Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
Prior to the cold snap, flowering had begun in some plasticulture strawberry fields in the region. As cold weather returned last week, temperatures dropped into the mid to low 20s requiring that row covers go back on. As a reminder, as buds become active but before flowers open, strawberry buds can survive down to temperatures of 22-27ºF, depending on just how close they are to opening. As flowers open, strawberries can only tolerate drops in temperature down to 30ºF. Small green fruit can stand temperatures down to 28ºF. During flowering and fruiting be prepared to freeze protect using row covers and in very cold conditions, sprinklers. It is critical to monitor temperatures. Temperature recorders that can be placed at crown height under the row cover are wise investments to do this monitoring.
When using sprinklers for frost protection, they must be used correctly. The idea is to slowly build up ice over the period when temperatures are below freezing over the plant or row covers. As ice is formed, some heat is released to the plant surface and to the surrounding air, due to the heat of fusion. To do this, sprinklers must be turned on before temperatures are at 34-35ºF. Use low volume sprinklers and apply irrigation throughout the night, building ice all night. Continue into the morning until ice has melted. An application rate of 0.15 inch per hour with no wind will provide protection to 22ºF. At colder temperatures or higher wind speeds more water will be needed.
It is recommended that for protection against frost above freezing, use sprinklers or row covers alone, for freezing temperatures in the mid to high 20s use sprinklers or row covers alone, for temperatures in the low 20s or below, use both sprinklers and row covers. A combination of row covers and sprinklers has been shown to protect below 20ºF.
Growers are strongly advised to subscribe to a weather forecast service to alert them of potential freezes in order to make frost protection decisions.
As fields are uncovered again, finish any cleanup of dead plant material and apply additional disease, mite, and insect controls. Fertigate nitrogen through the drip system, if it has not already been applied and monitor N levels using petiole and leaf samples. Petiole nitrate-nitrogen levels can be a good way to monitor and adjust your N program. At green-up, petiole nitrate-N should be around 1000-1500 ppm. As growth takes off, petiole nitrate-N should rise to above 4000 ppm. Petiole nitrate-N should be at 3000 – 4000 ppm during the first 4 weeks of picking and then decline gradually to around 1000 ppm at the end of harvest.
For more information about monitoring plasticulture strawberry nutrition, go the North Carolina Department of Agriculture & Consumer Services Agronomic Division plant tissue analysis site http://www.ncagr.gov/agronomi/pdffiles/sberrypta.pdf.