Growing grapes (Vitis spp.) isn't an exact science, and you can't predict exactly when the grapes will be ready to pick. When the grapes ripen depends on several factors, including the grape variety and how much sun the vine receives. In most cases, grapes tend to be ready for picking in late summer or early fall in the areas in which they grow of U.S. Department of Agriculture plant hardiness zones 2 through 10.
When Grapes Appear
In early to the middle of spring, grape flower shoots begin to appear. They grow through spring, and are pollinated. The pollinated blooms fall off the vine, leaving tiny grape berries behind, giving them the rest of the season -- sometimes all summer -- to develop and ripen.
Basic Picking Months
The best time to start checking your grapes for ripeness depends in part on the type of grapes you're growing. "Cabernet Sauvignon" grapes (Vitis vinifera "Cabernet Sauvignon"), for example, which grow in USDA zones 7 through 10, need a long growing season. You might be able to pick them as early as mid-September, but they might not be ready until the first two weeks of October. "Chardonnay" grapes (Vitis vinifera "Chardonnay"), which also grow in USDA zones 7 through 10, might be ready to pick as early as mid-August.
Factors of Ripeness
The grape variety gives you an idea of when to start checking your grapes for ripeness, but it can't tell you exactly when to pick them. Sun is a major factor in ripening grapes. If the summer was cloudier than normal, your grapes might ripen more slowly. Also, all grapes on the vine might not ripen on the same day -- bunches that sit on shady side of the vine could take longer than those that get more sun. Grapes tend to develop color well before they're ready to pick, so color alone shouldn't dictate when you harvest. Squeezing the grapes gently helps you determine when they become less firm and is often an indicator of ripeness.
Tasting for Sweetness
One of the easiest ways to tell when your grapes are ready to pick is to taste them, especially if you're unsure of your variety's general timeline. Start tasting the grapes when they look like they are the right color and feel slightly soft when squeezed. Continue tasting a grape daily or every other day until the grapes develop the sweet flavor you want. They don't continue to ripen after they're picked, so let them stay on the vine until the taste is right. For some varieties, this could mean tasting the grapes for several weeks, through September and into October.