Harvesting root vegetables is very straightforward, but there are some tricks to knowing when is the best time to harvest. Unlike the greens in your spring and fall gardens, once we harvest root vegetables, they are gone.
An easy way to tell if root vegetables are ready to harvest is by peeking just below the surface of the soil to assess the size of the actual root. This technique will be easiest with radishes, turnips, and beets. Basically, you monitor their growth and harvest at the optimal size for whatever variety you’re growing. At this point, having a record of not only what vegetables you planted, but the varieties of each vegetable will help in knowing what size to watch for. For example, a hakurei turnip can be harvested when it’s the size of a ping pong ball, but you’ll want to leave purple top white globe turnips until they are the size of softballs.
For carrots, it’s almost impossible to see how large the roots are just by brushing aside the surface-soil. So, in this case, it’s useful to find the carrot plant with the largest top (the part of the plant where the edible carrot that we know and love meets the leafy foliage). Pull that largest one out and assess the size of it. Is it the size you want? If so, feel free to start harvesting more whose tops have a comparable size to your test-carrot. If not, leave them in the ground for 1-2 weeks longer and test another one. Carrots have a nice range of temperatures at which they grow, but they will flower when left in the garden too long.