Both ginseng and lavender growers have invested in large amounts of row cover to protect plants from cold temperatures (Figure 1). Ginseng growers will use row covers when frost events are forecast. Lavender growers use row covers for winter protection, but may want to consider re-using them for frost protection as well if a major frost is in the forecast.
There is a considerable amount of labour involved to put row covers on the crop for a single frost event. Ginseng growers will need a day or two lead time before an event to get all of the cover on before the frost. Once it is on the crop, growers will also need to know when it can be removed. It can be difficult to find a forecast accurate and detailed enough to make these decisions in a timely manner. However, there are some things to look for in the forecast that could signal a significant frost event.
There are four main things that are required for a significant frost in the spring: a cold air mass, calm winds, clear skies and dry air. Cold air masses on their own do not necessarily lead to frost if it remains windy or cloudy overnight or if it remains humid. Normally on a clear night temperatures are coldest on the ground and warmer as elevation increases. This is because heat radiates from surfaces on the ground. Winds mix the warm air with the cold air on the ground, which usually prevents it from getting cold enough on the ground to cause damage. Any clouds will radiate the heat back to the ground, also preventing the surface from getting cold enough to cause damage, unless the air mass itself is exceptionally cold for this time of year.
The other requirement for significant frosts is a dry air mass. Drier air masses allow the temperatures to cool more rapidly. The dryness of the air mass is shown as either the relative humidity or the dewpoint. Relative humidity is the amount of moisture in the air relative to the maximum moisture that air mass can hold. Warmer air can hold much more moisture than cooler air. Consequently, as you cool an air mass the relative humidity will increase. The point at which that air mass cannot hold any more moisture is the dewpoint, or the point at which dew will form (also 100% relative humidity). When water condenses on a surface, heat is released that prevents the temperature from getting any lower until most of the moisture is out of the air. This is the principle behind irrigating for frost protection. As a result, the dewpoint provides a good indication of approximately how cold temperatures on the ground could get in most cases.
Both ginseng and lavender can tolerate light frosts. It is when the temperature on plant surfaces drops well below 0oC that damage can occur. If the forecast low is 0oC and the dewpoint is at -1oC, significant frost damage will probably not occur because the surface temperature is unlikely to go much below -1oC. In that case, fog may even form. However, on the night of May 23, 2015 when significant damage occurred to both ginseng and lavender, the dewpoint was -13oC the evening before the frost. In that case, the air mass was exceptionally dry and cold, and temperatures at the surface dropped as low as -9oC, even though the forecast low was in the range of -2oC. In some cases the air can be so dry that no dew will form, even though plants will be completely frozen and killed.
So when looking ahead in the forecast to see if a frost event may occur, look at four main things: Forecast cold night temperatures below 5oC, calm winds, clear skies and low dewpoints. Not all forecasts provide those details, and not all forecasts are accurate a few days in advance. We are currently tracking 5 main weather forecasting sites to determine which is most reliable over a one month span. Results of that tracking will be reported next week. The one weather forecasting site that does provide both hourly wind speed and dewpoint forecasts several days in advance is Weather Underground. Find out next week, how accurate different forecasting sites have been over the past month. A combination of weather forecasts may also be necessary to get an accurate forecast.
Also bear in mind that different locations have different microclimates that can be much colder than the forecast location. Also, low temperature forecast often under-estimate the cold in the Delhi region, an area that is known to be prone to frosts.