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cold and sunflowers
Most common sunflowers are annuals, which only live one year. If you have one of these, it is unlikely to do well indoors if you bring it in when temperatures gets colder, since at that time the plant is likely nearing the end of its life cycle. Plus, the added stress to the plant due to the move (unless, perhaps, it is already in a container) may shorten its days even more. Some people have had success growing sunflowers inside in winter under fluorescent lights in a warm spot. (These plants were grown as houseplants, and hadn't ever been outdoors.) In warm climates, sunflowers sometimes can be planted outside for bloom in winter, although they may not be as vigorous as summer plants. Perennial sunflowers will die back in winter and re-sprout in spring. Normally for these, you wouldn't have to bring it indoors when it gets cold. In fall, after the plant has died back, cover the soil with mulch to protect the roots over winter.