What was it all about?

Running from 2017 into 2018, this investment looked at protecting cropping opportunities and technologies for the vegetable industry in Australia’s tropics – identifying gaps in current information and assessing the practical and economic viabilities of protected cropping options in these regions.

In Australia, protected cropping of vegetables is largely located in temperate climate regions and in proximity to urban areas, while in warmer climate regions near and north of the Tropic of Capricorn, the use of protected cropping is scattered and relatively small.

Vegetable growers in these regions, the project team note, would benefit from technologies that can mitigate risks linked to climate variability, and that can help them address current and future market challenges and opportunities.

In the tropics, the availability of cost-effective protective cropping structures that are effective in removing heat from crops is paramount. To this end, the researchers looked at four greenhouse structure designs, and their benefits and drawbacks.

Full details are available in the project report but, as a quick look, the structures considered included…

• High tunnels– light structures with steel arches covered in polyethylene film. The researchers note that in the tropics, air temperatures can reach undesirable levels when tunnels are low (under 3m). For the tropics, tunnels need to be high, with vertical poles along their sides at least 2.5m in height, and with roof arches reaching a height of at least 3.5m at the centre of the arch. Among the findings, the researchers report tunnels have the lowest cost compared to the other structures investigated. “Tunnels are sometimes considered an entry level option for growers who aim to learn about protected cropping technology with very low investment costs,” they write. “Tunnels may also be an option for growers intending to have a small area of covered crops, or for growers who would like to take a step-by-step approach to adoption of different levels of protected cropping technology.” Because the frame of tunnels is made of light steel, only when wind speeds are low can they resist deformation. There may also be local government restrictions around their use depending on the location and cyclonic region.
  
  
• Passively ventilated greenhouses - compared to tunnels, high greenhouses covered with a polyethylene or polyweave film are much larger and comprised of stronger steel. The roof is curved, but arches are offset to create a roof vent. The extent of escape of hot air greatly depends on wind speed and wind direction. Coastal regions may have prevalent winds that increase the air exchange rate in these structures. Locating structures at higher altitudes in the tropics may also provide benefits for cooling. The researchers note that a passively ventilated structure with all desirable characteristics has been operating commercially in the dry tropics region since 2002, and crops such as cucumbers have been grown successfully to supply local and southern markets. More recently, in the Burdekin region, evaluations of capsicum, melon and eggplant production have been conducted under this type of structure with yields and fruit quality outcomes that are comparable to those obtained in tropical environments overseas.
  
  
• Retractable roof structures – tall structures with gutters at heights greater than 4m. The key characteristic of this design is that the roof cladding can be retracted. This makes them very efficient in letting hot air escape from the crop environment and can also close completely in minutes to protect crops from rain. The crop can be exposed to full sun under good weather conditions, or can be shaded with retractable screens. These structures have not yet been evaluated for vegetable production in the northern Australian tropics. In Bundaberg, Queensland, successful vegetable crops have been obtained in a 4-ha structure built in 2015. Recently, such structures have been built in temperate and subtropical regions for production of tomatoes, capsicums and leafy crops, as well as for protection from rain in cherry trees in Tasmania. Examples of good crop performance and crops harvested for long periods (cucumbers, eggplants and tomatoes planted in August 2016 and with some crops still being harvested in May 2017) were observed in Culiacan, Mexico, during the project’s study tour.
  
  
• Net houses – also known as screen houses or shade houses. The frame of these structures is made of poles (wood or steel), steel cables and wire. An insect exclusion netting covers the roof and sides of the structure. Cultivation under nets is done similarly to the way it is done in field crops, but higher trellis systems (either stakes or high wire and strings for support of plant canopies) are used to prolong the harvesting season. The main purpose of this protective structure is to provide shade to crops and exclude insect pests that damage or transmit plant viruses to crops. It is not designed to protect crops against rain water. As such, these structures tend to be used in desert areas or where dry seasons are long, with potential to evaluate them in the drier areas of the tropics, such as in areas inland from the coast.

The project’s report also looks at crops including capsicum, cucumber and eggplant as examples of vegetable crops suited to warm climates, which can benefit from a protected environment and specific agronomy practices (also looked at in the research).

Possible marketable yields are provided for these crops as well as estimates of production value for a range of size of areas that could potentially establish protected cropping systems.