Answer: Some varieties of grapes are better suited to different regions than others, so it is possible that your vine is not one of the best for your area and needs a longer season. Note too that color is not the only indicator of ripeness -- check the seeds and when they turn from green to brown and the cluster stem turns brown and wrinkles, that's when the grapes are ripe. Perhaps you have a wine grape rather than a sweet table grape.
In addition, regular care such as pruning, tying, weeding and spraying are required for the best possible production. If you do not normally fertilize, you might give the vine one tor two cups of a complete fertilizer such as 10-10-10 in the spring when the buds swell. Your County Extension (328-4800) should be able to give you the routine care recommendations for grapes in your area, including the suggested spray regime based on the local incidence of particular pests and diseases.
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