Frost damage in tender fruit and grapes
Frost damage from May 22-23rd has been on a lot of people's minds these days. You will start to see bloom kill or fruit kill at temperatures just below zero in tender fruit and grapes. In pears at -2°C, in apricots at -3°C in full bloom and in the shuck and at -2°C at green fruit, in peaches at -3°C at full bloom and -2°C at post bloom, in plums at -2°C at full bloom and post bloom, in sweet and tart cherries at -2°C for full bloom and post bloom. For grapes there is cultivar variation in spring cold temperatures but in Pinot Noir you will start to see damage at -2°C at bud burst and two leaf and -1°C at 4 leaf.
In some areas where temperatures dipped below freezing, frost damage was variable on tender fruit and is evident to see where air drainage can be improved. Wind machines were able to mitigate some of the frost damage in the Niagara Region and Norfolk County. Most grape vines in Prince Edward County were hit by frost, even in vineyards with wind machines, where the reported temperature lows were between -3°C and -5°C.
You may not be able to observe the extent of the damage on blossoms and young fruit until 2-3 days after the frost event. You will have a better knowledge of the extent of the damage after June drop, when you will better see fruit russeting, misshapen fruit, frost rings (Figure 1), reduced seed count and fruit drop. You may see frost rings on pears from dead petals of the blossom. You also may observe peaches or nectarines only mature to the size of a walnut. It is important to keep frost damage in mind for your thinning and pest management program.
Figure 1. A frost ring on pears.
For grapes, death of the primary shoot may result in development of a shoot from a secondary bud; however, secondary shoots do not produce as much fruit, if any (Figure 2). You may get up to 50% of a normal crop off of a secondary shoot. What can be done to mitigate the effects of a late frost in the future is to practice later pruning. Pruned vines have an earlier bud burst than unpruned vines. You can also practice double pruning where you spur prune to 5-8 buds. After the risk of frost has passed or basal buds have begun to burst, prune to two buds.
Figure 2. Frost damage on primary shoots of grape vines.
If you do have frost damage in your orchards and vineyards take advantage of this time and look at the effects of low lying areas, buildings, wind breaks and air drainage patterns on frost damage. For future frost mitigation strategies, select cultivars that bloom later, frost fans, cold air drains, sprinklers and burning hay.
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