Vermont orchards and vineyards are in varying states of vulnerability and likelihood of frost damage going into tonight’s expected cold event. At UVM orchard in South Burlington, we are at about 50% petal fall on McIntosh and the vineyard is at about one inch of shoot growth; inland and upland orchards are at or even approaching full bloom and vineyards are hovering around bud break. All orchards in the state are at risk for blossom tissue damage with temperatures at or below 28°F, and potentially a full crop loss around 24-25°F. Vineyards with fully emerged buds (1-2 leaves or more) are at the same risk, but those at bud burst or behind are less susceptible to damage and may do okay even into the lower twenties. In the Champlain and Connecticut River Valleys, expected lows are in the 30-32°F range, and orchards should be okay except possibly in low-lying pockets. Inland and upland, this event will likely show where site selection leaves orchards at most risk. Some inland orchard sites are calling for lows in the 24-25° range- that could cause substantial damage. Winds are expected to calm down overnight which lends to increased frost risk in low areas where cold settles. This does mean that methods that stir up air inversions may help- wind machines, helicopters, etc.- but few of our farms are set up for that aside from the lone wind machine I know of in Shoreham which I expect will be running in the early hours of tomorrow morning. There is little you can spray to affect frost tolerance of plants- some materials like some copper formulations and some potassium fertilizers are either labeled or purported to improve bud hardiness but not when sprayed right during a cold event and the science isn’t very good on that, either. Not to mention that sprayers will be icing up if you try to spray at 10-11 PM or later when the winds calm down, as that’s when the cold (below 32°, when spays will start freezing) is expected.
Growers could try burning round bales in the lowest / coldest parts of the plantings, starting around midnight or whenever the temperature starts to get down to 28°, but that isn’t known to be especially effective as the heat needs to radiate across a large area to really have any effect, and the smoke and near-range heat from burning bales can damage developing buds. If you have a Frost Dragon or similar device, tonight would be a good time to run it. But who has one of those in Vermont (I’d love to hear from you if you do).
The best case for most may be to wait this out and to conduct an assessment tomorrow afternoon or Friday. This article provides some good pictures of damaged tissues to compare yours against.
https://www.canr.msu.edu/news/assessing_frost_and_freeze_damage_to_flowers_and_buds_of_fruit_trees
Understanding the extent of damage will help with reporting for crop insurance purposes or to adjust you fruit (apple) or shoot (grape) thinning needs.
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