Weather forecasts from several different sources are in agreement with lows for tonight and again early next week reaching -3 to -4oC. Current indications suggest that the cold tonight will be accompanied by a breeze of around 10 km/hr. As a result, a temperature inversion, in which temperatures are coldest on the ground, will likely not develop. If the breeze occurs as predicted, this would prevent it from getting much colder on the ground. A dewpoint of around -13oC is in the forecast for later this evening in Norfolk County, which means the air could cool to that temperature before frost would form. A temperature inversion would have allowed temperatures on the ground to reach many degrees colder than the predicted low temperature, since predicted air temperatures are taken at a height of 1.2 m. A breeze would also prevent cold air from pooling in low areas and valleys.
The problem with a consistent breeze is that it causes problems for growers implementing many of the frost protection measures that would work in an inversion. Here are the issues with many of the frost protection measures:
- Wind machines. Wind machines work by pulling warmer air from up above and blowing it towards the crop, displacing the cold air on the ground. These only work if there is a temperature inversion AND air temperatures several metres above the canopy are still above freezing. Wind machines will likely not work with the current forecast for this evening, since neither an inversion nor above freezing temperatures above the canopy are forecast to occur.
- Irrigation through the frost. When water freezes it releases heat. If water is consistently added during the freezing process, this heat keeps the ice and the plant tissues underneath at 0oC. Plant material will not freeze at these temperatures due to the dissolved minerals and sugars in the tissues, which lowers the freezing point. In order to work, the irrigation must begin before it begins to freeze and continue uninterrupted until air temperatures rise above freezing again in the morning. Most growers do not have the capacity to keep the entire crop irrigated all night long. A consistent breeze and very low temperatures would make this strategy even more difficult since the amount of water applied would need to be even higher to keep up with the freezing and the irrigation would be required for a very long stretch. A breeze can also make the irrigation more variable over the canopy. For more information see this factsheet – http://www.omafra.gov.on.ca/english/crops/facts/frosprot_straw.htm.
- Irrigation ahead of time. Irrigation a day or two ahead of a frost can increase the amount of warmth the soil can carry and increase the humidity in the air above. This is helpful if done early enough for the sun to warm up the wet soil, so that warmth can be released slowly at night. Unfortunately, a breeze will constantly displace the air above the soil and negate most of the benefits from this method.
The final strategy growers can employ are row covers. At night, warmth from the soil is radiated upwards. On a clear night, that radiation is lost to space. On a cloudy night, much of the radiation is reflected back to the ground, preventing a temperature inversion and keeping the air warmer. Row covers work much like cloud cover to reflect some of the radiation back to the ground and keep some warmth under the covers. They also block some of the wind, preventing warmth underneath from being displaced by colder air. The windier it is, the less effective the row cover will be since it is likely to be flopping around in the wind, and the cold air will go through the cover more easily. With a light breeze, they should still provide significant benefits.
Row covers provide about 1 – 2oC protection, depending on their thickness. Doubling up the row cover can provide even more benefits because it add an extra layer of insulation. They can also be combined with irrigation ahead of time to increase the amount of heat the soil can hold and increase air temperatures under the cover at night. The irrigation will only help if the source is not very cold (e.g. <5oC) and there is enough sunlight and/or warm temperatures ahead of irrigation to warm the moistened soil. Warm temperatures are unlikely, but some areas could have enough sun to add some warmth under the cover. Plant material should also have enough time to dry before the freeze. Even at cold temperatures, the sun at this time of year can warm up the air under the cover by up to 10oC, which can then warm the soil underneath.
With the current forecast it appears the best method of protection for all low-growing specialty crops is row covers. This could be combined with moistened soil, if the irrigation can be done early enough and there is some sun to warm the ground up ahead of tonight. It is unknown if lavender can be damaged by the forecasted temperatures, but row covers will not hurt if you have the option to put them on. For ginseng, row covers will be the best option for protection, since ginseng that is above the straw may be damaged by the forecasted temperatures. For specialty tree fruit and nuts there is not much that can be done to protect the crop unless you have the capacity to irrigate over the entire canopy consistently throughout the freeze. Many specialty tree fruit and nuts are not at a really sensitive stage at the moment, so irrigation should only be employed if necessary, since the weight of ice on limbs could damage the crop as well. Hops that are damaged by frost can be replaced by new shoots emerging from the rhizome. As a result, frost protection of hops is not worth the effort.