This article was adapted from an article by Sean Westerveld, OMAFRA Ginseng and Herb Specialist out of Simcoe with additions from a 2015 article from Jennifer Llewellyn, former OMAFRA Nursery and Landscape Specialist

Although the weather forecasts generally still show low temperatures above freezing in the evenings this week, the risk of frost especially Wednesday night is relatively high. This is because of the following:

1. 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.
   
   
2. 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.
   
   
3. 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. Sometimes, it can help to apply fungicides for fungal pathogens such as Alternaria and Botrytis ahead of a frost event. If any frost damage occurs, the risk of fungal 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 ahead of a low temperature event allows some protection during the period that products cannot be applied.

Irrigate Your Crops!

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 crop ahead of time to increase soil moisture can slow the cooling on a clear night. It wouldn’t prevent cold air from neighbouring fields from flowing downhill into a production area, but might reduce further cooling once that cold air is in the crop.

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, irrigate as early in the day as possible so that the leaf tissues can dry.  Watering early in the day could also allow the wet soil to warm during the day and retain more heat at night.

Protecting crops with continuous irrigation

Continuous overhead irrigation is a common practice for protecting crops from frost damage, especially in the fruit industry (strawberries, apples, etc.). It has proven to be very effective as long as it is done properly. To provide full protection, irrigation must be started before it begins to freeze and continue right through to after the freeze is over. Ice will build up on the surface during the freeze, but this ice will remain at 0^oC, 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.  The weight of the ice can also damage tender new growth, therefore it has not been a common practice in the nursery sector to my knowledge.

Row Covers/Frost Blankets

Row covers/Frost blankets have proven to be the most effective for frost protection in low-growing crops, and are increasingly being used in horticulture. The disadvantages of row covers are the added cost for materials and labour for application and removal.  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. Higher priced items like Japanese maples often receive this royal treatment.