Historically, growers in tropical Australia have been slow to embrace protected cropping techniques, mainly because temperatures and light levels are suitable for open field fruit and vegetable production. Many growers associate protected cropping to costly high-tech glasshouses with full environmental control, but that is not necessarily the case. In this article, Queensland researchers demonstrate a new way of thinking about protected cropping in the tropics. By STEVEN CARRUTHERS
Greenhouses are used in many tropical regions of the world for the production of vegetable crops. The primary reasons for protected cropping in the tropics are for protection from heavy rain and wind, protection from extreme solar radiation, and pest exclusion. These are somewhat different reasons than for greenhouses in temperate zones, where controlling temperature extremes—particularly low temperatures—is the primary factor in greenhouse designs.
Because greenhouses in the tropics are used for different reasons than temperate ones, their design and construction is also different. An effective design has to be tall (sometimes up to 6 metres high), with a polyethylene roof and preferably have insect exclusion netting for side walls. Passive ventilation can be achieved with roof vents or in designs that have retractable roofs.
In North Queensland, researchers are now exploring the benefits of using low-cost protected cropping systems for high-value crops such as specialty melons and capsicums. Department of Agriculture, Fisheries and Forestry (DAFF) horticulturist and project leader, Dr Elio Jovicich, believes low-cost protected cropping could be the key to opening up diversity in Australia’s melon market.
“Rockmelons and honeydews are the two melon fruit types most commonly consumed in Australia, however, there is room for more diversity in our market,” Dr Jovicich said.
“While it is now possible to find piel de sapo (the Spanish name meaning ‘toad skin’) and small canary melons in supermarkets, other specialty melons such as galia and charentais types are generally absent in Australian markets,” he said.
Dr Jovicich says several growers in North Queensland have started exploring the potential of growing a greater variety of melon types because yields of some specialty melons have been unsuccessful when grown outdoors.
“The use of protective cropping has a high potential for improving fruit quality, increasing yield per square metre, allowing for off-season production and supplying niche markets in the Australian melon market,” he said.
“There are low-cost and effective systems available for warm environments that can moderate extremes of our variable climatic conditions and lead to high yields,” he added.
Early trials using low-cost systems have led to marketable yields up to 2.6 times greater than common yields of rockmelons grown in the open field.
“We have consistently seen results of two to four high quality fruits per plant using protective cropping, giving yields up to 8 kg/m2,” said Dr Jovicich.
“Melons have shorter cropping periods than tomatoes or capsicums, and growers could fit melon crops in between the harvest of these other crop species, or in crop rotation schemes that are compatible with market strategies,” he said.
Proof of concept
As a ‘proof of concept’, small trials are being conducted at Giru, a small town 54 kilometres south-east of Townsville. The structure being used is an existing high poly-tunnel, previously used to grow cucumbers in soilless media. It is a low cost design consisting of two bays—60 metres long, 6 metres wide and 3 metres high—with insect exclusion netting as side walls. The roof is covered with a semi-transparent UV stabilised polyfabric film, which creates some shading and diffuses light over the plants. The cultivation method is an open system with the drainage collected and re-used on an adjacent mango orchard or outside-grown vegetable crops. The only automation used in the trial is an inexpensive timer to control irrigation cycles. The complete nutrient solutions are prepared and stored in large tanks and the irrigation solution is delivered to pots containing pumice rock.
“The setup in Giru has been working well for the grower, but there are several improvements that can be made. When growing melons, we temporarily had to open sections of the sidewalls that were screened to allow the entrance of bees. With these first crops, we wanted to identify, which are major environmental constraints that would appear from growing melons and other specialty crops in the existing system” said Dr Jovicich.
Higher structures will also allow for trellising crops higher while avoiding extreme high temperatures in the crop canopies. In the tropics, tall passively-ventilated structures usually have a sawtooth roof design, which is created by roof vents (a series of vertical surfaces separated by a series of straight or curved sloping surfaces), which assist removing heat. In designs for warm environments, these roof vents remain permanently open but can be screened with insect exclusion nets. The slope of the roof reflects a high proportion of solar radiation away from the greenhouse, and natural ventilation increases when open vents face away from the wind—air flow over the roof causes negative pressure that sucks out warm greenhouse air. This also causes outside air to be drawn into the greenhouse through the open windward side wall, which mixes and cools the inside air before discharging out the roof vents.
There are also other structure designs that look very promising for growing crops in the tropics. For example, the high structures with roofs that can be retracted. In these designs, the roof is a polyfabric film material suspended on wires, and where the film can be folded or extended by electric motors. This allows for having partial or full sun over the crop and a rain protection cover when it is necessary.The roofs can have a slope and gutters to drain water from rainfalls.
In the tropics, the side walls of these structures are large for maximum ventilation, but are covered with insect exclusion screens. Roll-up poly films can be fitted on the side walls; however, all sidewalls will remain open during most of the year.
Sawtooth greenhouses and retractable roof structures are less expensive than glass or polycarbonate greenhouses, and they offer a number of advantages for growers in terms of extreme solar radiation and passive ventilation.
Other specialty crops
In addition to melons, the project has included research on specialty capsicums, cucumbers and eggplants with very encouraging early results. Argentine-born Dr Jovicich says while it’s still early days, past experience and promising results from the recent trials in North Queensland make protected cropping a complementary system to outdoor vegetable production in warm environments.
Before 2007, Dr Jovicich was involved in protective cropping research, development and extension (R&DE) in Florida, US, for many years where he gained experience working with Dr Daniel Cantliffe from the University of Florida.
“With capsicums, we had some crops planted in Giru in April and May last year, and we were still harvesting in early January 2015,” Dr Jovicich said.
