|Heat placement trials in tomatoes on raised troughs
By the late 1960s, an optimized weekly feeding schedule was fully developed, optimum levels in leaf tissues were known, and adverse effects of un-favourable levels were defined. The influence of the calcium level in fruit was related to a physiological disorder, blossom-end rot. These feeding schedules for cucumbers and tomatoes, with some modifications, remained in use until the mid-1980s when greenhouse vegetable production switched from soil to soilless systems to prevent soil diseases and avoid the need for soil disinfection.
ASSISTING THE SWITCH TO SEEDLESS CUCUMBERS
In the mid- to late-1970s, the greenhouse industry changed almost completely from the short American seeded-cucumbers to the long English seedless-
cucumbers. Most of the Ontario production at the time used Dutch varieties, which were quite susceptible to diseases, especially CMV, and did not have good keeping qualities. Wally Nuttall, a plant breeder at the research centre, evaluated a lot of long English cucumber cultivars for local suitability and developed a new seedless variety, ‘Harliton,’ which was intermediate in length, but resistant to CMV and powdery mildew, and had good yields and shelf life.
Crop management studies were started in 1980 when Dr. Papadopoulos joined the team. His research was initially focused on cultivation of greenhouse tomato and cucumber crops in soilless media, such as peat-based mixtures. A Harrow peat bag system was developed, and irrigation requirements for greenhouse tomatoes in peat bags were defined. Later on, extensive studies on rockwool, other soilless growth media, and pure hydroponic systems such as NFT were conducted. Superior fertilizer application recommendations on the basis of a “seasonal fertigation program” concept were developed and quickly adopted by the greenhouse vegetable industry. Nowadays, almost all the greenhouse vegetables are produced in soilless cultivation systems.
The idea of formulating fertigation programs spanning the whole life of a crop, with the ability to modify the nutrient concentrations according to crop growth stages and environmental conditions led to the development of the Harrow Fertigation Manager® (HFM) in the late 1980s by Dr. Papadopoulos and his co-operators in the private sector. The HFM is a patented, computer controlled, multi-fertilizer injector system for the precise application of fertilizers to any crop in accordance with preprogrammed seasonal fertigation programs. The system also proved to be a valuable research tool for studies on plant nutrition.
|High-wire English cucumber production on a raised
|Peat bag research in the early 1980s|
|a mini-greenhouse complex was constructed in the late 1980s.
The increasing cost of energy in greenhouse production in the 1980s led to the appointment of Tom Jewett as a greenhouse engineer. Many projects directed towards more energy-efficient systems in greenhouses at the time were done by contract research at a number of locations in Ontario. Jewett reviewed the proposed projects, provided engineering advice and reported on the projects. A major project found that extra heat in the greenhouse during the day could be withdrawn and stored as hot water in a large underground storage for use as a heat source at night.
Dr. Papadopoulos initiated a major project in 1987 on greenhouse cover materials.
With this project, nine mini-greenhouses with three different cover materials were constructed at the research centre. The project demonstrated that tomato and cucumber yields can be equally good under double-inflated polyethylene (D-poly) cover as under glass, plus a 30 per cent energy savings over glass. As a result of these findings, a large acreage of greenhouses constructed in Leamington are D-poly, which has resulted in great savings in construction (close to a billion dollars) and heating (approximately 30 per cent) costs, as compared to glass.
D-poly greenhouse utilization has become a unique feature in greenhouse crop production in North America. This area is still a very important part of the research program in Harrow. We are continuing to evaluate new greenhouse cover materials to improve greenhouse microclimate and reduce energy consumption. A project on energy curtains in D-poly greenhouse with sweet peppers has just been completed.
EXTENSIVE RESEARCH ON LIQUID FOAM TECHNOLOGY
Dr. Hao, who joined the research centre in 1996, also conducted a major project in co-operation with Laval University, Ontario Greenhouse Vegetable Growers (OGVG) and private companies on liquid foam technology during the last three years. The liquid foam can be injected into the space between the double layers of polyethylene films to improve greenhouse insulation during the night or as needed to reduce the heat loss to the outside. Foam insulation can reduce the heat loss through the greenhouse roof during the night by 50 per cent.
