Although the weather forecasts generally still show low temperatures above freezing every night this week in the ginseng production area (as of Monday at noon), the risk of frost especially Wednesday night is relatively high. This is because of the following:
- Longer range forecasts (beyond 24 hr) are often conservative until the event approaches. It has happened many times that forecasts show lows above freezing and then the forecast lows suddenly drop the day before the event.
- Humidity is forecast to be low, and we have not had rain in over a week, allowing the surface of most fields to be dry. Dry air cools much faster than moist air. Weather Underground, the only forecast I can find that includes dewpoint, shows the dewpoint as -4C Thursday morning. That means that dew or frost would not form until it hits that temperature and is often a good indication of how cold it could get on a clear and calm night at the ground level.
- All forecasts show clear skies Wednesday night.
The main factor that could prevent a significant freeze event is wind. Some forecasts show a breeze all night and others show dead calm. A breeze would mix up the air and often prevents a frost or freeze from developing at this time of year.
Now is the time to start planning for the potential for frost. It is best to apply fungicides for both Alternaria and Botrytis ahead of a frost event. If any frost damage occurs, the risk of both these diseases increases dramatically. Fungicides applied immediately after a frost event can cause phytotoxicity damage because open wounds on the plants allow the fungicide to be taken up more into the plant. Applying products now allows some protection during the period that products cannot be applied.
Row covers are the best option for protecting gardens from frost damage. If you cannot cover all of your gardens and have to prioritize, focus on low areas where cold air can pool on calm nights and seedling gardens that are at the highest risk of damage.
If you do not have row covers, irrigation is an option. Overhead irrigation ahead of a frost event may help a little by increasing humidity in the garden. However, to provide full protection, irrigation must be started before it begins to freeze and continue right through to after the freeze is over. This works well for other perennial crops but may cause higher risk of disease in ginseng due to the excessive water applied.