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The color of the vine also indicates when to harvest Concord grapes. Throughout the growing season, the vine and stems are a light to medium green. When the vine and stems begin to turn to a light then medium brown, it means the grapes are ripening and ready to harvest.
Test the plums for ripeness. The easiest way to tell when your plums are ripe is to lightly press the skin with your finger. If you make a slight indentation, the plum is probably ripe. If it's still hard to the touch, you'll need to wait a bit longer.