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There are many many varieties

There are many many varieties of sunflowers--from container size to mammoth.  Planting sunflower can begin anytime after soils have warmed to 50°F. If planted in early June, the large sunflowers (Helianthus annuus L.) in our area will be in full bloom after about two months in early August. For the timing of your wedding, you probably want to find a midseason bloomer versus an early or late bloomer. Check the seed packets to get an idea of how long it takes to get a plant into full bloom. Then back out from there.  We would stagger plantings to ensure you hit the right date.