Que signifie 'Some of the vegetables have gone to seed.'  ?

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It is discussing the situation on a farm or in a garden where someone is growing vegetables. Typically vegetables are grown for their edible leaves (such as cabbage or kale) or edible flowers/florets (such as broccoli or cauliflower). However it is a normal part of a vegetable's life cycle to develop seeds once they have flowered, so the timing of harvest is critical. Although some plant seeds might be wanted and collected for sowing the following year (or rarely to be eaten), saying that the plant 'has gone to seed' in many cases is a cause for disappointment as it means that the point of harvest has been missed and the plant is no longer likely to be tasty or useful.