May 5, 2017

Michigan frost forecast brings cautions for apple growers

Michigan State University Extension is urging Michigan apple growers to keep an eye to the weather forecast for the weekend – there are conditions predicted that might lead to a frost/freeze event.

When frost conditions occur in fruit orchards, what are activities that you can do to help minimize the potential damage and what are minimizing characteristics?

Apple Critical Minimum Temps for 10 percent Kill

Petal Fall 30˚ F

Full Bloom 30˚ F

Pink 28 ˚ F

Open Cluster 27˚ F

Activities:

• Frost fans will provide warming by mixing warm into the orchards. (Perhaps 5˚ F)

• Under-tree micro-sprinkler will provide heat from well water into the orchard. Start system a couple of hours before temps get too cold and freeze up the system. (Perhaps 4˚ F)

• Over-tree sprinkler will protect plants coated with ice. (Uses large amounts of water)

• Herbicide strips. Bare Soils will absorb heat and provide radiation heat to trees overnight. (Perhaps 2˚ F)

• Mowing orchards short. Short grass will allow sod to absorb heat all day from sunny conditions to release all night to the trees. (Perhaps 2˚ F)

• Nutrient sprays will strengthen fruitlets to resist freezing temps. (Perhaps 1˚ F)

• Burning wood and hay will provide heat to the orchard for only small areas near the fire. (Perhaps 4˚ F). Place small piles of firewood down the center of every or row and light every other pile at 3 a.m. and then at 5 a.m .start the other piles. Each will burn to roughly two hours.

• Wetted soil, run irrigation to wet the soil as much as possible before the frost event. Start trickle early enough to thoroughly wet orchard before the frost. Run irrigation (trickle) the all night of the freeze. Trickle will probably freeze up during the frost event. Wet soil absorbs heat and radiates heat best.

• Frost protection spray products can be useful but not consistent.

Minimizing Characteristics:

• Flowers pointing downward will not radiate their heat as much and flowers pointing toward the sky. These tend to survive cold frost events

• Abundant bloom. Numerous flowers will survive the light to moderate frost event.

• Flowers at a wide stage of development (Pink to Petal Fall) will have different critical minimum temperatures and will survive frost events.

• Abundant foliage. Leaves will provide protection to flowers hiding under the leaf. It will reduce radiation of heat.

Other Information:

• Smoke is of no value. The heat will radiate skyward right through the smoke.

– Philip Schwallier and Amy Irish-Brown, Michigan State University Extension