Figure 1 Hail Net over blueberries in Yarra Valley, VIC (Source: N Mann – October 2014)
Hail net is high tensioned mesh similar to shade cloth that is manufactured in various strengths, thicknesses and fibrous materials. The net is designed to hang suspended above the crop on wooden or steel poles. The net’s purpose is to protect the crop from hail damage first and foremost, however it can also prevent excessive wind damage to the crops. Unlike bird netting, a hail net does not prevent bird access to the crop and fruit. Although it can act as a deterrent. The netting above the crop also creates a form of ‘micro-climate’ in that it maintains a slightly cooler temperature in hotter conditions and a slightly warmer temperature in colder conditions. It also slows down the air movement creating a slightly more humid environment beneath the net.
In hail prone regions, hail netting is necessary otherwise the berry crops, flower buds, leaves and fruit can be damaged or lost completely resulting in little to no harvest and thus the grower’s return. Hail net does not prevent rain and the berry plants may still become wet and damage may occur from too much moisture. Harvesting may also be disrupted due to wet conditions which is disadvantageous.
Bird-netting is the most common form of protection on extensive production of blueberries as damage from birds can cause incredible crop losses affecting the yield and potential income of the grower.
Different coloured bird-netting is used around the globe and the selection of colour is usually a purely economic one. However, according to Retamales and Hancock (Hancock, 2012) blueberries can adjust their physiological processes and change to varying light levels offered by different shade-netting. There are advantages to selecting the correct colour and percent of shading for the specific climatic conditions and latitude that a grower is growing within and although fruit quality is not affected – there can be significant benefit to sustained increases in fruit yields (fruit weight) and the delays in fruit maturity which can be advantageous and profitable for growers (Lobos et al 2009).
The results from studies (Lobos et al, 2004) indicated that 50% white or 35% grey shade-netting had the best results because although 50% red net gave could results too – its colour greatly impaired the fruit colour. The 50-70% black net had the lowest impact on the yield and quality of the highbush blueberries. (Retamales et al 2004) In another study on Optical Manipulations of Insect Pests for Protecting Greenhouse Crops by Dr. David Ben-Yakir of ARO, The Volcani Centre, Israel presented at the Canadian Greenhouse Conference on 8 October 2014– found that yellow and pearl coloured netting attracted the insects likes aphids and thrips but these insects never penetrated the net to access the crop but remained on the cladding whereas there was more insect penetration under black and red net.
Like hail net, bird netting does not prevent losses from rain and wet conditions which causes diseases like botrytis and harvest may be disrupted due to wet fruit and labour unable to work in the rain. Most netting costs between $35,000-$50,000 per hectare for cables, poles, netting and construction. (Wilk, 2014)
Figure 2 White Bird Netting Over Blueberries (Source: N Mann New Zealand December 2014)
Low tunnels are another basic type of protection for berry production and mostly used in outdoor production of strawberries. The low tunnels are either made out of fleece or plastic. The protection may only be draped over the crop, as the covers are light and not much damage is sustained by the weight of these covers. Alternatively small hoop structures (either steel or bamboo) are erected over the beds and the plastic or fleece is secured over the hoops and suspended above the berries. The main purpose of low tunnels is to protect the crop from frosts, rain, birds and insects especially when it is at an immature and delicate stage of growth. This method of low tech protection is to assist with and brought to an advanced stage to coincide with ideal conditions, when this occurs these covers are removed and the strawberries flourish in the natural environment. Again the protection is used for timing and to enable growers to come into production earlier. Low tunnel technique is also adopted in some high tunnel structures. For example in Scotland and parts of the UK – it was observed that fleece covers were placed on young berry crops to maintain as much temperature as possible and protect the berry crops from the extreme cold which could damage or further delay spring growth when the temperatures began to warm up.
Another use for low tunnels is in organic berry production to protect against pests whilst the crops are immature and just establishing themselves – during this period the young berry crops can be susceptible to invasion or infection.
