Cold Injury to Apple Flower Buds: How and When to Assess the Severity

In Biglerville on March 27 and 28, 2022, we were at calyx green/calyx red peach bud stages and at green tip to half-inch green on apples. It got down to 20°F and 19°F those nights and killed some vulnerable blossoms. It has only recently warmed up so that the damage became apparent. My initial assessment is that there is some damage but that there also are still adequate numbers of live flowers to set a crop.

Growers are doing their own assessments now, but if you wait a week or ten days, you will have an easier time getting an accurate non-destructive assessment, and there will still be plenty of time to adjust thinning practices to suit the actual situation. Once damaged by freeze, the developing flowers will quickly die. When the buds get to tight cluster, damaged flowers will be shrunken brown mummies that can be flicked away with a fingertip. Live flowers will develop and bloom normally. Flower damage will be visible and can be objectively assessed from tight cluster through bloom.

Now go in your glove box or junk drawer and pull out the Equilli-Fruit disk you picked up at a twilight meeting or trade show. Select and measure the caliper of representative limbs at 1 inch from its origin. Match up the best fitting notch on the disk and read the corresponding the F value. This is the number of flowers that need to set to have a full crop. If the number alive is less than the F-value, this indicates a crop loss. If the number is greater than 100%, you will probably have to thin. We had a similar situation in 2012, but although we had 35-50% apple flower mortality, we still had 500 or 600 % flowers to produce a full crop.

Alternate figure showing placement of Equilli-Fruit disk. Photo: Melanie Schupp, Penn State