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You can dig up the tubers in fall, store them over the winter and plant them again next spring. In the second year, the plant grows taller, flowers and produces seeds before dying. Dahlias, on the other hand, are perennials. In their native warm climate, they re-sprout from their underground tubers to bloom each year.
Consequently, do dahlias bloom more than once?
Depending on where you live, that could mean that you can have dahlias in bloom for three, four or even five months. However, the most important word in that first sentence was can. Once you know how to grow dahlias, they can flower for extended periods but only if you, the gardener, help them do so.
Simply so, do you Deadhead dahlias?
The flowers produce seeds after they begin to wilt, which detracts from the plant's beauty and may shorten the blooming season. Removing, or deadheading, the spent flowers ensures a healthy, continual bloom. Inspect the dahlias at least once a week for dead or dying flowers.
Dahlias benefit from a low-nitrogen liquid fertilizer (similar to what you would use for vegetables) such as a 5-10-10 or 10-20-20. Fertilize after sprouting and then every 3 to 4 weeks from mid-summer until early Autumn. Do NOT overfertilize, especially with nitrogen, or you risk small/no blooms, weak tubers, or rot.