Answer: Watch the heads closely as they grow in size. It doesn't ripen, and so you could pick it anytime you want. However, you want it to reach maximum size without losing quality. Therefore, when the heads are just beginning to get "loose" (the buds are not as tightly packed) and the individual buds start to swell up like they are getting ready to open, you should go ahead and pick it.
Remember that most types will produce some smaller side shoots below where you cut the head off so don't remove the plants, but leave them for more harvesting later.
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