“It is the end of January and we are reaching up to 18 kilograms of red fruit per square metre with some cultivars, when a normal capsicum in the field will give you a yield of about 3 kilograms per square metre.
“Yield increases are a combination of an increase in fruits per plant, plant density, and the prolonged harvesting period. This can be achieved with cultivars bred for greenhouse production, and by trellising plants vertically and maintaining them healthy, so they can keep on growing and setting fruit,” he explained.
While the research team is keen for local farmers to test the concept themselves, Dr Jovicich says selling the idea to growers is proving a little difficult.
“When we talk to farmers here about protective cropping, many times they identify the system with glasshouse production, hydroponics and very expensive operations,” he said.
“But we are thinking about structures that are much cheaper,” he added.
“What we have to keep in mind is that the designs for structures used up here in tropical places have to remove the heat from the greenhouse in the best way possible.
“So the structures have to be very tall, they have to have a roof vent and openings all around. They have to protect plants from rain, so basically we are only creating a roof over the plants.”
Dr Jovicich says although he’s yet to thoroughly analyse the economic benefit of the protective cropping system, he’s confident the figures will stack up.
“We are growing speciality crops that should attract a higher value than the crops you are growing outside,” he said.
“The vegetables that are grown under protective cropping generally can be considered different commodities than those extensively grown outdoors. There is potential for marketing some of the produce in a different way; that would also attract a higher value.
“So while protective cropping involves more labour per square metre than in extensive outdoor vegetable crops, you are getting three, five or six times more yield per square metre. Well managed crops are also more efficient in the use of water and nutrients, based on the unit weight of harvested produce.
“We are soon going to start running some numbers, based on the yields we were achieving,” Dr Jovicich said.
South Pacific solution
The North Queensland project is part of the Pacific Agribusiness Research for Development Initiative (PARDI) with funding from DAFF and the Australian Centre for International Agricultural Research (ACIAR).
In 2012, Dr Jovicich (protective cropping project leader) and Shane Dullahide (PARDI crop component leader), met with industry representatives in Fiji and Samoa to discuss low-cost protective structures and research activities. Dr Jovicich said it was clear from the meetings that participants would make the most of the opportunities offered by protective cropping systems.
“The research began in 2013, first identifying a low-cost structure design, then setting up demonstration sites and more recently starting to validate crop growing systems that are likely to raise the production of high-value vegetables in the region and increase grower income,” he said.
In 2014, five structures, each covering a ground area of 360 m2, were built in locations with distinct environmental conditions in Fiji (Sigatoka, Koronivia, and Tavua) and Samoa (Nu’u and Tapatapao). Supply chain analyses and surveys, and advice from collaborating farmers, pinpointed the highest demand/high-value crops among buyers and consumers.
“These crops—tomato, capsicum and cucumber—are the subject of the first round of trials. Some growers have also decided to grow crops such as potatoes, herbs, leafy vegetables and red cabbage as they are easy-to-grow, high-value commodities that can rapidly bring returns while they learn how to grow more complex crops such as tomatoes and capsicums,” said Dr Jovicich.
The greenhouse design includes high roofs (up to 4.5 m), roof vents, shade screens and insect-exclusion netting. The greenhouse structures use only passive ventilation for cooling (no electric fans) and create an improved crop environment with respect to outdoor growing conditions and compared to the low-roof, walk-in tunnel designs.
“Walk-in tunnel designs are already used by some farmers and usually have roofs that are less than three metres high, no roof vents, and small open areas for lateral ventilation. Temperatures under these structures frequently reach levels that are higher than optimal for plant growth and fruit setting in vertically trellised crops such as tomatoes, capsicums, and cucumbers,” he explained.
Dr Jovicich said that the purpose of using taller and better ventilated structures, such as the ones designed for the PARDI project, is to demonstrate that crops can be trellised to higher levels, and thus, production can be increased with more harvests throughout the season. This environment is also better for short, leafy vegetable crops, and because crops are grown in soil, allow for using diverse plant species for crop rotation. In addition to providing improved growing conditions, the structures are designed to be partially disassembled when extreme weather conditions (i.e. cyclones) are forecasted.
Dr Jovicich says that training farmers is an important part of the project, given that the production system is new to the region.
“The project also provides training to farmers and local research and extension officers in new irrigation technologies (e.g. drip), and new plant growing practices (e.g. trellising and pruning). Our aim is to also manage pest and diseases with low pesticide use. We collaborate with other ACIAR projects on integrated crop management to monitor and manage pest and diseases during the trials.”
As well as ACIAR funding, several local commercial companies backed the research. Wah Sing Yee, Director of Marco Polo International Ltd, helped to import the structures and supported the Ministry of Agriculture (MOA) Fiji to import additional structures and irrigation components. Edwin Tamasese, a farmer and Director of Soil Health Pacific Ltd, set-up two structures, and assisted with the importation of seeds and irrigation components in Samoa. Mr Tamasese and Mr Munsami Naiker, farmer and Director of All Season Nursery in Tavua, Fiji, assisted with the production of seedlings for the trials.
“Currently, there are farmers that are also building structures with wood, which is fine as long as you keep some design considerations that help with the heat removal and thus provide an environment that leads to acceptable yields.”
Collaborator vegetable farmers in Fiji and Samoa have been growing their first crops under protected cropping and already are able to see first-hand the increases in yield and produce quality. Although farmers still have to learn and adapt many crop practices to their specific environments and farming scenarios, the design of the sourced greenhouses provide improved ventilation and crop protection, thanks to the combined efforts by ACIAR and DAFF and considerable local industry and business support.
February 2015 / Issue 152