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Autogrow Systems Ltd |
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Environmental control for Herbs
Temperature and light for herbs varies from herb to herb, but we generically run three different temperature "zones" during winter. Most herbs have a minimum light threshold to grow, with some needing a much higher level than others. The cost to deliver both light and heat to achieve plant optimum growth rates throughout the year is prohibitive in south eastern Australia so we just add heat, and hope that the sun shines brightly. Consequently, output per square metre falls by around 40% in winter. We take on customers based on our wintertime capacity because of this. The basic "temperature" zones (NOT CLIMATE) I would describe as: Tropical temp zone herbs - Basil, Lemongrass As a general comment there is an interaction between temperature and the amount of light received that is sometimes counter-intuitive. Basil for instance will not tolerate temperatures below about 18 degrees C when PAR levels are low and day-length is short. Yet during summer, when day length is longer, it will happily tolerate temperatures well below this overnight, without any ill effect. Our minimum winter day-length is about 9 hours effective sunlight assuming no cloud. Because we are close to the ocean and the topography of where the greenhouses are built, frosts are rare. The worst weather we suffer is extended periods of day time ambient air temperatures during the winter of 4 to 6 degrees Centigrade accompanied by strong winds, and temperatures down to 1 or 2 degrees C overnight with or without wind. Production systems:- Twin skin poly-houses – some old multispans with 50% white shade for summer, and some new Venlo types with 70% foil screen and open weave to allow ventilation at night. Growing system is recirculating nutrient delivered onto sand beds via drip emitters. Heating is delivered by two different methods - by hot water pipes in the sand bed (various methods including the new Rinnai instantaneous LPG HWS, heat pump and an industrial sized LPG heat exchanger system. We supplement the air temperature with open flame gas factory heaters (CRINGE results in high humidity levels and a requirement to dump air to control overnight CO2 levels and humidity. We are in the process of installing air heat exchanger systems to eliminate this problem.) TROPICAL ZONE – For Sweet Basil, commercial growth rates will not be achieved during winter in southern Australia without heating - we have a wintertime plant temperature threshold of 20 degrees C overnight, and 27deg C during the day. We try to keep the overnight to 22 degrees, but we can only afford so much gas. As the day length gets longer, and ambient temperatures rise into summer, the overnight temperature can fall lower without any detrimental effect on growth rates. In the middle of winter under heavy cloud, the light levels are just too low to achieve growth at all – basil has a minimum light requirement which is higher than all of the other herbs - exactly what light level that is has yet to be established. Post harvest, basil suffers chill injury below about 10 to 12 degrees C and goes black. If greenhouse temperatures fall below 18 degrees C, Botrytis gets into the plants, and below 15 degrees, basil will be lucky to survive, let alone grow. I talk in terms of PLANT temperature. COLD ZONE - Chervil grows happily at temperatures of down to 8 degrees C, and slows down from there, but is not bothered by cold conditions down to 0 degrees C. Coriander and Dill will also grow quite well down to the same temps, but will do better at 10 to 12 degrees C. MODERATE ZONE - we keep the plant temperature at around 15 degrees C overnight and as warm as we can get during the day, but try and maintain at least a 5 degree C differential day to night. Robert Hayes
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