Research on soilless cultivation, plant nutrition and environmental physiology have been further enhanced by Dr. Hao since his arrival. To address the environmental issues associated with leaching (waste) fertilizer solution into the environment, several projects were conducted to recirculate nutrient solutions. Thorough systematic investigation of crop nutrient uptake, seasonal fertigation formula and schedules, while maintaining the optimum levels of nutrients by matching the feeding with nutrient uptake, were developed for closed tomato production systems.
The systems achieved the same or better yield and quality than the open systems and reduced fertilizer and water use by 30 per cent.
Other studies on the nutrition of cucumber and sweet pepper in a closed hydroponic system context also led to the development of improved seasonal fertigation schedules for these crops. Nowadays, the majority of commercial tomato and sweet pepper production and about 50 per cent cucumber production are using, to a large degree, recirculated or closed soilless system, which not only reduce environmental pollution but also save on water and fertilizers. This area of research is continuing today, an irrigation project on super-saturation of nutrient solution with oxygen to improve rootzone environment has just been completed.
HELPING OVERCOME WINTER TOMATO QUALITY CHALLENGES
Poor tomato fruit quality in the winter (soft fruit of inferior taste) due to poor light conditions and low fruit quality in the summer (fruit cracking, russeting, blotchy ripening and sun scald) due to strong solar radiation and high temperatures became a serious industry concern in the mid-1990s. Two national collaborative national projects led by Dr. Papadopoulos were carried out to search for plant nutrition and greenhouse environment control strategies to improve greenhouse tomato fruit quality. The causes of poor tomato quality were identified and effects of pre-harvest factors on tomato fruit quality were reviewed. Variable electrical conductivity (EC) management strategies that modified feed EC according to seasonal and diurnal solar radiation were developed, which solved the long-standing conflicts between quality and yield with high EC (high EC generally leads to firm but small fruit and low yield).
Cultivar and N, Ca, Mg and K fertigation recommendations were also developed to improve fruit quality; e.g., using high Ca to reduce tomato fruit russeting and high Mg in later growth stages to improve fruit firmness. The work on tomato fruit quality is still continuing today, but the focus has been switched from fruit appearance, grades, firmness and shelf life to nutritional and health-promoting compounds.
Automatic microclimate and energy monitoring systems involving modern electronics and microcomputers for data collection were developed in the late 1990s for evaluating vertical greenhouse and plant surface microclimate profiles and energy use profiles in various types of greenhouses in Ontario. The data collected has provided important information for designing new greenhouse and heating systems, and for planning the energy supply to the greenhouse industry.
The microclimate within the canopy and at the plant surface can have a significant influence on the physiological processes of plants and epidemiology of pathogens. However, it is not normally measured because of the complexity of the measurements and the high cost of the sensors.
PLANT SURFACE MICROCLIMATE MODELS DEVELOPED
Plant surface microclimate (PSCLIMATE) models for tomatoes and cucumbers were developed in early 2000 by Tom Jewett, and Drs. Hao and Shipp (an entomologist). The model can precisely predict the vertical microclimate profile (air temperature and humidity, leaf temperature, wetness, vapour pressure deficit and solar irradiance) using routine recorded/monitored climatic parameters. It has proved to be a very useful tool for improving greenhouse climate control and for integrated crop and pest management.
The greenhouse program received a major boost with the opening of the north greenhouse complex in 1998. The complex consisted of 16 D-poly and eight glass greenhouses, which made the centre the largest greenhouse research facility in North America at the time. This state-of-the-art facility has made it possible for the team to conduct extensive and scientifically sound research on greenhouse climate control, environmental physiology and energy conservation.
Since 2002, Drs. Hao and Papadopoulos have carried out extensive research to develop cost-effective systems for year-round production of greenhouse vegetables, a prerequisite for successful market retention and for stable supply of high quality produce to Canadian consumers.
Supplemental light intensity, crop/intercropping scheduling, cultivar, plant density, fruit load, mineral nutrition, irrigation and energy balance were investigated to characterize the optimal use of artificial lighting, to optimize crop and fertigation management, and to identify crop planting and training systems that allowed for better light interception, higher productivity, and continued production of greenhouse cucumbers.
