Wait for it. Some vegetables give more clear indications of when they are ready to harvest. With onions, for example, the tops will fall over, letting the gardener know it is time to dig. Most guides recommend that new potatoes be harvested when the plants flower, but for mature spuds wait until the plant turns brown. Melons tend to fall right off the vine when ripe, and you want to catch them just before that moment. Muskmelons give off a rich aroma that says, “I’m ripe.”
Color and feel should be your guide for tomatoes. You want them to reach the correct color for the variety you’ve planted and be slightly soft. Generally, ripe tomatoes come off the vine with barely a tug.
Root vegetables, such as beets, carrots and parsnips, can be harvested at whatever size you like. And, parsnips and carrots often develop a sweeter flavor if left in the ground after a frost.
Winter squash needs to stay on the vine so it can reach its full color and have a skin that is so tough that it is difficult to puncture with your fingernail. Only then, should the squash be cut from the vine (with a stem on it) and allowed to mature in a cool, dry place.