The warm temperatures and sunny conditions last week have accelerated ginseng crop emergence. As of April 20, 103 Growing Degree Days (GDD) (Base 5) has accumulated at Delhi. This is 30 GDD above normal for this time of year. On average 4 GDD accumulate every day at this time of year, which would put 2023 about 7 or 8 days ahead of normal. It also puts 2023 in the top 5 for GDD accumulation to this point of the year over that period (Figure 1).
Figure 1. Total Growing Degree Days (GDD) base 5 accumulated up to April 20 for the past 21 years. The average for the period was 73.7.
The GDD calculation is based on the average daily air temperature for each day, subtracting the base temperature that is chosen. A base temperature of 5°C is often chosen for crops adapted to this climate, like ginseng. If the daily average temperature is at or below the base temperature, no GDD accumulate for that day. So, for example, if the high for the day is 10°C and the low for the day is below freezing, (the daily average is at or below 5°C) no GDD accumulate for that day.
The two factors that GDD does not include are soil temperatures and sunshine. If we have a harsh winter and the soil is solidly frozen down to a few feet deep, it will take much longer for soil temperatures to warm up even if air temperatures are higher than normal. Likewise, if we compare a cloudy day with an average temperature of 10°C and a sunny day with an average temperature of 5°C, the ground may actually warm up more on the sunny day than on the cloudy day, even though the cloudy day would have accumulated 5 GDD compared to 0 GDD on the sunny day. Ginseng emergence is entirely dictated by soil temperatures, so GDD is not always the best measure for ginseng crop development.
There is no reason to suspect that hours of sunshine have been any different than normal this year, but winter temperatures were definitely much above normal. Compared to the other years since 2002 with GDD accumulation at or above 100 GDD by April 20, this year is comparable to 2002, 2012 and 2017. There was a comparable mild winter in all of those years. It is probably not a coincidence that those also happen to be years in which ginseng had considerable frost events in May. Hopefully cooler temperatures in the forecast for the next two weeks will slow down emergence compared to those years.
Given that emergence is well ahead of normal, be careful with glyphosate applications normally done at this time of year. Ginseng plants can pick up residues off the straw and can be damaged if they emerge through the straw shortly after application. The mild winter may also have allowed diseases like Cylindrocarpon to advance over the winter. Although there are few fungicides to manage Cylindrocarpon, consider including those with suppression of the disease on the label (fludioxonil or captan products) in the early-season product rotation. To be effective, these will need to be washed into the root zone by rain or irrigation before they have a chance to dry on the straw.
For an up-to-date listing of the products available for control of Cylindrocarpon or any other ginseng pest, visit the Ginseng Crop Protection section of the Ontario Crop Protection Hub at https://cropprotectionhub.omafra.gov.on.ca/. You can also view a summary table of efficacy ratings for fungicides by clicking on “Efficacy View” at the bottom of the main page and selecting ginseng in the crop listing.