Fruit tree frost protection

Spring is almost here, and fruit crops will soon be in bloom.  Keep in mind that it’s just mid-March and we are not out-of-the-woods when it comes to cold weather.  During March and early April, we seem to always experience extreme weather changes that can be devastating to our fruit crops.

 Advective freezes occur when an arctic cold mass moves into a region.  It may encompass a large area and can last upwards of several days.  During an advective freeze, daytime temperatures will be cold, seldom getting above 50 degrees, the relative humidity will be low, and windy conditions will exist.  Cloud cover is also quite likely.   At night, it will get colder, remain dry, windy, and cloudy. 

 Radiation frosts comprise most of our cold weather events in spring.   Radiation frosts differ greatly from advective freezes in that they tend to be localized and of short duration, often only a few hours.  During the day leading up to the frost event, temperatures are apt to be warm, winds calm and skies clear.  At night, temperatures may drop considerably, often getting down to the critical levels for damage to many fruit crops.  An inversion layer is apt to develop, allowing several active frost-control techniques to be used successfully. 

 Frost protection techniques fall into two basic categories: passive and active.  Passive frost controls involve elements that are extremely important but may have already been decided. They include things like site selection, site development and maintenance, type of fruit crop being grown, variety/rootstock selection and cultural practices being used in the plantings.  Passive controls are good horticultural practices from other standpoints, as well as protecting against radiation frosts.

 Active frost control techniques are utilized specifically for frost protection.  They should be considered as being supplementary to passive controls.  They are expensive to install and utilize and each of them have definite limitations on their effectiveness.  Heat, wind, heat plus wind, row covers, water, and row covers plus water are the most common active frost control techniques.  For most homeowners with just a few trees, a frost blanket or an old sheet and possibly a heat lamp will be the easiest and most cost effective.  If put on and secured well, frost blankets or row covers will provide some protection. While a single lightweight row cover will only provide about 2 to 4 degrees protection, using a heavier cover, or using two lightweight covers together will increase the level of protection considerably.  If using row covers for frost protection, it is best to wait until later in the day to pull the covers over the crop being protected to allow as much heat to be absorbed from the sun as possible.