The Canadian Organic Grower www.cog.ca48 – Summer 2012

THE NORTHERN TOMATO

By Gwen O’Reilly

Tomatoes comprise the largest share of Cana-dian greenhouse production. In some prov-inces, 90% of all tomatoes are grown in green-

houses1—perhaps because many consumers justaren’t content to make do with salsa and cannedtomatoes all winter. Conventional greenhouse pro-duction is now a significant and growing part of theagricultural sector, accounting for $2.5-billion ofgeneral farm value.2 The demand for organic andregionally grown food is increasing by 15–25% an-nually, with 75–85% of the Canadian supply oforganic products being imported.3 It follows thatorganic producers in Canada are moving towardsyear-round production of warm weather crops.

But, that’s where it gets tricky for organic pro-ducers who want to venture into an industry domi-nated by conventional agriculture. Small or passivegreenhouses, like high tunnels and hoophouses,regulate the build-up of heat and moisture by vent-ing to the outside air—unfortunately, this is not anefficient option in year-round operations. Large-scalecommercial greenhouses are kept closed to controltemperature, humidity and carbon dioxide levels(plants need CO

2 to photosynthesize and grow).

These greenhouses need mechanized systems toheat, cool, dehumidify and provide additional light-ing for tropical crops like tomatoes. Year-roundgreenhouse production is thus a highly energy in-tensive effort—one that consumes 12% of the en-ergy used for agricultural production in Canada, andreleases 2 million tonnes of CO

2 (literally, “green-

house gases”) into the atmosphere as a result.4 Thegreenhouse industry, including organic producers,also produces effluent from nutrient solutions that

can pollute water bodies and groundwater.5 Alltogether, this means that your February tomato isexpensive in more ways than one.

Research is underway to create greenhouse sys-tems that are energy efficient and environmentallybenign. Rising energy costs have already resulted ininnovations among conventional growers such asthermal screens, better insulation, sophisticated tem-perature regulation and new structural designs. Or-ganic Science Cluster (OSC) researchers are takingthese a step further, looking at geothermal, biomass,and other sources for heat, cooling and dehumidifi-cation, as well as recycling, natural capture orremediation of greenhouse effluent.

Heating a translucent structure in winter is onething (tomatoes don’t do well below 16OC); but aclosed greenhouse environment presents additional

A HOT TOPIC IN A COLD CLIMATE

Conventional and organic vegetable production systems in Canada have manydifferences, but they share one major constraint: winter. Our less than tropicalclimate has fuelled the growth of commercial greenhouse production forvegetables such as tomatoes, peppers, cucumbers and lettuce.

Tomato plants growing in coco coir on the left and inorganic soil on the right.

Our Nature is Organicwww.cog.ca Summer 2012 – 49

challenges for organic growers.Unlike conventional producerswho usually use hydroponic sys-tems, organic growers must growtheir plants in a soil-based me-dium [see page 52 for details onorganic potting mixes]. Soil in-creases humidity and adds thechallenge of regulating pop-ulations of macro- and micro-organisms, including bacteria,fungi and insects. Fertilizer ornutrient solutions can cause saltto build up in the growing media(if recycled), or potentialgroundwater pollution if released.6

Heating, cooling anddehumidifying: NosweatOSC researchers in Quebec areevaluating two non-traditionalcooling and dehumidifying meth-ods in organic closed greenhouses.One is a geothermal cold water fansystem, where warm, moist green-house air is blown through a coilcooled by groundwater. The air iscooled and moisture condensesduring passage across the coil.

The second method carries theintriguing description of “waterdroplet curtain,” also called an“open water to air heat exchanger.”This consists of nozzles that sprayfine curtains of cold mist at the roof(hot part) of the greenhouse, be-tween rows of plants, to conductheat and condense moisture in theair. A rain shower on a hot day hasa similar effect. The falling water ac-cumulates airborne heat and hu-midity. The water then collects onthe greenhouse floor and is re-turned to an outside reservoir,where it is cooled before recirc-ulation.

Preliminary tests show thatboth the geothermal water fan anda droplet curtain can cool and de-

humidify as effectively as a vent-ing system. They have the addi-tional advantages of:1) preventing the heat loss thatcould occur in a vented green-house, and2) increasing the level of carbondioxide.7

Heat capture systems can helpregulate night-time temperatures.Water can be circulated during theday through hot areas in thegreenhouse to absorb solar radia-tion, and then stored in an insu-lated tank for recirculation afterthe sun goes down.8

Nutrient management:Too much of a goodthingFertilization in organic green-houses involves the use of com-post, kelp meal, or concentrated,reclaimed substances such as crab,shrimp or feather meal. Organicfertilizer run-off (which may con-tain high concentrations of ni-trate, phosphate and other

A constructed wetland is like a human-made swamp—waste water is circul-ated through gravel beds planted with species such as cattail, rush and iris.

nutrients) has the same negativeeffect on groundwater as that fromchemically derived compounds.What goes in must come out, sothe Organic Science Cluster re-searchers are:

1) investigating ways to reduce thenutrient load in greenhouse efflu-ent by adjusting fertilizer rates andtiming to better match the needsof the plants, and

2) examining recirculation andremediation of greenhouse efflu-ent.

