Some factors cannot be controlled when growing grapes

Unfortunately, there is not much you can do about that. It was poor pollination. Climatic factors have a significant effect on fruit set. Fruit set is greatly reduced when temperatures fall below 65 degrees Fahrenheit or exceed 100 degrees Fahrenheit during set. Rainfall or high humidity may reduce fruit set, hindering pollination. Rain can also interfere with the germination of pollen grains and inhibit fruit set. The berries appear to set and then fall off when they reach a small size.

Bob Morris is a professor emeritus in horticulture with the University of Nevada and can be reached at extremehort@aol.com. Visit his blog at xtremehorticulture.blogspot.com.