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Put the bags on the grape bunches when they change colors. Cut 2 inches (5.1 cm) slits in a paper bag. Pinch off any leaves directly around the grape clusters. Slide a bag over the grape bunches. Keep the bags on until you plan to harvest them. Use organza bags as an alternative to keep a close eye on the grapes.
Birds generally aren't attracted to grape bunches until they start maturing. When the grapes start to mature and change colors, bag them as soon as you can. Take a pair of scissors and make 2 2 inches (5.1 cm) slits on either side of the bag. Follow the center seam of the bag as a guide for where to make the slit. The slits are where the grapevines will poke through the bag. Leaves need direct sunlight and will wilt if left in the paper bag. Pinch or cut away any leaves that the paper bag would cover once you bag the grapes. Although leaves cannot grow in the paper bag, your grapes can grow without wilting. They may, however, mature slowly. Position the bunches so the vines rest near the side slits. Fold the bag's top flap over and tape or staple it shut to keep it in place. Repeat this process on all of your grape bunches until you've covered all of them. Avoid taking off the paper bags until you're ready to harvest the grapes. If wind, rain, or heavy weather conditions tear the bags off, replace them with new bags when the weather clears. If you're not sure whether it's time to harvest the grapes, take the bag off partially. Examine the grapes and, depending on whether they've matured, either harvest them or put the bag on again. Paper bags are bulky and can make it hard to tell how you grapes are growing. To monitor your grapes as they grow, pull an organza bag over a grape bunch and cinch it in place. Do not remove the bag until you harvest the grapes.  Organza bags are translucent fabric drawstring bags, and they are available online or at most craft stores. If you use organza bags, you do not have to pinch of any leaves.