OSC researchers Drs. MartineDorais and Valérie Gravel and col-leagues examined the impact ofrecirculation of effluents on toma-toes. The plants were grown invarious soil media over three years.Crop yields were high in spite ofincreasing salinization (mineralsalt build-up) in the growing me-dia.9 The researchers surmise thatthe salinization may have ulti-mately affected the total biomassof the tomato plants, but notyield.6

The Canadian Organic Grower www.cog.ca50 – Summer 2012

These OSC scientists are alsoassessing the use of constructedwetlands and passive bioreactorsto capture excess nutrients. A con-structed wetland is like a human-made swamp—waste water iscirculated through gravel bedsplanted with species such as cattail,rush and iris. This can be estab-lished within or outside of thegreenhouse (but wetlands don’twork when frozen). The plants andmicroorganisms in the wetland takeup and help break down excess nu-trients and some toxic compounds.The resulting run-off is cleaner andcan be released into natural water-courses without negative environ-mental impacts.5

Not only nutrients build up ineffluent—pathogens and phyto-toxins do as well, leading OSC sci-entists to examine whether artificialwetlands inoculated with biocharand ‘passive bioreactors’ can elimi-nate these additional problems.10, 11

Biochar—charcoal made fromburning woodchips, tree bark andother biomass—is the latest thingin amendments. As a soil amend-ment, biochar may increase micro-bial activity and nutrient exchange.Biochar is also used to filter impu-rities, including excess nutrientsand pathogens, from effluent. OSCstudies show that these techniquesare more than 99.99% effective inremoving Fusarium and Pythiumpathogens from greenhouse efflu-ent.12

Passive bioreactors more closelyresemble a septic tank than awetland, and are typically used totreat mine drainage. They use acombination of ecological (micro-bial) and geochemical reactions toremove metals from effluent andbuffer its acidity. Plants in these sys-tems add organic matter and stimu-late microbial activity, but are not themain uptake mechanism. Once es-

tablished, bioreactors are self-sus-taining and can function for yearswith minimal intervention.

Let there be lightOne significant issue for northerngrowers is supplemental lighting—short winter days do not a red to-mato make. Fruiting vegetablesrequire longer photoperiods toflower and develop a crop. Year-round, or even extended seasonproduction means adding lighting.OSC researchers led by Drs. SteevePépin and Martine Dorais com-pared the effect of supplementallighting in both organic and con-ventional greenhouses, and foundthat there was no significant differ-ence between organic and conven-tional tomato yields.13

High-intensity high-pressuresodium lamps can contribute heatas well as light to the greenhouse,and the researchers are experi-menting with novel ways to use thelights more efficiently. For exam-ple, they are evaluating the poten-tial for using lighting within thecrop canopy, instead of, or alongwith, above-canopy lighting.

Other established techniquesinvolve using photoperiodic light-ing, where supplemental lightingis used to bracket daylight hoursto extend photoperiods whendays are short. Supplemental light-ing can also be used to interruptdarkness for a few hours, toachieve a similar effect. For signifi-cant energy savings, lighting canbe cycled on and off every halfhour for 6–10 minutes during thephotoperiod. Higher intensity

lighting can also be used in lim-ited and targeted time periods, forexample when a crop is young.Increased light produces bettergrowth in young plants versus old,and since seedlings are usuallygrown at high density, there is lessarea to light.14

OSC researchers are not onlytesting new technologies, they arealso quantifying the environmen-tal impact of organic greenhouseproduction. Dorais and her teamevaluated the environmental foot-print of closed-loop, Northernorganic tomato greenhouse pro-duction. They evaluated everythingfrom raw materials extraction to thefarm gate, including infrastructureand equipment, climate control,waste management, fertilizers, pes-ticides and packaging. Comparedto a similar conventional operation,the organic system showed signifi-cantly less environmental impactin all categories, with the excep-tion of greenhouse structures andpackaging. The use of woodbiomass as a renewable energysource for heating reduced theCO

2 footprint of one kilogram of

tomatoes by almost seven timesthe rate of conventional produc-tion. The fertilizer assessmentdone for the organic crop alsoshowed significantly lower envi-ronmental impact in all categories,most notably eutrophication (sur-face water effects of fertilizer run-off).15

Researchers continue to lookfor better ways to produce North-ern organic tomatoes year round.However, these organically grown,February tomatoes are still moreinfrastructure- and energy-expen-sive than garden-grown, vine-rip-ened love apples. Until we all learnto survive on bottled, canned anddried tomatoes throughout thewinter, Canadian growers can use

Extended season

production means

adding lighting.