I think this is brussels sprouts? Do I just let it keep going, and the sprouts will come up the center? Hopefully? If I didn’t plant it too late? Do people do anything with all those big leaves?
With nasturtium below — I think nasturtium leaves are awfully pretty. They’re tender with a slightly sweet, green, tangy, and peppery taste, nice to add to salads, along with the yellow and orange flowers, not currently visible here. Nasturtiums are the official flower of Oak Park. 
