Frost Risk

Once again, a cold snap at the end of the long weekend and into next week could bring the risk of frost to frost-prone areas. As of May 15, none of the forecasts show freezing conditions, and dewpoints are forecasted to be 0 to -1°C, but there is a long history of weather forecasts underestimating the cold. This past Friday, temperatures at the Delhi Environmental Canada weather station were 4°C colder than the coldest forecast available the day before. This was due to much drier air than forecast that allowed temperatures to fall, especially in low and flat areas surrounded by dry, bare fields. Unfortunately, this means growers need to consider a worst-case scenario that is at least four degrees colder than the forecasts when considering whether to use row covers or other frost protection means. There are two things to keep in mind for the upcoming colder weather:

1. Ginseng plants will be significantly more advanced than they were during the last frost events, and the more advanced they are, the more sensitive they become to frost.
2. Most trees in the area will be in full leaf by the end of the weekend. This often adds humidity to the air that can increase the dew point by a degree or two and lower frost risk. This may be part of the reason that forecasts are showing higher night temperatures for the coming cold period, even though daytime highs are in the same range as last week. This effect may be lower in areas with a lot of bare fields and few forests or hedgerows.

Keep an eye on the forecast temperatures and dewpoints for early next week. It is best to check more than one site and then subtract a significant safety factor.

Dealing with Frost Damage

Some growers experienced frost damage this past weekend. Most of the damage consisted of split stems on older plants, but some direct damage to leaves was also reported. The combination of this damage and the current humid, warm weather will greatly increase the risk of Botrytis blight. Protect plants with a product with high efficacy ratings for Botrytis, especially in fields with frost damage.

Multiple nights of cold and frosty conditions could also cause stunting of some plants, resulting in a variable canopy. This may cause higher foliar disease risk due to slightly weakened plants and more difficulty in getting thorough coverage with fungicides once the canopy is fully developed. It is important to ensure thorough coverage of fungicides for all foliar diseases before the canopy fully closes.

Leaf Folders

In our research garden, more leaf folders have emerged within the past week. Some areas of the garden have about 10-20 leaf folders per metre of bed, which is enough to cause major damage to the plants. Leaf folders appear to have emerged first in areas with thin straw cover. It is unknown if this is due to warmer soils in these areas, which may have caused earlier emergence, or a preference of the adults to lay eggs in those areas. As a result of the staggered emergence of larvae, insecticides may need to be applied a few times over the next two weeks. Scouting is important for determining where to spray and when the caterpillars are finished emerging.