Figure 3 Low Tunnel Fleece covering young raspberry plants. (Source: N Mann United Kingdom April 2015)
This is the most commonly used greenhouse structure around the globe for protected cropping of berry crops. These structures are relatively easy to erect; require little to no planning approval as they are generally considered temporary structures; they are fairly robust in nature and low cost per m2 compared to multi-span greenhouses and glass greenhouses. There are numerous suppliers of high tunnels around the globe and there are just as many variations to the height, width, length, steel used, plastic thickness and gutter options of these tunnels. The level of the sophistication of the high tunnels varies greatly and it was observed that anything and everything is possible within these types of structures, some are fitted with automatic sides, doors and top opening vents powered by solar power to other tunnels just being the most basic with plastic covers manually attached and removed depending on the season and what outcome the grower is trying to achieve. Most high tunnels are used to keep the berry crops dry so that fruit does not get wet and damaged from fungal diseases like Botrytis cinerea. Also fruit can be harvested regardless of rain outside the tunnel which would otherwise mean harvesting may have to be delayed until the crop dried.
The tunnels may deter birds but unless they are totally sealed birds may still access the crop and berries causing extensive damage and losses. Some growers have put bird netting over the entire high tunnel structure at great expense to maintain a bird exclusion zone.
Figure 4 High Tunnel Structures over Strawberries. (Source. N Mann Spain March 2015)
Figure 5 White Bird Netting Over the Entire High Tunnel of Strawberry Crop. (Source: N Mann New Zealand December 2014)
Figure 6 High Tunnel With Plastic Covers Removed. (Source. N Mann Scotland April 2015)
Growers have the option to remove the covers to slow the crop down and delay the harvest period by maintaining the crop in cool conditions for as long as possible. The same technique may be used at the end of the season to stimulate and force the crop into winter dormancy earlier by removing the covers as the weather cools.
Figure 7 Solar Powered Slide Opening of High Tunnel. (Source: N Mann with Laura from Haygrove UK April 2015)
Figure 8 Laura from Haygrove UK demonstrating manual side opening of High Tunnels. (Source: N Mann UK April 2015)
There are steps to improve the control of high tunnels with electric and solar-powered systems. Computer-controlled equipment to open and close the sides, doors and vents (if available) is beneficial because if this is done by hand these can be slow, tedious and suck up man-hours which in countries like Australia is not realistic.
Cost for the most common high tunnels from Haygrove® and Quiedan® usually range between $80,000 – $85,000 per hectare including the supply and erection on site. (Wilk, 2014) Which begs the question of why grower do not invest the extra money for high tunnel over netting when the benefits are:-
- 35-50% increase in yield of Class one berry produce
- decrease the risk of severe weather events
- increase labour productivity as no weather disruptions to work activity
- decrease pesticide and fungicide use
- earlier crop development
- save water by reduction in evapo-transpiration
Telescopic tunnels are a variation on the normal high tunnels in that the legs of the tunnels can be extended or retracted depending on the season and how the grower is wanting to manipulate the crop. Basically the entire height of the tunnel can be lowered or heightened. Used mainly going into winter to maintain temperature of the berry crop as close to optimal temperatures as possible with minimum stress from severe cold conditions, the structure is lowered totally just above the crop. Incidentally, there some growers who have their table-top strawberries on telescopic table-top stands too – thus enabling the grower to lower the height of the entire crop to ground level where it is warmer in the extremely frosty months. In conjunction with the lowering of the tabletops and the entire structure– fleece covers can be placed on the crop as an added insulation and protection strategy against the cold. Once the conditions begin to warm up the structure may be gradually heightened to maximise the growth and warmth within the structure giving more control of the rate of growth and keeping temperatures and humidity as optimum as possible. The only disadvantage of telescopic tunnels and benchtops is that they are very labour intensive if the system is not automatically controlled. Approximately 50 man hours per hectare for lowering using a team of 18 people and 59 man hours per hectare to lift the telescopic tunnel for a team of 18 people. (Haygrove, 2015) With all this effort the crop can be advanced by a maximum of 2 weeks which could be mean a significant difference in return for the grower (Laura, 2015)
This type of control and manipulation of climate can be easily attained from a more sophisticated greenhouse structure but at a higher cost initially but there would be significant labour saving in the long run.
