The best time to harvest vegetables varies depending on the specific plant variety. In general, most vegetables should be harvested when they reach their mature size and color. For leafy greens and herbs, you can start harvesting when the leaves are large enough to use. Tomatoes should be harvested when they are fully ripe and have a vibrant color. It’s always a good idea to refer to the seed packet or gardening resources for specific instructions on when to harvest each vegetable.