Strawberry Frost Protection

April 8, 2024

Row covers are the favorite way to protect the plants from frost. 1.2-ounce row covers can increase frost protection by about 4-5 degrees and you can even double up if you want to.

Nutrient management also can improve resistance to frost – especially calcium, phosphorus, and silica, and if not organic, use 1-2 lb. urea. Copper has also been known to help frost resistance.

Sprinklers are also commonly used for frost protection. This can be very effective if used correctly. A general rule of thumb seems to be that once it hits 34 degrees air temperature, you want to start sprinkling – but that is not always correct. It is actually 32-34 degrees wet bulb temperature that really matters.

Wet bulb temperatures factor in the dew point along with the air temperature to give a more accurate picture of when the sprinklers should or should not be turned on. To find the wet bulb, you take the dew point, subtract the air temperature, divide by 3, and add the air temperature. This equals your wet bulb temperature. Example: If your dew point is 22, and your air temperature is 34, the wet bulb temperature will be 30 degrees.

So why does this matter? In the example just given, if you were to turn on the sprinklers at 34 degrees air temperature, you would be much too late to save your blossoms and fruit, because it would be below freezing when you factor in the dew point, which would have given you a wet bulb temperature of 30. Knowing the wet bulb factor, you would have wanted to turn on the sprinklers at about 38-41 degrees air temperature instead.

The long and short of all this is that you NEVER want the wet bulb temperature to go beneath 32 degrees. If you have a weather forecast that gives you dew point and temperature, you can do the wet bulb calculation yourself – it’s very easy, as described above. Otherwise, if you are serious strawberry grower that will use sprinkler irrigation to save a crop in the case of cold weather, you may want to get a tester that gives you the dew point and/or wet bulb calculation. They are available for under $100.

Again, when sprinkle irrigating for frost, we recommend to never let the wet bulb temperature drop beneath 32 degrees. Once started, you want to continue sprinkle irrigation onto the sun comes out and melts the ice. Some growers are sprinkle irrigating directly onto the strawberries. Others are sprinkle irrigating onto row cover, which I like much better because it gives you additional protection.

One last thing: when using row covers, you want to take them off whenever you can while the strawberries are blooming, due to pollination. My understanding is that better pollination triggers larger berries, so this becomes very important – as does having high brix during bloom so that bees want to work the flowers. A bee will go to the flowers that have the highest nectar flow because that is where they have to work the least.