When I got home I did some research and learned that you can make 'capers' from either the flower buds (harvested when the buds are slightly loose and green) or from the seed pods (also green but larger pods that appear after the pretty purple flowers fall off).
I'm lactofermenting the flower buds using this recipe.
Here are some pics of the process so far:
Flower buds:
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I discarded these. They were too close to flowering:
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'capers' in their salty brine:
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In a few weeks I will forage some milkweed seed pods and experiment with the recipe in Sandor Katz' book Wild Fermentation.