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It could be that your plants
It could be that your plants need larger pots, especially if the varieties that you have are the larger types of sunflowers. Also, be careful to water at the base of the plant; water on the leaves during hot sun will burn them. Make sure the pots offer good drainage—that there are drainage holes at the bottom, so the soil isn’t soggy. Have the plants gone through any environmental changes when you brought them home? For example, have nights been cooler? Is the site you selected much hotter or are they in more direct sun for a longer time than when they were at the store? If so, then it could be the plants are under stress and are trying to acclimate to their new space. Does it look as if the old flowers are going to seed? If so, you can deadhead them to encourage blooming. Hope this helps!