Frost Warning Tonight

This article was adapted from an article by Sean Westerveld,                                             OMAFRA Ginseng and Medicinal Herb Specialist out of Simcoe.

                                                                                                          Sean is a great writer and posted a very detailed article on Frost Protection for Ginseng on OMAFRA’s Specialist Crops Blog today.  I took the liberty of re-posting it here, and adapting it a bit for nursery production.


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.  Its sunny and windy so foliage should dry more rapidly than normal.  Watering early in the day could also allow the wet soil to warm during the day and retain more heat at night.

Frost Protection Products?

Various spray-on products have been promoted for frost protection. These have not been tested for use on ornamentals for phytotoxicity or whether they provide satisfactory benefit.   Some growers are in the process of testing these products.

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.

Continuous Overhead Irrigation

Continuous overhead irrigation is a common practice for protecting crops from frost damage, especially in the fruit industry. 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.  The weight of the ice can also damage tender new growth.

Some nursery growers have found overhead irrigation to be an unsatisfactory method of frost protection, especially if the frost event is extreme or more than a few hours.

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 other growers. Growers have experimented with different techniques such as fans on the edge of the production area. If temperatures are cold enough, this may 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 production area over time to ensure the method is working.



About Jen Llewellyn

OMAFRA Nursery and Landscape Specialist @onnurserycrops
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