Before grapes fully mature, a number of changes take place with the grape, grapevine and grape seed. Look for these clues to determine harvest time.
The color of the grape will turn from green to red, blue or white depending on the variety. A white coating, called a bloom, will also be visible on a fully ripened grape. The grape seed and the stem that holds the cluster both turn from green to brown.
Squeeze a grape with your fingers. A grape ready for harvest will be slightly less than firm to the touch.
Tasting the grape is the best way to determine if it is ripe. Mature grapes will taste sweet and not acidic. Taste the grapes at the bottom tip of the cluster as these ripen last. A ripened grape will be 18 to 22 percent sugar with a pH level between 3.0 and 3.5. Portable handheld tools to measure sugar and pH in grapes are available from online retailers.
Be aware that weather and climate can impact harvest dates. Consider picking ripened grapes ahead of forecasted rainfall. If rain occurs, wait two days for grapes to dry before harvesting. If frost hits, pick ripened grapes immediately to preserve their quality.