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After zinnias flower, cut off the old flowers (a process called “deadheading”) to encourage more flowers to form. Zinnias are annuals and will die with the first fall frost. If you want them to reseed, let the last flowers of the season mature fully and scatter their seeds.
Also asked, should you cut back zinnias?
Prune zinnia stems and blooms frequently to encourage thicker growth. Allow the plant to grow several inches before pruning. Prune back the stems using floral pruning shears. Plants purchased full size from a garden center should be trimmed back to about one-third of their height.
Also asked, do zinnias bloom again after cutting?
Zinnias bloom more profusely once cut back.
Zinnias are annuals, so they'll grow for one season and produce seeds, but the original plant will not come back in subsequent years. Double-flowered zinnias have numerous rows of petals and their centers are not visible.