Scrape the soil away from around the area where the turnip greens protrude from the soil. For best results, use your fingers to move the soil, so you avoid damaging the turnips.
Assess the size of the turnips after you remove the soil. Generally, turnips are ready to harvest when the roots reach at least 1 inch in diameter. Optimal turnip size is about 1 1/2 to 3 inches.
Use a trowel to dig up a turnip in the optimal size range, and assess the turnip for ripeness. A ripe turnip will feel firm and heavy in your hand.