Question: I've got Thompson grapevines about 3 years old on an arbor along the north side of a block wall. The vines grow well, but the grapes are only about a quarter of grocery-store size. Last fall, I cut the vines back some, but was afraid of overdoing it. What's the correct way to do it?
Grapevines are just starting to produce when they are 3 years old and will continue to be very prolific producers if they're pruned correctly.
Thompson seedless grapes need cane pruning. Most other grapes can be spur pruned. Fruit in 2009 will be borne on canes that grew during 2008. Fruit is not produced on wood older than this.
This past year's growth should not be cut back more than about 18 or 24 inches long, which needs to contain about 12 buds. To increase the size of the berries, remove some of the fruiting canes so there are not so many of them, and when the berries are the size of petite peas, pinch off the bottom third of the berry cluster. This will allow more food generated by the leaves to be distributed to fewer berries.
There are other techniques, such as the use of hormones or girdling, that should not be left to the inexperienced homeowner. Delay pruning grapes until mid-March and danger of cold weather has passed.
Bob Morris is an associate professor with the University of Nevada Cooperative Extension. He can be reached at morrisr@UNCE.unr.edu.
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