COUNTRY LIFE IN BC • MARCH 2020


Growing degree days not just for tree fruits A more variable climate boosts the importance of understanding climatic influences


by MYRNA STARK LEADER


ABBOTSFORD – Anyone growing crops should understand and use growing degree days (GDD) to help with production according to Gary Judd, a research scientist and entomologist at the Summerland Research and Development Centre. “Growing degree days are impacting all agriculture,”


says Judd, who has 40 years’ experience in horticultural pest management. Judd says an understanding of growing degree days can improve crop management. Applications include adjusting crop planting times to avoid pests, predicting crop maturity and harvest dates, making more informed varietal and planting decisions, scheduling weeding, predicting the timing of pest populations and beneficial insects and forecasting diseases and their severity. “All plants and animals have a minimum and a maximum temperature for development,” Judd told a roomful of organic growers at the Pacific Agriculture Show in Abbotsford at the end of January. Between the upper and lower thresholds is the optimum temperature for a plant to grow. Growing degree days measure the plant’s exposure to this optimum level of heat. Growing degrees are calculated by adding the day’s maximum temperature and the daily minimum, dividing them by two, then subtracting the lower threshold temperature. For example, a summer day where the maximum is 30°C and the low is 10°C and the crop in question requires a temperature of at least five degrees to grow will accumulate 15 growing degrees (in Celsius; the degrees accumulated in Fahrenheit would be different, something growers need to watch when comparing results). The science helps to explain why an annual plant may not be at the same stage of development on the same calendar date in different years. It may flower earlier, for example, or set fruit later. Judd notes that growing degree days are a better way to understand plant development than, say, information on a seed package. The package


instructions might say 70 days to maturity but development could take anywhere from 60 to 90 days. To emphasize his point, Judd showed the difference temperature has on the number of thrips on onions. If on June 1 there is one thrips and the temperature is 68°F, by July 10 the model predicts 210 thrips. If, however, the temperature on June 1 is 86°F, the single thrips becomes 250,047 insects by July 13. BC tree fruit growers have access to the Decision Aid


System (DAS) developed at Washington State University. Something similar for other horticultural crops would be useful but doesn’t exist yet. While scientists know how to use degree days very


GARY JUDD


well, Judd says the challenge has been transferring the knowledge to growers. He believes tools like the DAS should should broaden the use of this crop management approach, at least for tree fruits. He sees the benefits of the tool in a changing climate, particularly for organic growers who generally have fewer pest control options compared to conventional growers and need tighter management programs to prevent problems from getting out of hand. Accurate outputs from GDD models on which to base these types of decisions require two things. The first is weather data. This can be collected from


weather stations but BC’s many microclimates mean it’s most accurate when collected at individual farms. Good modelling depends on having accurate high and low temperatures. The second thing required is a degree day calculation method. You need


crop/pest temperature thresholds and there must be a defined start and stop date for measuring. For annual vegetables, this could be from January 1 until bud break, or from transplant date to flowering. For perennial woody crops, the window might be from January 1 until petal fall, the fruit achieves full colour or a set sugar content. Dates linked to pests could also be used, such as first trap catch, or first weather event. “The oldest reference for degree days is about 1969 so it’s been around for a long time but we’re still learning to use them,” says Judd.


41


Customer Service order@norseco.com


514 332-2275 | 800 561-9693 450 682-4959 | 800 567-4594


Our Team of Experts Ben Yurkiw


British Columbia ben.yurkiw@norseco.com 604 830-9295


Proud of our roots since 1928


Martin Deslauriers Sales Manager


martin.deslauriers@norseco.com 438 989-4863


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52