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hardlikearmour is a trusted home cook.
You can see the top of the beet and when it gets large enough you pick it. There is size variation amongst the different varieties of beets.
I don't know when you planted them, but when you thin (each beet "seed" is actually several seeds), you can put the greens in salad. Later on, when the little ball of the beet is evident (feel around carefully), you can thin again, dicing the baby beets and chopping the greens to steam away on top. When they are fully grown (about 45-50 days), the beet itself is pretty visible at the soil line. The greens may have toughened by then. The ripe beets will rest comfortably in the garden for quite a while, getting bigger but generally still good to eat. You can continue to plant beets (depending on where you live, but in New England there is still time--my fall crop is just up) as they prefer the cooler weather of fall.
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