Plastic Tunnels with Shade Netting
In Portugal at Sunshine Fruit in the Algave it was observed that 40% shade-netting was put over the plastic high tunnels for the 3-4 months in the height of summer to increase the quality of the raspberries which would have otherwise suffered damage from pro-longed exposure to high radiation in the long summer days which can cause white/bleached drupelets. (Hall, 2013). Hugo from Sunshine Fruit said the 40% shade-netting could go on top of the plastic tunnels as soon as temperatures reached 27⁰C as there was adequate daylight hours and the extended day-lengths had an adverse effect on the quality of the raspberries (soft skins and pale fruit) therefore the shade-netting was essential in this region. (Hugo, 2015)
“A greenhouse is one part of an integrated intensive plant production system. It is a critical component that dictates the success or otherwise of the business enterprise. A successful greenhouse based business is dependent on achieving the right balance between the horticultural, engineering and financial aspects of the facility.” Explains Geoff Connellan at the PCA Conference 2015. (Connellan, 2015)
Mexico is a good example of a country that is embracing protected cropping as shown in Figure 15 below. (Olson, 2013)
|BERRY||Protected agriculture share of production||Ave. labour requirement (worker/ha)|
Multi-Span Plastic Greenhouses
Although not commonly setup for growing berry crops initially, there is a trend in various countries where small and medium sized multi-span plastic greenhouse growers are adapting these greenhouses to grow berry crops from previous crops of flowers, cucumbers, capsicums or tomatoes which are becoming less viable for the small to intermediate growers (Mann, 2015).
Growers are attempting to grow strawberries, blueberries, raspberries and blackberries in these semi-sophisticated structures with positive results. There is greater air volume as most of these structures are at least 4 – 5 metres from ground level to the gutter with the more advanced structures being at least 6 meters to the gutter. A larger volume of air has the advantage of gradually increasing or decreasing in temperature as opposed to a smaller air volume that rapidly spikes in either situation. There is more control of the climate within these structures due to computer controlled automated vents, shade-screens, fans and other equipment common in these types of structures – some of the structures visited have inclusions such as pad and fan systems for added cooling, hydronic heating, fogging systems, etc – giving greater control and better manipulation of the environment to maximise yields and minimize stress through less disease pressure and thus increase and intensify production and yields. Birds are not problematic for the berry crops cultivated in these structures; IPM strategies can be successfully implemented providing greater control of pests without pesticides; crops do not suffer scorching; and pollinators (with the help of landmarks) can be very effective too within these improved structures. Opening vents and sidewalls (with birdnet already in place) can be advantageous for humidity and freedom of the pollinators to forage without hindrance.
Cost per m² ranges between A$85-A$100 but this varies considerable between suppliers and what is inclusion are offered within the structure and the quality of the steel and plastic. There are options to get much cheaper imported structures from places like China and to purchase second-hand structures which could greatly reduce the cost per m².
Figure 10 Seven Month Old Blueberry Bushes in Croft Multi-Span Greenhouse. (Source. N Mann NSW Australia August 2014)
State of the art technology glass greenhouses are being used in various parts of the world (where adverse weather conditions and or seasonal change can be severe) to produce top quality, consistent crops of strawberries. These are high investment projects and in the Netherlands at Van Gennip Kwekery (photographed below) it was explained that the return on investment is targeted at 15 years. The technology included in this type of growing environment are:- hydronic heating, hanging gutters, pipe-and-rail, shade-screens, energy screens, supplementary lighting, CO2 enrichment, foggers, precision irrigation, filtration and sterilizing system, recirculating and recycling of water and nutrients, air-flow fans and energy buffer tanks.
€300m² is a realistic cost for this type of structure and contents inclusive of the substrate, plants and their royalties. Control of the climate is precise for manipulating and creating optimum growing conditions to maximum plant health, development and eventual yield.
The response from the supermarkets to the production of strawberries from these projects has been extremely positive as supply is consistent, quality is first class and spray residues are minimal to non-existent.
