Modern knitted or woven nets may have a square, hexagonal or diamond-shaped mesh and are all made from woven monofilament strands without knots. The most common nets are made from high-density polyethylene (HDPE) and these have a good durability. These nets have a manufacturer’s warranty (in some cases) of 10 years. However, given Australia’s higher incidence of UV than in Asia or Europe, we believe an effective life of around 8 years is appropriate. Use nets with woven salvaged edges, which give extra strength and ensure the net material will not unravel. The salvaging normally forms an eyelet that a wire can be threaded through for easy attachment to the support wires or cables
Choice of mesh size usually depends on three factors
1. They type of pest, or pests, you wish to control.
2. Your ability to manage any changes to orchard micro climate that may occur after netting the orchard (nets with smaller mesh sizes have a great impact on the orchard microclimate than nets with larges mesh sizes).
3. Financial factors: Nets with a small mesh size are more expensive than large mesh nets. In addition, they are heavier and some can hold hail (and therefore may need stronger, more expensive structures to support them).
Net with a 40mm mesh size will exclude larger birds (such as lorikeets, parrots, crows, ducks), but smaller species (such as silver-eyes) will only be excluded by nets with a mesh size of 15mm or less.
In the past, other types of net have been used to protect orchards. The nets may still be available, but they are not as suitable as knitted nets for orchard netting. Extruded nets consist of a grid of extruded plastic. Extruded nets are cheap, but not as strong as knitted nets, and do not last as long. Knotted nets are made from monofilaments twisted together to form multifilament strands (rope), which are knotted together to form a square or diamond mesh net with a knot at each corner. In our opinion knotted nets are better than extruded nets but not as good as knitted nets.
Traditionally, white nets were cheaper than black nets but had a life expectancy about two-thirds of black nets, the carbon black allowing greater UV Stabiliser and had the best durability. However, modern white nets incorporate chemical UV stabilisers that do not colour the material have comparable life spans to black nets. White nets stand out more and may act as a physical barrier as the pests can see them easily. A recent example of this, is a customer who included chickens in his orchard structure, which attracted eagles. The eagles damaged the roof netting while trying to attack the chickens. When the customer threw a white net over the roof, the eagle stopped Whereas black nets disappear into the background and may be more pleasing to the eye.
Net panel widths should coincide, if at all possible, with tree row widths or multiple of these. Panels are usually 10-15m wide but can be wider. Wider nets are generally favoured where available as they reduce installation costs because there is less joining, however very wide nets can be difficult to handle when erecting. With flat canopy designs, panel widths greater than 10m may require a support wire down the centre of the panel to reduce sag.
Plan the position of doors carefully for efficient access. Doors can be as simple as a curtain drop of net with a pipe weight at the bottom. A more elaborate hinged-gate design, consisting of a metal frame covered with the net material, can also be used.
Quad nets have a main mesh plus strands of monofilament that stretch across the apertures of the main mesh and reduce the hole size. The strands of monofilament are known as cross-stitches . Hence, Quad nets have quadruple cross-stitched nets or (four cross stitches). Cross stitched nets have a major advantage – the hole size of a large mesh net can be reduced without adding too much weight or excessively reducing light transmission through the net.
In areas where hail storms are less regular, flat canopy netting is used. The net catches the hail and stops it damaging the crop. With this design the net gives under the weight of the hail and stretches (usually returning to tension after the hail has melted). The cables holding the nets are positioned over the tree rows and the nets sag into the inter-row space between tree rows, thus minimising damage to trees.
Nets with small holes can limit access or exclude bees and other pollinators. Bees are able to pass through holes as small as 12mm diameter and bird exclusion nets (which have mesh sizes greater than 12mm) are unlikely to limit bee movement in and out of the orchard. Bees usually fly up and down though the canopy net when entering or leaving an orchard, than going through the side netting (P Warhurst 1998). In orchards enclosed by nets with a mesh size smaller than 12 mm, beehives can be successfully introduced during the flowering period. or alternatively, sides can be designed to left lift up.
Department ofPrimary Industries and Fisheries
To net or not to net
By Peter Rigden