HORTICULTURE HORTICULTURE
Late Spring Frost in Grapes
Mark Longstroth
On June 3 1998, a severe frost struck Southwest Michigan. It was a classic radiation frost affecting low-lying areas. Clear skies allowed the ground to cool by radiation to the sky. The ground then cooled the air in contact with it through conduction. Cold air accumulated close to the ground and as the amount of cold air increased it formed deep pools above the ground. The cold dense air began to flow downhill where there was vertical relief. It also collected in any flat areas even those which were located in higher areas without air drainage. As the ground cooled air temperatures dropped below freezing and damaged grapes. Low temperatures were reported into the 20s on June 3 and 4.
Low spots in vineyards were affected.
Air drainage in this vineyard was impeded by a road fill which blocked cold air flow out
of the vineyard and backed air up this draw damaging vines much higher than the level of
the road.
Orchards and vineyards on flat areas without air drainage were also
affected.
Damage varied greatly within vineyards.
And within the same plant,
leaves on the outside of the canopy were killed
Or most of the canopy was killed.
Sometimes leaves were only damaged.
The leaf on the left is relatively undamaged while the one on the right shows the
puckering typical of frost damaged leaves.
Flower clusters below leaves were might be killed.
leaving the leaves relatively undamaged
Sometimes damage was extensive, but new growth began quickily
New bud growth can be seen in the top center of this photo.
More protected clusters deeper in the leaf canopy survived
More protected clusters deeper in the leaf canopy survived
These fruitlets survived, but show russeting from the frost.
On severily damaged vines, regrowth began almost immediately.
At extreme left a dormant bud has begun to swell. At the right a new shoot have
broken from buds at the base of this years leaf petioles.
By the end of the year damaged areas were almost invisible as new growth had completely covered the damage.
