Environmental Canada has issued a frost advisory for most of the ginseng production area for overnight tonight (May 30-31, 2024). Patchy frost is possible in the area. Here are some reminders on assessing the risk of frost in your area.
Environment Canada issues advisories and warnings on a county level. If there is a risk anywhere in the county, generally the whole county gets the warning. Within Norfolk County, there is a lot of variability caused by factors such as proximity to Lake Erie and the amount of vegetation in the area. Generally, the highest risk areas are the areas around Delhi and west and southwest where soils are the sandiest, fields are flat, and a large number of fields are still mostly free of vegetation. These dry soils tend to radiate more heat on clear nights and there is less moisture to buffer the drop in temperatures. Cold air also flows downhill into the lowest areas, increasing the risk in these areas but only if the dewpoint is below freezing (see below). Surface temperatures on Lake Erie are now between 10 and 15°C and that relative warmth will likely contribute to more air flow at night and lower risk for areas closer to the lake.
The dewpoint is the most important measurement for determining frost risk. The dewpoint is the temperature the current airmass would have to cool to before dew or fog would form. At this time of year, the dewpoint is often a good indication of how low temperatures could get at ground level on calm and clear nights. If the dewpoint remains above freezing, then the risk of frost is low. The current dewpoint at Simcoe (2 pm) is -0.5°C but the dewpoint can still change if drier air is still moving in. Dewpoints tend to rise slightly at night when winds calm and the air near the ground absorbs some moisture from the ground and any nearby vegetation.
When forecasting frost events, it is best to look at a range of the available forecasts. Environment Canada is currently the coldest forecast. For Simcoe or Delhi, here are the current (1:30 pm) forecasted low temperatures for overnight tonight from the main sources:
Environment Canada (Simcoe and Tillsonburg): Low 2°C; no dewpoint forecast
Meteoblue (Simcoe): Low 7°C; calculated dewpoint (based on relative humidity) 2.9°C
Weather Underground (Delhi): Low 5°C; dewpoint 2°C
Weather Network (Delhi): Low 5°C; calculated dewpoint -0.8°C
Accuweather (Delhi): Low 4°C; calculated dewpoint 2.2°C
The shade cloth should provide some protection from frost, so the most at-risk areas would be the edges of gardens that can radiate heat outwards. Keep in mind that ginseng is probably slightly less sensitive to frost now that plants are fully expanded compared with newly emerging plants. Stems are sturdier now and less likely to kink if the stem is damaged by frost. Also, the full leaf canopy will add more humidity into the air, raising the dewpoint, and would act as a blanket protecting the stems from freezing. The exception is in seedling gardens, which do not have a full canopy.
The biggest risk of frost damage at this time of year would be the botrytis blight that develops on damaged tissues when temperatures warm back up. This can be counteracted with timely fungicide applications.
Although the current forecast is for a marginal frost event, forecasts can change and can be wrong, so take any precautions necessary to protect high risk areas, or if that is not possible or practical, ensure plants are protected with fungicides before the event or after any damage occurs. Avoid any pesticide applications within several hours of a damage event to allow plants to heal the wounds. Otherwise, there is a risk of phytotoxicity from excess absorption of the chemical into the plant.
