AMES, Iowa — Home gardeners can successfully grow grapes in Iowa with a good planting site, hardy varieties (cultivars) and proper culture. Horticulturists with Iowa State University Extension and Outreach share grape harvest time tips. To have additional question answered, contact the ISU Hortline at 515-294-3108 or email@example.com.
Grapes should not be harvested until fully ripe. The best indicators of ripeness are color, size and flavor.
Depending on the cultivar, the berry color changes from green to blue, red or white as the grapes approach maturity. At the fully ripe stage, the natural bloom on the berries becomes more pronounced. However, color should not be the sole basis for harvesting grapes. Many cultivars change color long before the grapes are fully ripe.
Size and firmness are other useful indicators of ripeness. The individual berries should be full-sized. They also become slightly less firm to the touch at maturity.
The final and most reliable test for ripeness is flavor. Taste a few grapes when size and color indicate the fruit is approaching maturity. If they are not sweet, leave the clusters on the vines. Grapes do not develop full flavor when harvested before completely mature.
Several factors could be responsible for the uneven ripening of the berries within a cluster. Possible causes are over-cropping (too many grape clusters on the vine), a potassium deficiency, moisture stress or 2,4-D damage.
Over-cropping is the most common cause for home gardeners. An average grapevine may have 200 to 300 buds that 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 buds that should remain on a grapevine after pruning.
Grapes can be stored for one to two months at a temperature of 32 to 36 degrees Fahrenheit and relative humidity of 90 to 95 percent. For most home gardeners, the refrigerator is the best grape storage location. Place grapes in perforated/vented plastic bags and store in the refrigerator.
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.
Visit the Yard and Garden FAQs website, https://expert-hort.sws.iastate.edu/, for more information on growing and caring for grapes and other gardening topics. The publication ‘Growing Grapes in the Home Garden’ can be downloaded from the Extension Online Store at https://store.extension.iastate.edu/.