A few bok choy (Brassica rapa [Chinensis Group]) flowers before the harvest don't have to mean total failure. The flower stalks indicate the first stages of the end of life for this cool-season vegetable, meaning no more leaves for the harvest. When the first yellow flowers appear, you have a few options to make the best of the situation.
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A Good Time to Harvest
When one or two bok choy plants send up a flower stalk, the rest of the plants generally aren't far behind. At this point, harvest the rest of the bok choy plants by cutting each plant at the base, slightly above the soil line, with a knife. Cut carefully away from yourself to avoid injury. Wash the harvest knife in hot, soapy water or wipe the blade with rubbing alcohol. Use bok choy fresh or store it in the refrigerator for two to three weeks.
Pick the Edible Flowers
The bok choy's edible flowers taste best right before they open. If your bok choy crop is starting to produce elongated stalks and flowers from the center of each plant, pluck off the flower stalks and add them to salads. If the plant has only just begun to flower, you may still get a leaf crop. Pick a few leaves and taste them. If they're woody, pick the rest of the flowers and toss the plants on the compost pile. Otherwise, harvest the best tender leaves and flower stalks before discarding the plants.
Collect the Seeds
When bok choy flowers, you can harvest the seeds if you're growing an heirloom variety. (Seeds from hybrid plants may not germinate; if they do, second-generration plants may not grow true to type.) Allow a group of plants to remain standing -- ideally two to three or more for pollination purposes -- until the flowers turn into seed pods. As the seeds mature, the pods turn from green to brown as they dry out. Cut off the pods and put them in a paper bag for a few weeks then break open the pods and dump the seeds into a separate paper bag. Store them in a dry spot until next planting season. Properly stored seeds can remain viable for up to five years.
How to Prevent Bolting
Bok choy bolts (flowers and then goes to seed) rapidly in hot weather, but you can slow the process by growing from seed in late summer and early fall. In mild climates, plant bok choy in late fall and grow it through the winter. Bolting also occurs when optimal spring temperatures turn cold for a prolonged period and then suddenly warm again. Plants that are larger and more mature are affected the most by these changes in temperatures.
You can even grow bok choy through the winter in areas with mild frosts. Bolting is less likely to occur by planting transplants that haven't been stressed, keeping the garden weed-free and maintaining consistent water practices. Also, look for bolt-resistant cultivars like Joi-Choi and Chin-Chiang.