As sow thistles get a bit older, and start to flower, the leaves become tougher, more bitter and a tad fibrous. The prickles can also become stiffer and less pleasant to eat. At this stage it's best to cook the leaves completely before you do anything with them. But this is the stage when the buds are also ripe for harvest.
The buds don't taste all that different from the rest of the plant, a mellow sweetness within, sort of like corn, would be the only difference. Otherwise it's that same rich green, slightly bitter flavor, rather like kale.
One of the most popular ways to eat any slightly bitter bud is to pickle it, and make sort of a mock caper--it's really crazy how much they tastes like them when done!
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