback. But there is general agreement that nets pay for themselves when they are used appropriately. Horticultural adviser Anton Müller shares data from trials he conducted with draped nets on Granny Smith apples.

“If your yield was around sixty tonnes, you could make an additional R100 000.00 per hectare with your nets, which is roughly the cost of the draped nets You pay for the net in one year and it lasts twelve to fifteen years. The question isn’t how much it costs, but how much money I’ll get back.”

Orchards under nets require careful management as the trees grow more vigorously. This is a potential advantage when establishing orchards under fixed nets. “The growth you get from your trees is substantially better,” says Brink. “The day that the trees come into production, you have a higher production because the trees are larger.”

The downside of too much growth in older trees is reduced yield. “Farmers who are considering nets must implement very good summer pruning,” advises Du Plessis. “I think anyone who uses nets and doesn’t have optimal summer pruning will eventually run into fertility problems.”

The effect of nets on colour development can be positive — think greener Grannies — or negative. Brink warns that you sacrifice red colour in bicolour apples, although less so with white or translucent nets. “Your production gains have to be enough to justify that loss of colour,” he says. Some growers also report that apples are slightly smaller under nets. Brink believes this could be due to over-irrigation, insufficient fruit thinning or increased tree vigour.

Colour development in plums can likewise be less intense under nets. However fruit tend to be larger.

To fix or not to fix

The majority view is that fixed nets are the preferred option for protection against hail and wind damage. Both fixed and draped nets are suitable for controlling sunburn.

Fixed nets are available as so-called retractable nets that can be pulled back to expose trees to full light. Retractable nets are more expensive than non-retractable nets.

Retractable nets are the recommended option for trees that require pollination by bees. “Bees and nets are not compatible,” says Stander. Nets also impact the quality of flower buds, and this can lead to reduced fertility and yields over several seasons.

Stander advocates retracting the nets immediately after harvest and leaving the trees exposed until pollination is complete. “If you have fixed nets that can’t open, they need to be fairly high,” cautions Stander. This allows room for bees to move over the rows.

Growers who experience snow should also invest in retractable nets. “We made the mistake of not having retractable nets where there’s snow,” recalls Willie Kotze, technical adviser at Dutoit Agri. “You have the risk of the whole structure collapsing. That’s not much fun.”

Draped nets are supported by the tree itself and don’t require an expensive structure to keep them in place. So could they be a better option than fixed nets for sunburn? It depends, says Müller. “If you’re in the Grabouw area there’s no real reason for a fixed structure. You only need to control sunburn at certain times of the year. But for hail you need nets for a much longer period.”

Draped nets are especially valuable for older trees. Müller recounts that his initial research into draped nets was prompted by old Granny Smith trees. “Their harvest was lost because of sunburn, and everyone was saying you should take them out. But the market is clamouring for Grannies. With a net we can keep those orchards for many more years.”

Does net colour matter?

Professor Wiehann Steyn, research and development manager at Hortgro Science, recommends that growers stick to monochromatic nets — black, grey, white, and translucent. The difference between these shades is the amount of light they let through.

Steyn stresses that growers need to be clear about the problem that the net is intended to solve. Is it sunburn on Grannies? Consider a black net. Worried that a black net will reduce the productivity of your Goldens? A grey net may be a better option. And for damage that isn’t caused by light — for example hail — look at white or translucent nets.

“The one thing we’ll have to keep in mind in future is the environmental impact,” says Steyn, “the visual impact.” The Italian province of South Tyrol allows only black nets for aesthetic reasons. “Black is a little extreme for our blush cultivars. Grey is a potential compromise between the visual disturbance of white nets and too much shade cast by black nets.”

Pest control under nets

“If there is a pest and you cover it with a net, it will cause problems,” warns Brink. “You need more monitoring. But you don’t necessarily need to spray more.” Fixed nets improve spray conditions and can facilitate pest control. It is possible to spray effectively through draped nets by using higher air volume settings.

Fruit flies and codling and false codling moths appear to be less numerous under nets.

Growers need to be aware that chemicals break down more slowly under nets. “The market is fanatical about residues,” cautions Stander, “so you must adapt to ensure that you stop spraying a few days earlier.”

Stander also raises concerns about potential residues in draped nets. “If the spray accumulates in the nets and it touches the fruit it could leave a residue.” Similar problems have occurred in the past with chemicals transferring from crates to fruit.

What lies ahead for nets?

Everyone agrees that the number of orchards under nets will proliferate further. But, as Kotze points out, there are some constraints. “This is a growing industry and there’s a shortage of contractors to erect nets. And there aren’t enough dwarfing rootstocks.”

Kotze explains that the success of apple trees under nets relies on dwarfing rootstocks. “We tested nets in the past, but we could never control the growth of the trees and we lost productivity. Now that we’re on the right rootstocks we can put the trees under nets.”

He describes the next challenge as sourcing trees in the numbers that he needs to establish new orchards under nets. “We’re reliant on the right rootstocks and this is likely to limit the expansion.”

Climate change is set to boost net use even more. Everyone agrees that nets lower the water requirements of trees — but by how much? Work done in Australia found reductions of up to 25% but a study funded by Hortgro, and the Water Research Commission indicate savings of only 12%. “This was in the mild Witzenberg valley,” reports Steyn. “Savings tend to be greater under more extreme conditions.”

Stander is positive about the expansion of nets. “Nets will play a much more prominent role, given the drought in the Klein-Karoo and the growers that need to generate income. If water is limiting your new plantings, then you have to increase the yield of what you have. You have to increase your pack-outs and how do you do that? By reducing wind damage and sunburn.”

Image supplied by Wiehann Steyn | Hortgro Science.