It is frost season again, and there is a lot of uncertainty at this time of year. Ginseng in older gardens is emerging through the straw and could be susceptible to freezing conditions. Here are some considerations for forecasting and dealing with frost in ginseng:
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Reliable Weather Data
The Environment Canada Delhi weather station is once again showing temperatures much colder than they should be, which is due to the ginseng shade that surrounds the weather station. Last night it showed a low temperature of -4°C, which was 3 to 4 degrees colder than all weather stations located nearby, including Simcoe, between Simcoe and Delhi, Brantford, and London. Normally, it is about 1°C colder in the Delhi area on calm and clear nights, but not 3 or 4 degrees colder.
Last year, we collected temperature data from different heights in the ginseng garden next to the weather station, near the Environment Canada weather station in the open, and then at different heights on a grass lawn down the road. On every calm and clear night, the temperatures at all levels under the ginseng shade were 2 to 5 degrees colder than the actual air temperature (normally collected at eye level). Normally, in open areas, temperatures on the ground are a few degrees colder than eye level. It appears that the ginseng shade acts like a new ground level, and everything below it becomes the same temperature it normally is on the ground. This effect is probably more noticeable in flat and sheltered locations, since gardens on a definite slope would allow for cold air drainage at night as the cold air flows downhill, and more open sites would allow any breeze to dislodge the cold air under the shade. The Delhi Station is very sheltered due to tall hedge rows and probably traps the cold air more than most other sites.
What does this mean?
Prior to last year, the Delhi weather station was displaying the proper air temperature (at eye level) during the growing season. On calm and clear nights, it was often 3 or 4 degrees colder in a ginseng garden than the weather station readout. So, if the Delhi temperature was 0°C, it was often -3 or -4°C within ginseng gardens. These temperatures would lead to only minor damage.
Now, you can consider the Delhi weather station as basically being inside a ginseng garden, so the temperatures are the same. So, if it shows -4°C at Delhi, it is likely also -4°C in a sheltered ginseng garden. The risk to the ginseng will not be any different than what it was in the past at when the Delhi temperature showed 0°C, but the temperature displayed for Delhi will seem more alarming. You will need to recalibrate your thinking when using the Delhi weather data, which is used by Environment Canada and several other weather forecasting sites to represent anywhere in Norfolk County.
Keep in mind that this effect only occurs when it is both calm and clear. If there are any clouds or wind, then the temperature readings at Delhi would be accurate. You may also consider having your own thermometer out in the open at eye level to get an accurate reading of actual air temperatures, or using one of the other Environment Canada weather stations like Brantford or London.
Forecasting Frost
There is the continued risk of frost for the next two nights, and then the cold air may return later next week. Keep an eye on the forecast for planning frost mitigation measures like row covers. For forecasting frost, it is best to look at multiple weather forecasts. The dewpoint often provides a good indication of how low the temperature can get if it is calm and clear. Some sites (e.g., Weather Underground) provide dewpoint forecasts. Other sites (e.g. Meteoblue, The Weather Network) provide a relative humidity and temperature forecast. With these two numbers, you can use an online dewpoint calculator to determine the dewpoint.
Row Cover Cautions
Row covers provide 1 to 2 degrees of protection from frost, which is often sufficient to protect against damage. However, they also act as greenhouses during the day and can be up to 10 degrees warmer during the day in full sun. Since ginseng shade blocks 80% of the sun, the warming effect is likely not as extreme in ginseng, but it could still be a few degrees warmer under the row cover during the day. This could cause the plants to emerge faster than they otherwise would, which may make them more susceptible to any future frost events over the next few weeks. This effect would be minor if the covers are only on for a day or two, but could be more dramatic if they are left on for extended periods. Diseases can also progress under the cover, and you will be unable to apply fungicides while the cover is on. Although it is labour-intensive to take them on and off, it is best to only cover the plants for the period of the current frost risk and avoid keeping them on during sunny and warmer weather, even if there is another potential frost event in the forecast for later next week (which is still uncertain).
