One or two nights of below freezing temperatures have been predicted for many areas of the state this coming weekend.
In most areas, buds and blossoms on tree fruit and small fruit crops are at their most susceptible point in regards to cold damage. For tree fruits approaching full bloom and for a period beyond it, 28 degrees F for 30 minutes or longer is considered to be the point at which 10 percent of the buds and blossoms will be killed and 24 to 25 degrees F for 30 minutes or longer is considered to be the 90 percent kill point. For many small fruit crops like strawberries and blueberries, the critical temperature is a bit higher - about 30 degrees. F. For periods where the subfreezing temperatures last longer, or where there are back-to-back cold events, damage may be more severe. The tolerance of the buds and blooms to cold will also be influenced somewhat by the overall health of the plant.
Protection of the blooms from frost/freeze damage can be achieved in many cases, and often just a difference of a few degrees will mean the difference between no damage or minor damage and a severe loss. There are several thoughts on frost protection - some effective and some totally ineffective or worse.
For small trees or small fruit crops, covering the canopy of the plant with a sheet or blanket will slow down reradiation of heat accumulated in the soil during the day back up into the atmosphere at night. Plastic sheeting is not as good a covering as cloth as burn on the plant will occur where the plastic rests against new plant tissue in subfreezing temperatures. The more of the plant canopy that can be covered, the better, however, the cover does not need to completely envelope the crown of the plant from ground to ground. The cover serves as a way to slow down heat loss from the ground under the plant. Covers should be put over plants late in the afternoon to early evening to take advantage of as much heat accumulation in the ground during the day as possible. Covers should be removed the following day before temperatures get too high. A small, supplemental heat source such as a trouble light using an incandescent bulb suspended in or under the plant canopy and under the blanket covering the plant will provide some added benefit. More intense heat sources should be avoided.
Washing the frost off a plant in the morning is ineffective as the damage will have already occurred by that time.
Overhead irrigation is the most effective way to protect tender buds from cold injury, however, it is also the most difficult to do effectively and the penalty for not doing it right can be devastating. Therefore, this is not recommended for non-commercial growers. To be effective, overhead irrigation must begin before temperatures drop to the freezing point and continue nonstop throughout the night until active melting begins the following day, often midmorning or later. Icing down a plant and stopping the irrigation while temperatures are near or below the freezing point will cause far more damage than would have occurred if nothing had been done. Evaporative cooling on the ice will lower the temperature of the ice and the plant tissue under it to levels well below the air temperature. Also, the weight of the ice load can cause substantial damage to the plant.
The critical temperatures for damage with developing fruits shortly after blossom are the same as for bloom. It is important to keep in mind that a healthy fruit tree can set a full crop on a small percentage of its blooms. Following a frost, it takes a while before the amount of damage can be assessed. With this in mind, it would be a mistake to quit caring for the crop immediately after the cold event because there may be more surviving buds than thought and by the time this can be recognized, other factors such as insects and diseases may have damaged the remaining fruits.