Figure 11 Glass Venlo Greenhouse. (Source: N Mann The Netherlands March 2015)
Plastic Retractable Greenhouses
“Retractable roof greenhouses will cause a rewriting of crop management strategies, as well as guidelines for how greenhouses are built and where they are built.” (Vollebregt, 2002)
Plastic retractable greenhouses are becoming a popular option globally albeit expensive for the berry sector and demonstrates a lot of potential. In particular, countries such as Australia stand to benefit immensely from this option due to a unique situation. Bumble bees are neither native nor currently present on mainland Australia and due to stringent quarantine regulations, they are forbidden from any form of introduction onto Australian shores. There are currently feral bumble bee colonies in Tasmania – but strictly prohibited for commercial breeding and utilisation. Bumble bees are globally acclaimed as superior commercial crop pollinators (Koppert Biological Systems, 2015) and unfortunately due to their “non-existence” here, European honey bees are an alternative option. (Steen, 2015) These European honey bees are ubiquitous in Australia but have been noted for their poor navigation when present under plastic-enclosed greenhouses. This retractable greenhouse provides an ideal solution to overcome this pollination issue, as the movable roof vents and side-walls fully withdraw to provide and allow for natural, direct sunlight to fully penetrate the greenhouse berry crop and environment creating no adverse impact on the European honey bees. (Tjeerd Blacquière, 2006)
There are also studies that support the full spectrum of sunlight and direct UV within a greenhouse can destroy some fungal diseases like botrytis and powdery mildew by reducing the “half-life” of the spores. (Len Tesoriero, 2015) Another advantage of extensive roof opening is that humidity can be reduced significantly and quickly allowing more air movement around the berry crop with the hot humid air exhausting and replaced by denser cold air. This reduces fungal diseases from sporulating in high humid conditions which is a common occurrence in normal enclosed greenhouses. (Len Tesoriero, 2015)
Benefits of retractable greenhouse growing systems (Powerplants, 2015):-
- Reduction in fertilizer usage – as roof tops close during rainfall reducing leaching of nutrients
- Reduction in risk from catastrophic loss – from major disease outbreak because crop management is supreme; from extreme weather even cyclones with wind speeds up to 200kph as the rooves and sidewalls retract in minutes;
- Reduction in fungicide use between 50-100%
- Reduction in growth regulator usage between 50-100%
- Reduction in pesticide use between 10-100%
- Reduction in herbicide use between 50-100%
- Increased life of the roof covering
Figure 12 Hydroponic Strawberries. (Source: Cravo Website 2015)
The retractable greenhouses made from polycarbonate are extremely robust structures and able to withstand severe wind and climatic conditions which plastic structures cannot tolerate. The vents are vertical opening allowing rapid release of humidity and increased air exchange as well as allowing natural light to enter the crop. An added benefit from retractable greenhouse rooves is the exposure of the plants to natural light which stimulates the strengthening of the cell cuticles of the plants to become stronger and more resilient to disease as they are less permeable and susceptible to infection. “Retractable roof greenhouses appear to optimize both rate of photosynthesis and the reduced levels of beneficial plant stress to ensure that it develops the strength, disease and insect resistance not present when plants are grown inside conventional greenhouses”. (Vollebregt, 2002) Additionally, “plant responses in retractable roof greenhouses include stronger root systems, reduced internode lengths, thicker cuticles, fewer root and foliar diseases, fewer insect pests, and less stress and show less shock following transplanting.” (Vollebregt, 2002)
Figure 13 Polycarbonate Greenhouse with Retractable Roof. (Source. N Mann Watsonville, USA October, 2014)
Considered the “Rolls Royce” of protected cropping structures – the Glasshouse with a Retractable Roof gives the grower the ultimate in climate control – these structures are fitted with all the latest growing tools like hydronic heating, pipe and rails, shade screens, energy screens, foggers, misters, CO² injection and fans. An added advantage is the ability to get natural sunlight into the structure and onto the crop and to exchange air quickly with the outside environment. A disadvantage of this system is the access from birds and insects is made possible through the open roof tops. Incidentally some beneficial insects and pollinators may escape through the rooftops too – this can be counter-acted with bird or insect netting.
Figure 14 Glass Greenhouse with Retractable Roof. (Source: N Mann Ontario, Canada 2014)