Grapes are self-fertile, so one plant will successfully produce fruit. Thinning the fruit increases berry size and hastens maturity. The best time to thin fruit is at blossom time when whole clusters of flowers can be removed. Generally a plant with one cluster per shoot may ripen two weeks before an over-cropped plant. Pruning away weak and thin shoots that cause shading to the fruit cluster will hasten fruit ripening.
Grapes are especially sensitive to drought.
Keep them well watered. Sufficient water will help the fruit to reach their maximum size and hasten maturity. Water the plants until late August, then withhold water to allow the plants to harden off for winter. Once grapes are picked, the ripening process stops. If frost is expected, the fruit can be protected by covering them with blankets or sheets.