OPTIMIZED YEAR-ROUND CUCUMBER PRODUCTION
An optimized year-round, high-wire production system on raised-gutters (troughs) for long English cucumbers was developed, which achieved very high yields (100 per cent increase over the conventional system). A twin-head, high-wire cucumber production system was also developed, which achieved the same fruit yield as the single-head, high-wire production system while cutting the crop start-up costs by half. Lighting strategies were also devised to increase energy use efficiency in winter months. Several cucumber operations in British Columbia (B.C.) and Ontario have adopted the technology. Very high yield (280 cucumbers m-2 year-1, a 100 per cent increase) has been achieved in commercial greenhouses that adopted the systems.
This area of research is continuing today on mini-cucumbers.
The escalating energy prices in the early years of this decade brought back the attention on energy conservation after the industry had enjoyed a good break from the reasonable energy price during the 1990s. Since Tom Jewett has moved to another government department, Dr. Hao took on the additional responsibility on greenhouse climate control and energy conservation. Using the principles of greenhouse environmental physiology, micrometeorology and control engineering, and the concept of plant-based climate control, Dr. Hao initiated a series of projects in 2003 to develop integrated climate control strategies to improve energy use efficiency and to reduce greenhouse gas emission.
RESEARCH ON GROW-PIPE HEATING BENEFITS
The raised-gutter cultivation system (i.e., to grow vegetable plants on troughs hung 60-130 cm above the ground) became popular in the early 2000s as it facilitates nutrient recycling, intercropping, and labour work efficiency. However, the system has changed the heating and microclimate profile in the greenhouse. Growing media is now farther away from the heating pipes on the ground, requiring increased heat utilization to maintain adequate plant microclimate and root temperature. Dr. Hao evaluated various heat placement options, including the use of a grow-pipe, at the research centre and in commercial greenhouses, and identified the best heating pipe placement for the raised-gutter cultivation system. The new heat placement maximized the heat transfer to plants instead of the air, achieved higher plant temperature with same air temperature, reduced the humidity inside the crop canopy and the incidence of Botrytis, and increased early and total fruit production without additional energy consumption. The grow-pipe now has been widely adopted in tomato production.
Dr. Hao is also actively involved in searching for alternative energy sources for the greenhouse industry. Biomass is greenhouse-gas “neutral” since it only releases as much CO2 as that absorbed by the plants during the photosynthesis process. In co-operation with scientists at Natural Resources of Canada (NRCan), they are developing a flue gas scrubbing system to clean the flue gas from biomass combustion, so that it can be used for CO2 enrichment in greenhouse crop production.
‘DYNAMIC PLANT-BASED ENVIRONMENT CONTROL’
The greenhouse is to provide a good environment for the plants to have good growth, and high quality of produce. Any energy conservation measure that compromises fruit yield or quality is not going to be acceptable. Based on this premise, a four-year national study on “Dynamic Plant-Based Environment Control to Improve Energy Use Efficiency in Greenhouse Crop Production” was initiated (as reported in the June 2009 issue of Greenhouse Canada) in 2008. The study will develop dynamic greenhouse temperature control strategies, new lighting strategies, online crop monitoring systems, and root zone environment management strategies tailored for dynamic greenhouse climate for improved energy use efficiency in greenhouse vegetable production.
Consumer demand for organic produce has dramatically increased in the last few years. However, the lack of dependable information on production methods limited the organic greenhouse vegetable production. To address this need, Dr. Papadopoulos initiated a major project with the support of Ontario and British Columbia greenhouse growers, and private companies, in 2006 to develop integrated methods for the organic production of vegetables in greenhouses. The focus of the project was to identify commercially available organic media, formulate new organic media, develop appropriate nutrient feedings and assess the feasibility of the organic systems, and to provide much needed recommendations to the greenhouse industry. The tomato and sweet crops grown with the organic system developed achieved similar fruit yield as the conventional rockwool production system over the full growing season and thus provided a viable alternative for greenhouse growers who are looking for a transition under the present marketing condition.
Over the last 50 years, the greenhouse industry has grown from small family based operation to large enterprises, from low-tech glasshouse with little environmental control to high-tech modern greenhouses with complex climate control system, from soil to soilless production, and from short production cycle to year-round production of high quality produce. The greenhouse program at Harrow has also kept growing and improving, providing vital support to the industry.