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Tradition of innovation

August 18, 2009  By Drs. Xiuming Hao and Tom PapadopoulosDrs. Xiuming Hao and Tom Papadopoulos

The plant nutrition research on greenhouse vegetables was initiated when Dr. Gordon Ward was transferred to Harrow from Ottawa in 1960.

Heat placement trials in tomatoes on raised troughs


The plant nutrition research on greenhouse vegetables was initiated
when Dr. Gordon Ward was transferred to Harrow from Ottawa in 1960. In
collaboration with local growers, he systematically investigated the
levels of nutrients in plant tissues, determined the total nutrient
uptake by the plants, and then established a feeding schedule with
soluble fertilizers that would maintain rapid growth and good sustained
yields of tomato and cucumber when grown in soils. Much of the precise
nutrient analysis would not have been possible without complicated
laboratory equipment, and Dr. Ward was prompt to take full advantage of
such new analytical instruments of the time, including the amino acid
analyzer, gas chromatograph and absorption spectrophotometer.


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.

In the mid- to late-1970s, the greenhouse industry changed almost
completely from the short American seeded-cucumbers to the long English
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
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

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.

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.

 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 (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.

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.

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

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.

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.

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