With frost in the forecast for Friday night, there have been questions about various frost protection methods in ginseng. We don’t have any specific research results for ginseng, but there has been work done for other crops. Here are the main methods used and advantages and disadvantages for their use on ginseng:
Row Covers/Frost Blankets
Row covers have proven to be the most effective for frost protection in low-growing crops, and are increasingly being used in ginseng. There are no studies in ginseng yet to determine the exact temperature difference between covered and uncovered gardens. However, in crops grown in open fields the protection ranges up to 6oC, depending on the thickness of the cover. The disadvantages of row covers are the added cost for materials and labour for application and removal, and that they interfere with fungicide applications. Because it takes time to apply the cover to the field, they have to be applied based on a forecast for frost conditions and forecasts can change over time. However, the protection they provide would probably be far better than any other existing method.
Continuous Overhead Irrigation
Continuous overhead irrigation is a common practice for protecting crops from frost damage, especially in the fruit industry (Figure 1). It has proven to be very effective as long as it is done properly. In order to be effective irrigation has to start before frost conditions begin and continue without interruption until temperatures go back above freezing. The reason it works is because water freezes at a higher temperature than leaves or fruit and heat is released during ice formation which prevents the ice from cooling further. Ice will build up on the surface during the freeze, but this ice will remain at 0oC, above the freezing temperature of the plant. As soon as irrigation is stopped, even for a few minutes, the water can fully freeze and quickly cool below freezing causing the plant tissues beneath to freeze as well.
Although this method has been effective for other crops, it has major disadvantages for ginseng: 1) Massive amounts of water can be applied with continuous irrigation that will increase the risk of disease, 2) The tender tissues of ginseng might not be able to withstand the weight of ice that accumulates on the surface on a really cold night (Figure 2), and 3) Most growers do not have sufficient irrigation capacity to continuously irrigate all of their acreage all night.
Air Movement Techniques
Large fans or other methods that encourage air movement are common and effective for many orchard crops. The coldest air on a calm, clear night occurs right at the soil surface and temperatures rise with elevation (i.e. temperature inversion). These techniques move the air around to mix the cold air at the surface with the warmer air from above. Many of the machines used in orchards are fixed structures, which doesn’t work well for ginseng growers who move gardens around. In addition, the shade structure can block the air movement and prevent good mixing. Growers have experimented with different techniques such as fans on the edge of the garden, but the shade structure can again interfere with air mixing and can result in a re-circulation of the same air. If temperatures are cold enough, this can result in even more damage because wind-blown cold air will cool more effectively than still air. There may be techniques to encourage more mixing with the warm air above, but growers should always monitor air temperatures in and out of the garden over time to ensure the method is working.
A wet soil can hold up to four times more heat than a dry soil. Frost conditions are often worst during dry periods because of this issue. Previous research has suggested that irrigating a field ahead of time to increase soil moisture can slow the cooling on a clear night. This method may be less effective with a straw layer, because the straw reduces heat transfer from the soil to the air. It also wouldn’t prevent cold air from neighbouring fields from flowing downhill into a ginseng garden, but might reduce further cooling once that cold air is in the garden. Although this hasn’t been tested for ginseng, it has been dry anyway, so there is no major downside to irrigating at this time. This technique, if effective, would work with any irrigating method. However, leaves must be dry when entering the freezing period or the water droplets on the leaf can freeze at a higher temperature than the leaves would freeze and can damage the leaf tissue beneath. If overhead irrigating, do not irrigate any later than the morning before the frost so the leaf tissues can dry. This could also allow the wet soil to warm during the day and retain more heat at night.
Various spray-on products have been promoted for frost protection. These have not been tested for use on ginseng for phytotoxicity or whether they provide any benefit. Furthermore, some of these products would need to be registered for use on a crop, and are not currently registered for ginseng. These would not be advised until further research is conducted