Answer: Although it is tempting to try to bring plants into bloom while they are still indoors, it's not usually a good idea to do so in that artificial environment. Your zinnias will grow best outdoors in real, full sunlight and will not hit their stride until they are fully acclimated to the out of doors and have become well adapted to their growing location.
Younger plants adapt faster and survive the move outdoors better than older plants. A good strategy is to have them filling their pots but not crowded when it is time to set them outdoors, usually a week or so after the last expected frost when the weather has settled. (Zinnias like hot weather, so do not let them get frosted.) You will need to acclimate them slowly to the harsh realities of outdoor life, and then, once your zinnias start blooming they will bloom for the rest of the summer and long into fall. Removal of spent blooms and maintaining good fertility and regular watering will help keep them blooming their best.
The length of time it takes for them to come into bloom will depend to some extent on their growing conditions, but an average would be about eight to ten weeks. Oddly enough, stress can trigger bloom while at the same time weakening the plants in the long run so early patience is really the better approach for maximum bloom here. Enjoy your zinnias!
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