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I have been growing Romanesco - that funny Italian cross between cauliflower and broccoli - for the first time this year. The heads have started to appear, but it has just occurred to me that I have no idea how to tell when they are ready for harvesting. I don't want to leave it too late, in case they go to seed, or something nasty like that. On the other hand, I want to give them as long as possible to bulk up.
Does anyone have any advice on the best time to pick?

Two days before they started bolting...  :-X
but seriously as soon as you feel the head is big enough...

It is not a variety I am familiar with but like any other cauli/broccoli they are made up of 'florets'

Initially these florets are close packed and you dont normally see the join/s but once they are up to their optimum size the florets will start to grow apart, this is the stage you are looking for, i.e. when you see the hairline cracks.

But beware once they reach this stage they will soon bolt and run to seed so keep a watchful eye on them.