Source: Iowa State Extension
Home gardeners can successfully grow grapes in Iowa's climate. Grapes can flourish in a backyard garden or a vineyard, but obstacles like insects and knowing the proper harvest time can keep them from reaching their full potential.
ISU Extension and Outreach horticulturists can help answer your questions about issues with grapes and proper growth. To have additional questions answered, contact the ISU Hortline at 515-294-3108 or firstname.lastname@example.org.
Grapes must be harvested at the right stage of maturity to insure high quality. There are several indicators of grape maturity. Berry color, size, sweetness and flavor are the most useful indicators.
Depending on the cultivar, berry color changes from green to blue, red or white as the grapes approach maturity. Color alone, however, should not be the sole basis for harvesting grapes. Berries of many cultivars change color long before they are fully ripe. At maturity, individual berries are full size and slightly less firm to the touch. As a final test, taste a few grapes for sweetness when berry size and color indicate the fruit is approaching maturity. Harvest grape clusters when the berries are sweet.
When harvesting grapes, remove clusters with a knife or hand shears. Sound grapes can be stored in perforated plastic bags in the refrigerator for up to two months.
Several factors could be responsible for the uneven ripening of the berries within a cluster. Possible causes include overcropping (too many grape clusters on the vine), a potassium deficiency, moisture stress and 2,4-D damage.
Overcropping is the most common cause for home gardeners. An average grapevine may have 200 to 300 buds which are capable of producing fruit. If grapevines are not pruned properly in late winter, the number of fruit clusters may be excessive. The vine is unable to ripen the large crop properly, resulting in uneven ripening of the berries within the clusters. In Iowa, 60 is the maximum number of fruiting buds that should remain on a grapevine after pruning.
The best way to prevent birds from eating the grape crop is to place netting over the grapevines. Netting can be purchased at garden centers or through mail-order companies. When placing netting over the grapevines, make sure the netting is secured to the ground to prevent birds from entering from below.
Yellow jackets are a type of wasp. In early summer, yellow jackets forage for caterpillars and other “meat” items and are beneficial predators. However, in late summer they prefer sweets such as soda pop, candy and the sweet juices of fruits and vegetables.
Discourage yellow jackets from feeding on the grapes by harvesting the grape clusters as soon as they ripen. Remove any over-ripe or damaged fruit from the garden area. Do not leave beverages, candy or other food items in the vicinity of the grapevines as they may attract yellow jackets to the area.