What’s important to look at if concerned about potential frost?

Two things-the developmental stage of your buds and the critical temperature for frost damage
– For deciduous like pome and stone fruits, dormant buds are protected enough and not too much affected by the frost and freeze conditions. However, once the buds swell and become ready to burst, low temperatures are a major threat for the developing floral organs and the fresh vegetative tissues. For the northern parts of Virginia, and from what I see in our apple orchard at AHS Jr. AREC, I would not raise any red flags regarding the frost that might occur tonight (March 10) or over the weekend. Why? because most apple varieties are still between the silver tip and the green tip stage and we would need some degrees between 5-10 F or below to worry about bud damage, which is not the case based on the weather forecasting for this period.
Here is the table that has been developed by WSU for the critical temperatures that might cause damage to apple and other fruit trees.

To download the whole table, use the link https://extension.usu.edu/files/publications/factsheet/pub__5191779.pdf

– Another source that may be useful for some of you is the apple freeze risk tool developed by Cornell University. Link:

CSF Apple Stage / Freeze Damage Probability

In this tool, you can select the location (using ZIP code), the variety (only three apple varieties), and whether you would like to see the full season trend (choose full season) or the current (this is the default setting). The tool takes into consideration the minimum temp., the developmental stage, and the temp. where 50% buds could be killed due to frost. If the minimum temp line (the blue line), crosses with the critical temp (the orange line), then the expectation is 50% bud damage.
If you look at the chart above, you can easily realize why I am stating that it might be useful for SOME (not all), because we at Winchester area are not even close to the tight cluster in Red Delicious (or any other variety) as this tool suggests; and hence the prediction of the 50% damage during the weekend (March 11, 12) is unrealistic. However, if this tool hits the right stage for you, I would say use it as a reliable source for frost prediction. Otherwise, use the above-mentioned table to assess the damage, if any.

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